Carine Milcent
email: carine.milcent@psemail.eu

Binzhen Wu

How Do You Feel? The Effect of the New Cooperative Medical Scheme in China

Keywords: Subjective health, Rural China, Health insurance, New Cooperative Medical Scheme. JEL Classification: D82, H52, I18

During the 2003-2006 period, subjective health status in Chinese rural areas improved. We used a unique household longitudinal survey to analyze how the introduction of a public insurance system has contributed to the change. This program is based on a doubly voluntary process: counties decide to launch, then households decide to subscribe. We disentangle two channels of influence of the insurance: the insurance effect of the coverage and a general equilibrium effect on all residents in the insurance-adopting counties. The empirical findings include, first, a positive extensive margin: individuals feel better about their health status when covered by the NCMS. However, there is no intensive margin: an individual's self assessed health status does not improve with the number of years enrolled in the program.Second, we find a positive general equilibrium effect of introducing the NCMS program on non-participants in NCMS counties. This effect accumulates over time.

Introduction

In recent years, the self-reported health of the Chinese rural population has worsened. The percentage who declare themselves to be in bad health rose by about 15 percentage points between 1991 and 2004 [START_REF] Milcent | Industrialisation and Inequality: Healthcare Use in Rural China[END_REF]. Over the same period, there has been a drastic decline in the utilization of health care [START_REF] Bloom | Health Sector Reform: Lessons from China[END_REF][START_REF] Hesketh | Health in China: The healthcare market[END_REF][START_REF] Gao | Changing access to health services in urban China: implications for equity[END_REF]. A common explanation of these two phenomena is the rise in the price of health care and the absence of a health insurance market. One of the major causes of impoverishment in rural China is now the financial hardship caused by ill health [START_REF] Liu | What is wrong with China's health system[END_REF]. In 2003, 80% of rural residents in China did not have any health insurance [START_REF] Wagsta↵ | Extending health insurance to the rural population: An impact evaluation of China's new cooperative medical scheme[END_REF]. In the same year, a new health-insurance system, the New Cooperative Medical Scheme (NCMS), was introduced in rural areas. By 2009, more than 94% of eligible rural residents (833 million people) were covered by the NCMS (China Statistical Yearbook 2010). This paper uses a unique household longitudinal survey which allows us to distinguish both i) the e↵ect of the health-insurance market on self-assessed health (SAH), and ii) the e↵ect of insurance coverage on SAH.

Individuals do of course have some idea about their own health status, but this knowledge depends on their medical know-how and the frequency with which they go to the hospital or to the doctor. The need of individuals for healthcare is determined by their objective health, which is only partially known to them. The perception (so the self-assessment) that individuals have of their own health does not determine their need for healthcare but it determines their demand for healthcare access. The role of public insurance is to improve the a↵ordability of healthcare, and allow individuals to fulfill their demand for healthcare access without, or at least with less regard to, their budget constraints. Hence, public insurance is a political tool which responds to healthcare demand based on individuals' subjective health. The NCMS program o↵ers a quasinatural experiment which allows us to address a novel question: How does the introduction of a health insurance program change individuals' views of their subjective health?

Moreover, insurance has the e↵ect of modifying the individual's perceived health, via greater use of healthcare. On the one hand, we expect the individual's health to improve, via the greater use of healthcare associated with looser budget constraints; on the other hand, more frequent healthcare use should also give individuals more accurate knowledge of their objective health status, which will also a↵ect their perceived health. Moreover, the presence of health insurance may also a↵ect the supply of healthcare. As individuals become less budget-constrained, the supply of healthcare may become both better-quality and more expensive. As such, the mere presence of health insurance may have an impact on the healthcare demand of those who are not covered by the insurance. The budget constraints of the latter may be a↵ected because of these changes in the supply of healthcare; a resulting e↵ect on their subjective health may then be expected. Last, as the use of healthcare rises, it is likely that a greater amount of information about health will circulate between individuals. These assumptions will be tested in this paper.

The implementation of NCMS health insurance program is not unique. There are di↵erent programs targeting interventions for ethnic minority and low-income populations (Medicare and Medicaid programs and now, Obama public healthcare plan in US; La Carte d?Assurance Maladie (CAM) in Burundi; the Policy on National Health Insurance (NHI) from 2011 in south Africa). However, most of the studies are from developed countries. Substantial empirical studies have discussed public health insurance plan at a cross-sectional level. The widely known RAND Health Insurance Experiment is that higher cost sharing lead to reduced healthcare expenditures [START_REF] Manning | Health insurance and the demand for medical care: evidence from a randomized experiment[END_REF]. Because the NCMS program is on voluntary basis with a low coverage rates and target the low-income populations, it may recall the more recent Obama public healthcare plan in US. According to our knowledge, this is the first paper focusing on this type of insurance program on enrolee population and non-enrolee population in the long run.

Although the NCMS has been widely seen as one of the most important reforms in rural China since the economic reforms of the 1980s, the systematic empirical evidence on this issue is very limited, with a noticeable exception in [START_REF] Lei | The New Cooperative Medical Scheme in Rural China: Does more coverage mean more service and better health?[END_REF] finding of no significant improvement in self-reported health status or sickness or injury in the past four weeks after families acquired insurance coverage. Similar to the studies on public health insurance programs in other countries, the few papers that have evaluated the NCMS program have all focused on its e↵ects on healthcare utilization or the healthcare spending of households. However, it is not clear whether more (less) healthcare utilization means better (worse) self-assessment of health status, 1 not to mention that evidence in the literature on the e↵ect of the NCMS on healthcare utilization is actually mixed [START_REF] Wagsta↵ | Extending health insurance to the rural population: An impact evaluation of China's new cooperative medical scheme[END_REF][START_REF] Lei | The New Cooperative Medical Scheme in Rural China: Does more coverage mean more service and better health?[END_REF]Bai and Wu, 2010). Despite the fact that the main purpose of the NCMS is to improve the a↵ordability of healthcare services, empirical evaluations have not found solid evidence of a decrease in out-of-pocket health expenditures (per visit) owing to the NCMS [START_REF] Wagsta↵ | Extending health insurance to the rural population: An impact evaluation of China's new cooperative medical scheme[END_REF][START_REF] Lei | The New Cooperative Medical Scheme in Rural China: Does more coverage mean more service and better health?[END_REF]Bai and Wu, 2010). It has been suggested that suppliers may have exploited the benefit by increasing the price of healthcare services or increasing the number of visits or unnecessary tests or procedures for each patient, resulting in an increase in total healthcare expenses. This increase can also a↵ect both people's health status and their assessment of it. For example, a decrease in the a↵ordability of healthcare for the uninsured may have a negative e↵ect on their views of their health status. Moreover, the NCMS program a↵ects not only insured families, but also non-participants in the counties that have introduced the program (labeled as NCMS counties). We call these e↵ects general equilibrium e↵ects. These generally do not directly result from better healthcare. There are also other general equilibrium e↵ects, including a reinforcement of awareness of health risks when the government tries to convince people to participate to the program, the spillover of health-related knowledge, and peer-comparison e↵ects when exogenous shocks a↵ect the health status of individuals in the community.

Due to the fact that the NCMS has been rolled out sequentially in a series of counties, 2 and that counties and households decide whether to participate voluntarily, three types of households can be distinguished in each year: the insured, non-participants in villages within a NCMS county, and the non-exposed in villages within counties that have not launched the program (labeled as non-NCMS counties). Three e↵ects of the program can be identified after addressing potential selection biases: the insurance e↵ect, which works only on the insured, by comparing the insured with non-participants; the gross e↵ect of the program, by comparing the insured with the nonexposed; and the spillover e↵ect, by comparing non-participants with the non-exposed. Previous studies have focused on either the insurance e↵ect or the gross e↵ect of the program: our paper distinguishes between them.

This paper draws upon a unique dataset that combines a longitudinal survey carried out in rural China (National Fixed-Point Survey, or NFS) from 2003 to 2006 and a household survey on the NCMS that we conducted with a subsample of the 2006 round of the RFPS. The way the NCMS was introduced provides a quasi-natural experiment which can be used to examine how this public insurance program changed people's self-assessment of health. Nevertheless, because county enrollment is not completely random and participation is voluntary, the main challenge of the study is selection bias among both counties/villages and households. To deal with selection bias, we combined a matching method with the di↵erence-in-di↵erence framework. We found, first of all, that individuals feels better about their health status when they are covered by the NCMS. However, there is no additional gain in self-assessed health status with additional years of enrolment in the program. Second, we find a positive general equilibrium e↵ect of the introduction of the NCMS program on non-participants in the NCMS county, which accumulates over time.

Third, besides the insurance program, household composition (family size, number of young and old members) and socio-economic status (income, occupation, and education) also have strong e↵ects on self-evaluated health status.

The paper is organized as follows: Section 2 summarizes the literature and describes the Chinese context; Section 3 describes the data and gives preliminary statistics; Section 4 explains the econometric strategy; Section 5 presents the results, and Section 6 concludes.

2 Literature Review and Chinese Context NCMS programs have several key features: 1) the program is targeted at rural residents; 3 2) participation is voluntary but must be at the household level; 3) participants are required to pay flat-rate premiums, but their contributions are heavily subsidized by governments; 4) the programs mainly reimburse large expenses so as to ease the economic burden resulting from catastrophic disease and alleviate illness-caused poverty; 5) the programs are operated at the county level rather than the township or village level. The primary goal of the NCMS is to reduce impoverishment resulting from illness and improve the a↵ordability of healthcare (Central Committee of CPC, 2002). Local governments have been granted autonomy to design, implement and supervise the programs.

Participation in NCMS insurance is voluntary but the unit of participation is the household.

There is reason to assume that household subscriptions are based on the average health status of members of the household, but not that they are based on every member of the household having a poor health status: the system thus reduces adverse selection bias [START_REF] Dror | Field based evidence of enhanced healthcare utilization among persons insured by micro health insurance units in Philippines[END_REF]. However, although rural inhabitants are required to participate as household units in order to reduce adverse selection, the elementary conclusion drawn from previous studies is that the system cannot prevent adverse selection [START_REF] Wang | W Hsiao Adverse selection in a voluntary Rural Mutual Health Care health insurance scheme in China[END_REF]; Wagsta↵ et alii, 2007;[START_REF] Zhang | 0.6%' Chronic'disease' 7.2%' 4.6%' 7.8%' 7.5%' Household'size' 4.6' 4.6' 4.6' 4.6' Percent'of'members'older'than'65' 33.0%' 40.0%' 32.0%' 32.7%' Percent'of'members'younger'than'10' 47.0%' 48.0%' 49.0%' 48.1%' Age'of'head'of'household' 51.7' 50.2' 50.5' 50.9' Head's'years'of'education' 6.7' 6.5' 6.5' 6.6[END_REF]. 4 In this paper, we control for household selection bias.

While local governments have some discretion over the level of financing of the program, the standard in 2003 was for each participating household to pay at least 10 RMB (about e1 and about $1.2) for each household member every year, with the local government providing more than 10 RMB for each person per year. The central government would also match with 10 RMB per year for each beneficiary living in the central and western provinces. As of 2006, while individual contributions generally remained at the existing level, subsidies from local and central government increased [START_REF] Wagsta↵ | Do health sector reforms have their intended impacts? The World Bank's Health VIII Project in Gansu province, China[END_REF]. NCMS insurance mainly provides financial risk protection to patients with catastrophic health problems. Many services-particularly outpatient care-are not covered, deductibles are high, ceilings are low, and coinsurance rates are high. In fact, NCMS pilot counties were not randomly selected. Instead, a complex set of criteria, including local interest and capacity, level of economic development, and the status of the delivery system were considered. The voluntary nature of NCMS raises concerns about an adverse selection that may represent a serious threat to the financial sustainability of the NCMS. In this paper, we control for the county selection bias: in other words, we control for the e↵ect of living in a county selected to have an NCMS on self-assessed health (SAH).

Along with the NCMS, local governments provide some supporting policies, such as improvements in rural healthcare (delivery) networks and health service provision, the strengthening of pharmaceutical governance, and supply chain construction. These improve the quality and delivery of healthcare service and also benefit families who choose not to participate the NCMS. Therefore, the adoption of the NCMS program by the county may have a positive e↵ect on healthcare access for both the NCMS-insured and the NCMS-uninsured.

NCMS insurance programs make healthcare services accessible to those who were previously not able to a↵ord them due to not being covered by any insurance scheme. As a result, providers have a greater population of potential patients. According to the theorical literature, these healthcare providers may have an incentive to increase the price of their healthcare services to maximize profits to the greatest extent compatible with the continued financial viability for patients of being treated in their facilities. [START_REF] Feldstein | The Rising Price of Physicians' Service[END_REF] showed that physicians raise their fees when insurance becomes more extensive-i.e., when a large part of the population comes to be covered by health insurance. [START_REF] Chiu | Health insurance and the welfare of healthcare consumers[END_REF] uses a formal model to show that the introduction of insurance increases the equilibrium price of healthcare. In such cases, it is uncertain whether or not insurance will reduce the financial burden of sick people. [START_REF] Chiu | Health insurance and the welfare of healthcare consumers[END_REF] shows that if the supply of healthcare is su ciently price-inelastic, this increase in price always leads to a reduction in consumer welfare. Table 2 displays the descriptive statistics on self-reported health status. It is based on the answer to the question "How do you feel about your health status?" and there were five options: Excellent, Good, Fair, Bad, Very bad. As can be seen in the table, more than half of the sample felt excellent about their health, while about 12% of individuals reported fair or worse health.

Data and Descriptive Statistics

Over the years 2003-2006, the trend in the number of people who felt excellent was positive.

Average SAH went from Good to Excellent. Table 2 also relates the subjective health status to enrollment status in the NCMS. The results highlight that both the insured and non-participants experienced an improvement in health status over the period studied. In fact, non-participants experienced even greater increases in subjective health than the insured. This improvement is at 1% level significant for the former whereas this is not significant at 10% for the insured ones. In contrast, the non-exposed experienced a deterioration in subjective health, significant at 1%-level.

However, this might result from dynamic selection, as it is possible that people who continue not to participate are those with excellent health, and that villages in counties which continue to be excluded from the NCMS program are those with bad average health. Table 3, Column (4) gives preliminary statistics on individual, household and village variables on the whole sample (2003)(2004)(2005)(2006)). An inhabitant with no education is the reference. Women made up 47.5% of the sample, and the average age was 37. About 27% of the sample are doctors, and the villages on average deliver information on health more than 3 times a year. We also have information on chronic disease (such as Hypertension, Heart diseases, Nephritis, and Diabetes), other serious diseases (hepatitis A, hepatitis B, phthisis, AIDS, schistosomiasis), and activities of daily living (ADLs: cleaning yards, bowing and bending knees, bathing, feeding, dressing, getting out of bed) for each individual in 2007. Specifically, 7.5% of the sample su↵er from a chronic disease. One half of the rural inhabitants in the sample have a primary school education or less.

The proportion of laborers was around 55%. The percent of minority ethnic households was about 10% of the sample, which corresponds to the o cial statistics. Di↵erences were observed between the samples: among non-participants and the non-exposed the percent of minority ethnic households was more than 15%. Average household size was around 4 members. On average, households included at least one member below the age of 10 years, and at least one over the age of 65 years. In China, the size of the household is restrained by the one-child policy. While in rural areas, the policy constraint is less strictly enforced than in urban contexts, households still face monetary penalties according to the number of excess children. Moreover, children are quite often raised by their grandparents. For reasons related to finances, tradition, and registration permits ("hukou") 7 , the children remain in their hometown and may be a source of concern for their elderly caretakers.

Wubao individuals are those classified as under the poverty line as defined by the local government. They are then eligible for public policies aimed at helping the poor. They made up around 0.2% of our sample. Similarly, 18% of villages were considered pingkun, meaning that the village as a whole was under the poverty line as defined by the local government. In such cases, the whole village is eligible for governmental help, which can be financial or involve infrastructure improvements. Thirty-eight percent of villages were classified as xiaokan by the local government.

These villages possess enough wealth to be above the poverty line defined by the local government. 8 Fifteen percent of the households lived in the suburbs and 14% were located in a "township". 87% of the towns are in areas administratively defined as "agricultural regions". In topographical terms, half of the villages are located in hill or mountain areas.

The data presented in Table 3, Columns (1) to (3) also shed some light on the di↵erences between the insured, non-participants and non-exposed rural inhabitants. Since the number of counties and households participating in the program increased over time, 2006 includes more observations for the insured group, while for 2003 there are more for the non-exposed group.

To ensure that the values of the variables, which may change over time, is comparable between these three groups, we focus on their values in 2003. Subscribing households generally had higher income and include fewer members who are older than 65 or younger than 10. The heads of these households were slightly older, more educated, less likely to be female, single, or a non-agricultural worker. They were also more likely to include members of the Communist Party, and less likely to be members of a minority or in poverty (wubao). Among non-participants and the non-exposed the percentage of ethnic minority households was more than 15%, but in the insured group it was under 10%. Villages in NCMS counties were also much richer and had fewer clinics but a higher rate of child vaccination than those in non-NCMS counties. They also included fewer migrants, more laborers, had higher average education levels and were less likely to be located in mountain, western or central areas.

Econometric Strategy

The subject of our modeling is the response to the survey question "How do you assess your health?", where answers range from very bad health (coded as 1) to excellent health (coded as 5) on a five-point scale. Y is measured on a discrete ordinal scale. We use an ordered probit model. Our empirical analyses exploit the quasi-natural experiment arising from the introduction of NCMS programs to examine their e↵ect on the self-assessed health of individuals. As mentioned above, each period includes three types of households: participants in the NCMS program, nonparticipants within NCMS counties, and non-exposed households located in non-NCMS counties.

To learn how the introduction of the insurance program a↵ected individual self-assessed health, we begin by applying the di↵erence-in-di↵erence (DID) framework. More specifically, the e↵ects of the NCMS are identified through di↵erences between insured and uninsured households in changes in SAH over the time periods before and after the local launch of the NCMS. The advantage of this double-di↵erence framework is to eliminate time-invariant selection bias. This is crucial in the present context because participation in the program is voluntary, meaning that households which chose not to participate may di↵er from participants in both observable and unobservable characteristics. Moreover, program distribution over counties may be non-random. As a result, the members of insured and uninsured households may have had di↵ering SAH even in the absence of the NCMS. The double-di↵erence method can deliver unbiased and consistent estimates as long as the changes in household SAH over time would have been parallel were there no NCMS. Su cient conditions are: (a) that trends in period-specific unobservables are the same in treatment and control groups, and (b) the expected change in idiosyncratic errors is zero for both the treated and untreated. Hence the claim that double di↵erencing makes it possible to estimate the impact of a program if the unobservables that di↵er between the treated and the untreated are timeinvariant [START_REF] Wagsta↵ | Extending health insurance to the rural population: An impact evaluation of China's new cooperative medical scheme[END_REF].

The introduction of the NCMS program may have had two types of e↵ects: one, that of insurance coverage, has been widely studied in the health insurance literature. The other is the e↵ect of the introduction of a large-scale public insurance program, and other concomitant changes in healthcare provision, on both, participants and non-participants. Indeed, the introduction of the NCMS coincided with other relevant changes: in particular, governments implemented supporting policies to improve the quality and delivery of healthcare services. In addition, anecdotal evidence indicates that the price of healthcare services increased after the introduction of the NCMS (Mao, 2005). To identify the insurance e↵ect of the program, which results only from the insurance coverage itself and excludes the e↵ects of the aforementioned contemporary policies and changes, we use the double di↵erence method to estimate the di↵erence between participants and non-participants in NCMS counties, because both groups were a↵ected by these changes. By contrast, the double di↵erence between non-participants and the non-exposed yields what we call the spillover e↵ect of the NCMS on non-participants. Refinements such as matching DID and tests of the identification assumptions are discussed in Section 6.

Di↵erence-in-Di↵erence Model

To facilitate understanding, we first present the model where the treatment is participation in the program given that the county has enrolled. The regression model for the double-di↵erence comparison between participants and non-participants is as follows:

Y ⇤ ihjt = X ihjt + HHinsured ht + ⌧ t T t + HH h + ✏ 1 ihjt (1)
HHinsured ht = 1 if the household h partipates to the NCMS program at the year t such as t > t 0 (2)

where Y ⇤ ihjt represents the SAH of individual i of household h located in village j in period t. Here, t 0 is the year just before the first year of the household's participation. HHinsured ht is the binary variable that indicates whether household h has subscribed to the NCMS in year t. T t includes three year dummies that control for the trend over time that is common to all groups. This assumption of a common trend for all groups is discussed in the Robustness section. HH h includes all the household indicators. HH h controls for all time-invariant household characteristics. X ihjt includes observable individual variables that vary over time and may a↵ect SAH (See Table 3 for details). represents insurance coverage e↵ects, as it shows the di↵erence in outcomes between participants and non-participants.

We now include the non-exposed.

Y ⇤ ihjt = X ihjt + HHinsured ht + ⌧ t T t + HH h + CountyEnrolled jt + ✏ 2 ihjt (3)
CountyEnrolled jt = 1 if the county j enrolled in the NCMS program at the year t such as t > tt 0 (4)

Here, tt 0 is the year just before the year of the county's enrollment. The SUTVA assumption is discussed in the Results section.  gives the spillover e↵ects of the NCMS program showing the di↵erence in outcome between the non-exposed and non-participants.

Furthermore, we assume a common trend for all groups, attributing any di↵erence in trends between the treatment and control groups that co-occur with the NCMS program to the program.

This assumption may be debatable, so we attempt to check its robustness on the results by performing a placebo test using a "fake" treatment group. Here, we present the results for the year 2005. The "fake" treatment group is the 2006-NCMS-households (i.e. households who had not yet subscribed in 2005) and the control group is the group of households without NCMS in 2006 (and thus also without NCMS in 2005). On this analysis there is no significant e↵ect of the NCMS program. Hence, our original DID is not biased (results are available on request).

We also make the stable unit treatment value assumption (SUTVA), which implies that i) the treatment status of any county does not a↵ect the potential outcomes of another county (noninterference); ii) the treatments for all counties are comparable (no variation in treatment). The supply side may be thought to change in non-treated areas because of the attraction of suppliers to NCMS-counties due to the increase in a↵ordability of the healthcare for the NCMS-insured.

Thus, as a robustness check, we control for supply side variables. The results are shown in Table 4, last column. We then compare the NCMS e↵ect with and without this control (Columns ( 5) and ( 6)). The absence of change in the NCMS e↵ects suggests that the SUTVA assumption is valid. 9

5 Results and Discussions

NCMS Insurance Program

We now present the results on di↵erences in dynamic changes in individual SAH. The di↵erencein-di↵erence (DID) model applies a fixed-e↵ect regression that controls for fixed e↵ects of county, household and year. All the time-invariant e↵ects of household characteristics are controlled by household fixed e↵ects. In our context, the program is voluntary and households chose whether or not to participate. All the time-invariant e↵ects of county characteristics are controlled by the county fixed e↵ects. Similarly, enrollment in the NCMS program was not compulsory for counties.

Counties chose whether or not to enroll. Note that households did not change county during our period of observation. Hence, there was colinearity between county fixed e↵ects and household fixed e↵ects. We thus only consider the household fixed e↵ects that include the county fixed e↵ects. On average (or median), household fixed e↵ects are computed on 14 observations. The yearly time trend in SAH that is common to all individuals is controlled for with year fixed e↵ects.

The reference is a NCMS non-exposed man aged between 20 and 30 years old, non-educated, in 2003, living in a non-governmental township, in eastern and plains areas.

Controlling for the selection issue changes the results on the coverage e↵ect and the spillover e↵ect ( The amount of time that has passed since the NCMS program was launched may impact its spillover e↵ect on SAH. In the first year following its launch, the NCMS may have had a lesser impact than the following years. Several explanations may be advanced for this observation: as the years pass, increasing interest in health, increasing trust in the NCMS program, 10 as well as improvement in the community's feelings of satisfaction with respect to public provision of healthcare and healthcare access (by central and local governments) through the NCMS program.

We then allow for the possibility that the average treatment e↵ect varies over the course of the implementation cycle of the NCMS program by considering an additional e↵ect: number of years following the first year of county NCMS enrollment. Columns (3) and ( 5) give the results of this analysis. There is a highly significant positive association between seniority of NCMS county enrollment and self-assessed health. This additional e↵ect dominates the overall spillover e↵ect of the NCMS program.

Similarly, the decision to subscribe may a↵ect the coverage e↵ect on SAH. To assess how this e↵ect varies over time, we also consider an additional variable, distinguishing the years after a household subscribes from the NCMS subscription by itself. This seniority e↵ect is comparable to an intensive margin e↵ect. Columns ( 4) and ( 5) give the results of this analysis. The seniority of household NCMS subscription (more than one year) has no e↵ect on the SAH. Therefore, what really matters is the fact of subscribing, and not the length of subscription. In other words, there is an extensive margin but no intensive margin.

The model now controls for both the seniority of the launch of the NCMS program and the seniority of the NCMS household subscription (Column ( 6)). The seniority of household NCMS subscription (more than one year) still has no significant additional e↵ect beyond the overall coverage e↵ect. Therefore, the length of a household's coverage by the NCMS program does not improve self-reported health. One explanation is the high correlation between the coverage e↵ect and county enrollment after one year. Indeed, as shown in Table 1, from 2004, when a county had enrolled in the NCMS program, the percentage of subscribing households jumped to around 90%.

The spillover e↵ect increases with the seniority of the county's enrollment in the NCMS program.

Hence, the earlier the county enrolled in the NCMS program, the stronger the e↵ect.

Individual Characteristics and Geographical E↵ects

This section presents an overview of determinants of individual subjective health. Readers who are interested only in NCMS program e↵ects may skip this section. Results are presented in Table 6. First, as expected, age has a negative e↵ect on health index. 11 Women's self-assessed health tends to be poorer than that of men. Being single also decreases health status. This could be due to greater feelings of insecurity or due to taking less care of their health. The survey includes questions regarding individual physical health and individual health behavior. SAH is worse when the individual su↵ers from a chronic disease or common serious illnesses (hepatitis A, hepatitis B, phthisis, AIDS, schistosomiasis). 12 We tested the e↵ects of variables such as social class and labor market status to capture individuals' economic and financial status. Working in the industrial sector or having a non-agricultural job significantly improves SAH. The model reveals that education and income are strongly correlated with self-assessed health. The higher an individual's level of education, the better their health. This correlation partially results from an "age cross education" e↵ect, with older respondents being less educated on average due to increasing access to secondary and higher education over the past thirty years. Higher household income is associated with better perceived health. 13

Because of colinearity problems, focusing on the e↵ects of the geographical variables and household variables requires to regress the model without county fixed e↵ects and household fixed e↵ects. The results are displayed in Table 6.

Perceived health deteriorates with increasing household size. This may be explained by family composition. As explained above, the children of internal migrants remain in their hometown,

i.e. their village, with their grandparents. This may also be a cause of worry or fatigue in elderly grandparents. 14 SAH worsens with the number of old people (over 65 years old) in the family and with the number of young people (less than 10 years old) in the family.

Members of ethnic minorities are more likely to report being in bad health. Even after controlling for the level of village income and village status (as being under the poverty line as defined by the province), minority status remains a significantly associated with worse health status. Being a very poor family as defined by the local government (wubao household) has no significant e↵ect after controlling for household income and for being in a pingkun village (or not). A village classified as pingkun by the local government receives some subsidies. Higher household income is associated with better perceived health.

Moreover, having a doctor in the household or a household member with training in primary care is not associated with better perceived health. In contrast, having a cadre of the Communist Party in the household makes individuals more optimistic on their own health. A household member having a party position might help other household members to get a job and to resolve certain issues involving local government.

The estimated coe cient of living in an agricultural region on reported health is positive, whereas living in a "townships" deteriorates self-assessed health. Township areas o↵er more healthcare facilities and/or more access to healthcare facilities, but the price of healthcare is also higher than in the countryside. Furthermore, some other elements whose aim is to improve healthcare access are associated with this program. These include the expansion of healthcare facilities, free health examinations (in general, one per year), and medical training education. The presence and extent of these elements depends on decisions of the local government, and their e↵ects are unclear. While vaccinations for children improve individual health, the e↵ect of free clinical examinations and health training education are more ambiguous. Having access to more information on health status may also make people more aware of their health problems.

Robustness

The results of the robustness tests are presented in Table 5.

To test whether the identification assumption of the baseline model holds, we apply the same model to alternative samples. The primary concern with the baseline model is that households that enrolled in the NCMS may not be comparable to households that did not. Particularly, we are aware that households that had not joined by 2007 15 In this paper, we use an ordered probit model with fixed e↵ects. As mentioned above, household fixed e↵ects are computed on an average/median of 14 observations. However, for some households, the data includes only 2-5 observations. Therefore, we may have an incidental parameters problem.

One robustness check is to run the same model using only data from households with more than 14 observations. We call this sample E.

Analyses with di↵erent samples (Sample B, Sample C, Sample D, Sample E) confirmed the extensive margin e↵ect. In all samples, the intensive margin has no e↵ect on subjective health.

The other factor whose e↵ect is consistent for all the samples, is the spillover e↵ect for villages in NCMS counties with seniority of enrollment.

In the literature on the e↵ects of NCMS, authors such as [START_REF] Wagsta↵ | Extending health insurance to the rural population: An impact evaluation of China's new cooperative medical scheme[END_REF] consider the adverse selection issue-i.e., the possibility that insured individuals have specific characteristics that differ from those of non-subscribers. Table A.1 in the appendix gives the regression of household participation decisions against observable household characteristics. This analysis confirmed that most of the di↵erences in household characteristics are significant. Table A.1 also displays the regression of the county enrollment. Part of the di↵erences in county and village characteristics are significant . Moreover, the evidence for adverse selection is mixed: the participation rate increases with the share of members with good or fair health status, when the reference group is the share of members with excellent health status; however, participation rate actually declines when the share of members with bad health status rises.

We couple the DID approach with matching to reduce the selection bias on the observables.

The matching method can address two types of potential biases in the baseline method: bias caused by di↵erence in the supports of the observable covariates between treated and untreated groups, and bias due to the di↵erence between the two groups in the distribution of the related observables over their common support [START_REF] Smith | Does matching overcome LaLonde's critique of nonexperimental estimators?[END_REF]. The matching method applied here is propensity score matching. We estimate the propensity score in two steps. First, in each year, we estimate the probability of household enrollment in the villages where the county has launched the program. This is estimated on the basis of households in the NCMS counties. We predict the probability of enrollment of households in non-NCMS counties on the basis of this estimate. Second, we estimate the probability of county enrollment in the NCMS in each year.

For both steps, a probit model is estimated. For the comparison between the insured and nonparticipants, the probability predicted in the first step is the propensity score used in the matching.

For the comparison between the insured and non-exposed, both the similarity between households and that between counties must be considered; hence, the (composite) propensity score in each year, i.e., the product of the two probabilities predicted from the aforementioned two steps, is used in the matching estimation. Figure A1 shows the histogram for the (composite) propensity scores for the three groups: the insured, non-participants, and non-exposed. As expected, the distribution of the propensity score is more skewed to the right for the insured than for nonparticipants. Nonetheless, the region of common support is adequate. The propensity score distribution for the non-exposed is more skewed to the left than the one for non-participants because of the lower possibility of county enrollment.

Given that we are considering more than two periods, the traditional DID matching method requires modification. We first estimate the insurance e↵ect in each year from 2004 to 2006. Then we also determine an alternative measure: a weighted average of the insurance e↵ect over these three years calculated by weighting the e↵ect in each year on the basis of the ratio of the number of treated in that year to the total number of treated over the three years. The standard errors are bootstrapped with 100 replications. For each year, we use five-nearest-neighbor matching with replacement and caliper 0.01, and impose a common support condition. The distance between the propensity scores is measured by the Mahalanobis metric. We confirm that the results are not sensitive to the number of neighbors (including 3 and 10) or the choice of caliper (including 0.005 and 0.0025). Another complication here is whether to consider experienced participants (those who are in their second or later year of enrollment in the NCMS) among the treated. The counties that launched the NCMS earlier may have di↵erent characteristics that the ones who introduced later the NCMS. To test the robustness of our results, we also study the case where all the experienced participants are excluded. In this case, we focus on the first-year e↵ect of the NCMS on the SAH.

To summarize, Case 1 : On the whole sample, probabilities are computed year by year. Case 2 :

On the whole sample, a weighted average of the insurance e↵ect is computed. Case 3 : Excluding all the experienced participants, probabilities are computed year by year. Case 4 : Excluding all the experienced participants, a weighted average of the insurance e↵ect is computed.

In all cases, the result with the DID matching model is the same. NCMS household subscriptions improve individual subjective health but seniority has no e↵ect. A spillover e↵ect is also observed but this e↵ect depends on the duration of county enrollment: the e↵ect of the NCMS program increases with the time that the county has been enrolled in the NCMS.

Conclusion and Extension

During the 2003-2006 period, subjective health status in Chinese rural areas improved. We use a data for these years from a household longitudinal survey to analyze how the introduction of an insurance system, known as New Cooperative Medical Scheme (NCMS), contributed to this change. The health insurance program was launched sequentially over counties beginning in 2003, and households voluntarily choose whether participate in the program after it has been launched by their county. These facts make it feasible to disentangle two channels of influence of the NCMS on self-reported health status: one is the insurance e↵ect of the coverage, which can be estimated by comparing insured households with non-participants in counties that have launched the program (NCMS counties); the other is the general equilibrium e↵ect that a↵ects all residents of NCMS counties, which can estimated by comparing non-participants to non-exposed households (located in counties where the program has not been implemented). The longitudinal data also allow us to examine how the e↵ect changes with the duration of households' subscription to the program. In this paper, the variable of interest is the subjective health in itself. We do not consider subjective health as a proxy of objective health. We focus here on perceived individual health, a determinant of demand for healthcare access and treatment.

The finding of adverse selection is in line of several paper as the ones of [START_REF] Wang | W Hsiao Adverse selection in a voluntary Rural Mutual Health Care health insurance scheme in China[END_REF] or [START_REF] Zhang | 0.6%' Chronic'disease' 7.2%' 4.6%' 7.8%' 7.5%' Household'size' 4.6' 4.6' 4.6' 4.6' Percent'of'members'older'than'65' 33.0%' 40.0%' 32.0%' 32.7%' Percent'of'members'younger'than'10' 47.0%' 48.0%' 49.0%' 48.1%' Age'of'head'of'household' 51.7' 50.2' 50.5' 50.9' Head's'years'of'education' 6.7' 6.5' 6.5' 6.6[END_REF].

The empirical findings include, first, a positive extensive margin: individuals feel better about their health status when they are covered by the NCMS. However, there is no intensive margin:

there is no additional gain in self-assess of health status when the individual enrolls in the program for more years. A subsidized of the voluntary-based health insurance scheme provides appropriate amount of benefits that can o↵set the cost experienced by the participants. These high-risk buyers, namely, the less healthy population that have an incentive to buy more coverage than the more health population improve their feeling of health status with the enrolment. This feeling of improvement with the enrolment is unchanged over time. If the government is able to go on to subsidize the NCMS i.e. financially sustainable in the long run, the measure would ensure the benefit for the weakest part of the population in terms of health status.

Second, we find a positive general equilibrium e↵ect of introducing the NCMS program on non-participants in the NCMS county, which accumulates over time. Therefore, besides the likely positive e↵ect of a more aggressive subsidy or reimbursement policy (reducing co-payment rate), the existence of this public health insurance scheme lead to an improvement of the subjective health for the non-participant households that may justify this program in the long run.

Third, besides the insurance program, household composition (family size, number of young and old members) and socio-economic status (income, occupation, and education) also have strong e↵ects on the self-evaluation of health status. Not only is larger household size associated with worse self-assessed health, but the composition of the household also has an e↵ect: a larger number of old people is associated with worse perceived health, while the number of young people has a significantly positive association with perceived health. Both education and income have an e↵ect on SAH. A high level of education and a relatively high income lead to a better perceived health.

Controlling for wealth and geographical area, it turns out that ethnicity still has an impact: Being part of a minority has a significant negative e↵ect on self-reported health. Having a position in the Communist Party has a positive e↵ect on subjective health.

The paper studies the impact of the New Cooperative Medical Scheme on self-assessed health status. The change in self-assessed health status could be caused by real change in objective health or by the change in pure perceptional (or both). A natural extension of this study would be to get data with extensive information in objective health measurement in order to conduct an analysis on objective measures of health levels.

In the same way, we control here for all the aspect of county di↵erences in supply market, we use a county fixed e↵ect model. Then, we capture all di↵erences between county. However, we cannot explicitly determine which variables of the market supply influence the subjective health.

With data on details on the implementation of the NCMS program county by county will allow to assess the e↵ect of di↵erent types of health insurance scheme on health, namely, the pure part of subjective health and the objective health. As a next step, with appropriate variables, it will be interesting in showing the channels through which the impact of supply market on the subjective health took place.

Our findings focus on the role of the NCMS program in improvement in SAH in Chinese rural areas. Understanding the e↵ect of the implementation of a health insurance system on selfreported health may help policy makers to design insurance schemes with increased participation rates and ensure that rural populations gain better access to health services and enjoy greater protection against the cost of illness.

2. 1

 1 The New Rural Community Medical System (NCMS) Along with the economic reforms of the 1980s came the dissolution of the rural Cooperative Medical System, and illness emerged as a leading cause of poverty in rural China. The high cost of healthcare has deterred many families from obtaining necessary healthcare. In response, the Chinese government started pilot programs of the New Cooperative Medical System in 2003.

  There are more than 2800 rural counties in China. The NCMS pilot program began in 310 rural counties in 2003. It expanded to 617 counties in 2005, 1451 counties in 2006-i.e., 50.7% of the total number of counties-and started to spread across the country in 2007. By the end of June 2007, the program had expanded to cover 84.9% of all rural counties and 82.8% of all rural residents. It has covered the whole rural population since 2010. Provincial and county governments retain considerable discretion over the details of the pilots, including the placement of the pilot program.

  Our data combines the results of the longitudinal Rural Fixed-Point Survey (RFPS), carried out between 2003 and 2006, and those of a supplementary household survey aimed at evaluating the NCMS. The RFPS has been carried out each year since 1980s under the responsibility of the Chinese Ministry of Agriculture. Over the years 2003-2006, the same households were surveyed, with weekly book accounting recorded by these households as the primary information source. The sample was selected on the basis of a multi-stage stratified random sampling strategy. The 2006 round covers 26 provinces and 19,488 households. It provides information on individual demographics, including health status, and household characteristics, including income and expenditures. 5 The supplementary survey was conducted by Tsinghua University in May 2007. It surveyed a subsample of the 2006 round of the RFPS, collecting detailed information about the time when a county and a household enrolled in the NCMS. The final sample excludes some outliers, such as households who have exited the NCMS or who participated in some cooperative insurance program between 1993 and 2002. Also excluded are households who purchased commercial insurance in 2007. The final sample includes 520 villages, 17,715 households and 71,866 individuals over the 4-year sample period. 6 Table 1 shows the participation rates of counties and households between 2003 and 2006. The percentage of our sample counties enrolled in the program increased over the years, from about 16% in 2003 to about 77% in 2006. Similarly, the enrollment of individuals also increased over the years, from 9% in 2003 to 72% in 2006. These numbers are consistent with the national data (Chinese Statistical Yearbook 2010). In the villages within counties which had launched the NCMS (called NCMS counties), the majority of households participated in the program, with the participation rate increasing from 64% in 2003 to 95% in 2006. Moreover, most households started to participate in the first year that their county launched the program-the participation rate by county in the year following the launch of the program was 64% in 2003 and 96% in 2006. Nevertheless, these numbers also indicate that quite a few households chose not to participate in the program or delayed in participation. Over the four years from 2003 to 2006, about 12.8% of households in NCMS counties did not participate.

  may have already been covered by other insurance policies or had very specific concerns regarding participation. Therefore, in sample B, we excluded counties that had not participated by 2007. Given that the program was first piloted in July 2003, we exclude observations for 2003 from counties that launched the program in 2003, in order to avoid complications resulting from the start of the NCMS in the middle of that year. As a result, the year 2003 can be treated as the year where no counties introduced the NCMS: sample C. However, the counties that enrolled in 2003 were selected by the central government on economic and social criteria. The year 2003 was an introductory period for this reform. These counties may have some specific characteristics that are not well captured by the DID method. In sample D, data from 2003-enrolled counties are eliminated for all years.

Table 4

 4

, Columns (1) versus (2)). When no selection bias is considered, coverage appears to deteriorate subjective health, whereas the presence of general public health insurance leads to an improvement in SAH. Taking selection bias into account leads to a reversal of the coverage e↵ect of the NCMS program: household NCMS subscription improves individual SAH. The spillover e↵ect of the NCMS program does not significantly a↵ect self-reported health. To sum up, after controlling for di↵erences in county (household) characteristics according to the year of NCMS program introduction (NCMS household enrollment), a household's coverage by the NCMS improves perceived health.

& & &

& & &

Notes

1 On one hand, more healthcare utilization can lead to better objective and hence subjective health status; on the other hand, more utilization of the healthcare may lead to more awareness or anxiety of latent diseases (Koszegi, 2001).

2 A county is composed of villages

3 The program will also be o↵ered in urban districts and county-level cities that include rural residents.

4 For a more extensive review of this literature, see [START_REF] Zhang | 0.6%' Chronic'disease' 7.2%' 4.6%' 7.8%' 7.5%' Household'size' 4.6' 4.6' 4.6' 4.6' Percent'of'members'older'than'65' 33.0%' 40.0%' 32.0%' 32.7%' Percent'of'members'younger'than'10' 47.0%' 48.0%' 49.0%' 48.1%' Age'of'head'of'household' 51.7' 50.2' 50.5' 50.9' Head's'years'of'education' 6.7' 6.5' 6.5' 6.6[END_REF]. So far, we explained that the enrolment is at the household level. Actually, the situation was more complex. This rule was not enforced strictly [START_REF] Zhang | 0.6%' Chronic'disease' 7.2%' 4.6%' 7.8%' 7.5%' Household'size' 4.6' 4.6' 4.6' 4.6' Percent'of'members'older'than'65' 33.0%' 40.0%' 32.0%' 32.7%' Percent'of'members'younger'than'10' 47.0%' 48.0%' 49.0%' 48.1%' Age'of'head'of'household' 51.7' 50.2' 50.5' 50.9' Head's'years'of'education' 6.7' 6.5' 6.5' 6.6[END_REF] 5 There are quite a few mistakes in the identification code used to track individuals and households. We used conservative rules based on individual's age, sex, and education to match individuals and households over years.

If more than half of the household members could not be matched across two years, we excluded the household from our sample. Altogether, we deleted around 8% households from our sample due to the inconsistency of the identification code. 6 The exclusion of these households has no e↵ect on the results. Results using the whole sample are available upon request 7 Migrant workers' children have very limited access to public schools in the regions where they work 8 The criteria defined by individual local governments for classification as wubao, xiaokan and pingkun are set up at the local government level and they are highly variable. 9 We did the test for the previous year, too. We obtained that there is no significant e↵ect of the NCMS program for the year 2004 but the e↵ect of the NCMS program is significant at 12% for the year 2003. Results excluding the countries that introduced the NCMS in 2003 (Sample C and Sample D) allow to get more robust estimations.

10 Indeed, di↵erent local health insurance programs had previously been launched, with poor results in terms of improving healthcare access. As a result, public health insurance programs were widely considered to be untrustworthy.

11 As is well-known, in China there is massive internal migration of healthy 15-to 45-year-olds from rural to urban areas. Because of missing healthy people aged between 15 and 45 years, a polynomial specification for age is not relevant here. The results of the model with polynomial specification for age are available on request.

12 The data does not provide detailed information on these pathologies.

13 Here we do not test the income-related heterogeneity in self-assessed health.

14 The children of internal migrant workers have very limited access to public schools in the areas where their parents work. 15 The data also contains information on subscriptions for the year 2007 Insured' 4.37' 4.32' 4.33' 4.36' 4.38' NonRparticipants' 4.39' 4.25' 4.27' 4.42' 4.48' NonRexposed' 4.33' 4.34' 4.34' 4.33' 4.27 'of'15'years,'health'selfRassessment'may'be'debatable.'Individuals'may'be'too'young'to'selfR assess' themselves.' Dropping' these' observations' does' not' change' the' results.' We'also'regress'the'model' with'age'divided'into'groups.'For'individuals'born'in'1958R1983,'we'divide'age'into'10Ryear'categories,' except' over' age' 40' where' we' divide' into' 5Ryear' categories.' Considering' age' group' dummies' does' not' significantly' change' the' results' on' the' other' independent' variables.' We' find' that' members' of' the' lost' generation'are'more'optimistic.' 'The'lost'generation'is'defined'as'the'birth'cohorts'born'in'1948R1957.'Most'members'of'this'generation' experienced'disruptive'education'and'sendRdown'policy'(Hung'and'Chiu,'2003).'Using'4Ryear'panel'data' from'the'longitudinal'National'FixedRpoint'Survey'(NFS)'2003R2006,'we'defined'4'groups'of'age'following' Huan ' (2009).' The' members' of' the' oldest' cohort ' (Cohort' 1,' born' in' 1943R1947)' came' of' age' as' young' children' and' teenagers' during' the' New' China' (1949' People's' Republic' China)' and' they' either' finished' their'high'school'education,'college'education,'or'entered'the'urban'labor'force'before'the'beginning'of'the' 1966R1976' Cultural' Revolution.' Cohort' 2' (1948R1952)' members' experienced' the' early' stages' of' the' Cultural' Revolution,' in' which' sendRdown' policy' was' applied' seriously.' They' graduated' from' junior' or' senior' middle' schools' in' 1966,' 1967,' and' 1968.' Many' of' them' spent' 5R10' years' in' intensive' labor' on' farmland' before' they' were' allowed' to' return' to' the' cities.' Cohort' 3' (1953R1957)' members' completed' secondary' education' and' spent' a' few' years' in' the' countryside' during' the' late' stages' of' the' Cultural' Revolution.' Cohort' 4' (1958R1962)' members' spent' their' childhood' during' the' Cultural' Revolution.' Disruption'of'education'and'sendRdown'policy'were'specific'to'Cohorts'2'and'3,'which'are'called'the' 'omitted'category'is'illiteracy'or'years'of'education'less'than'3.'' b.R'The'omitted'category'is'farmer.'The'regression'for'village'enrollment'is'based'on'countyRlevel'data.'' '


Stéphane Roux

Clément Jailin

Arturo Mendoza

Jan Neggers

François Hild

Olivier Allix

A Mendoza Quispe

O Allix Lmt

From big data to key data

Keywords: Image-based identification, Model reduction, Fast 4D identification

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction Today, tomography based mechanical tests offer the ultimate access to a full 4D characterization. This dream becoming reality alas comes with a huge cost, that of the acquisition of a large number of 3D-volume data as the sample is loaded. A large number of tomographies, each composed of a large number of radiographies, each with an ever increasing definition, not only come with a long acquisition time, but also with very severe constraints and challenges both for the specimen for which no relaxation or creep is tolerated, and for the experimental set-up that should remain free of drift for days. Even when considering 2D tests, it becomes customary to process thousands of high definition images, leading to huge data processing (computation) costs.

This observation prompts for a proper methodology to handle those experiments, and new ways to benefit from these remarkable new opportunities but at mastered costs.

Spatio-temporal framework One first observation is that time being sampled at a fast rate, displacement fields as sought using DIC or DVC have often a temporal evolution that is smooth, thus it is advantageous to introduce a time evolution that is parameterized coarsely in time. Namely, the displacement field may be written as

u(x,t) = ∑ i, j u i j φ i (x)ϕ j (t) (1)
where the time functions ϕ j (t) span numerous time frames. The fact that the reference image does not evolve in time provides an efficient formulation, whereby the computation of the kinematic unknowns scales linearly in the number of temporal unknown. This route was explored in Ref. [START_REF] Besnard | A space-time approach in digital image correlation: Movie-DIC[END_REF]. Note however that at this stage, this feature reduces the number of unknowns, but not the amount of data to be handled.

Pyramidal description

The regularity of the displacement field is also a motivation for using a pyramidal approach where a rough determination of the displacement is first sought based on a coarse image stack. The latter corresponds to a space and time convolution of the image stack by a mollifying kernel, followed by a suited subsampling. Such a processing can conveniently be iterated, eventually with different reduction factors in space and time, leading to a pyramidal construction [START_REF] Hild | Multi-scale displacement field measurements of compressed mineral wool samples by digital image correlation[END_REF]. When the space-time displacement has been obtained at a specific level of the pyramid (starting from its top), it may be used as an accurate pre-determination of the next finer level. Most of the computation time is naturally spent at the bottom level, and a suited pre-determination reduces the number of needed iterations at this stage.

PGD approach More than often, the chosen kinematic basis has a dimensionality that is much larger -not to be limiting -than the effective number of degrees of freedom that are needed to describe the studied case. In order to exploit this "simplicity", the Proper Generalized Decomposition (PGD) is an elegant method that has been introduced in the domain of model reduction. It consists of adding "modes" progressively, one at a time, and as long as the residual level necessitates. Such an approach for DIC and DVC (in the space dimensions) has been pioneered by Passieux et al. [START_REF] Passieux | High resolution digital image correlation using proper generalized decomposition: PGD-DIC[END_REF][START_REF] Gomes Perini | A Multigrid PGD-based Algorithm for Volumetric Displacement Fields Measurements[END_REF]. In our case, a mode is the product of a temporal a j ϕ j (t) function and a spatial b i φ i (x) field. The PGD approach naturally leads to a sparse representation of the displacement field [START_REF] Neggers | Big Data' in experimental mechanics and model order reduction: Today's challenges and tomorrow's opportunities[END_REF]. Here again, a net gain results from the reduction in the number of degrees of freedom that are introduced.

Data pruning In all the previous items, the entire set of data is used. However, because of a large redundancy, not all data are equally important, or useful. Hence, one can be dispensed with some data, either in the processing to speed up the exploitation of an experiment, or better, in the acquisition, to perform much faster experiments, an appealing route for tomography-based mechanical tests, whereby only few radiographies may be used to compute a displacement field, once the full microstructure is known) using a Projection-based DVC [START_REF] Taillandier-Thomas | A soft route toward 4D tomography[END_REF].

Gathering all the above ingredients offer the opportunity to perform few-minute 4D in-situ experiment and analyze them successfully [START_REF] Jailin | In situ µ-CT-scan mechanical tests: Fast 4D mechanical identification[END_REF][START_REF] Jailin | ???? Photomechanics Conference[END_REF].


Eloïse Bihar

Timothée Roberts

Yi Zhang

Esma Ismailova

Thierry Hervé

George G Malliaras

Jozina B De Graaf

Sahika Inal
email: sahika.inal@kaust.edu.sa

Mohamed Saadaoui

Keywords: Inkjet printing, organic electronics, tattoo, textile, electrophysiology

Driven by the ever-growing needs for developing low cost, easy-to-use, noninvasive diagnostic tools, biomedical devices that can be integrated on human skin or textiles have begun to emerge. These "wearable" devices should couple electronics directly to the human skin and detect a variety of biologically relevant signals such as the neuromuscular activity. In this work, we develop a simple, low cost and customizable device to perform electromyography (EMG) measurements based on electronics

Introduction

One of the greatest endeavors of current medical diagnostics research is to develop devices that can record vital signals from the body in real time whilst being minimally invasive [START_REF] Gambhir | Toward achieving precision health[END_REF]. The sensor technologies thus move from lab-on-chip platforms to wearable electronics, indicating that sensors should be integrated with the skin or clothes that surround the skin, allowing for personal health monitoring as well as drug delivery systems [START_REF] Yeo | Multifunctional Epidermal Electronics Printed Directly Onto the Skin[END_REF][START_REF] Bandodkar | Tattoo-based potentiometric ion-selective sensors for epidermal pH monitoring[END_REF][START_REF] Kim | Non-invasive mouthguard biosensor for continuous salivary monitoring of metabolites[END_REF][START_REF] Parrilla | A Textile-Based Stretchable Multi-Ion Potentiometric Sensor[END_REF][START_REF] Imani | A wearable chemical-electrophysiological hybrid biosensing system for real-time health and fitness monitoring[END_REF][START_REF] Tee | An electrically and mechanically self-healing composite with pressure-and flexion-sensitive properties for electronic skin applications[END_REF][START_REF] Wang | Skin electronics from scalable fabrication of an intrinsically stretchable transistor array[END_REF]. Electrically interfacing the skin with electrodes for monitoring the electrical activity related to functioning of cells is a well-established practice in clinic, albeit performed with bulky rigid electronics that are not comfortable to wear. For instance, standard devices to record muscle activity, i.e. electromyography (EMG), are nonpolarizable Ag/AgCl electrodes which are applied on the skin typically with gels that allow for ion/water penetration and therefore reduce skin/electrode impedance. The performance of these electrodes depends on the gel-material and as such they pose several limitations including discomfort, allergies or irritation when in contact with the human skin, as well as limited stability A c c e p t e d M a n u s c r i p t related to gel drying out during long-term applications [START_REF]Risk of skin reaction when using ECG electrodes[END_REF]. New materials and concepts have emerged in order to address these aforementioned limitations, aiming to develop devices that can adhere seamlessly to skin as well as adapt to its movements. Conformable electrodes based on smart textiles are prime devices generated to fulfill these requirements [START_REF] Tao | Smart Fibres, Fabrics and Clothing: Fundamentals and Applications[END_REF][START_REF] Park | Smart textiles: Wearable electronic systems[END_REF][START_REF] Matsuhisa | Printable elastic conductors with a high conductivity for electronic textile applications[END_REF][START_REF] Yapici | Graphene-clad textile electrodes for electrocardiogram monitoring Sens[END_REF]. Someya and coworkers, for instance, demonstrated new conductors made of highly stretchable materials composed of Ag flakes mixed with fluorines. These materials could be integrated into garments via screen printing and used for recording electrophysiological signals [START_REF] Matsuhisa | Printable elastic conductors with a high conductivity for electronic textile applications[END_REF]. Graphene has been another material integrated with nylon fabrics [START_REF] Yapici | Graphene-clad textile electrodes for electrocardiogram monitoring Sens[END_REF].

In parallel to the development of smart textiles, a new generation of devices that can be fully conformable, known as electronic skin or e-skin, [START_REF] Tee | An electrically and mechanically self-healing composite with pressure-and flexion-sensitive properties for electronic skin applications[END_REF][START_REF] Wagner | Electronic skin: architecture and components[END_REF]15] has emerged. The pioneering work in this field incorporated electrophysiology, temperature and strain sensors made of gold, thin silicon, gallium arsenide in the form of nanoribbons and nanomembranes [15]. These materials were deposited directly on the skin by dissolving a water soluble film of polyvinyl alcohol, leaving the electronics adhered to the skin by van der Walls interactions [15]. Recently Bao's team proposed a new route for the fabrication of "epidermal" circuitry comprising transistors arrays, based on stretchable polymer materials such as carbon nanotubes and conjugated polymer/elastomer mixtures, and developed integrated touch-sensors able to detect the footprint of an artificial ladybug [START_REF] Wang | Skin electronics from scalable fabrication of an intrinsically stretchable transistor array[END_REF]. These devices were transferred to the skin using a water-soluble sacrificial layer as well. A promising substrate material for device integration has in fact been the commercially available tattoo paper. If the electronics can be integrated on the paper, it can be transferred to the skin by dissolving the water-soluble sacrificial layer (typically made of starch-dextrin) of the paper.

As such, the electronics conforms onto the complex features of the skin and represents an ideal disposable system for performing noninvasive daily diagnostics of a variety of biomarkers. These tattoo integrated electronic platforms exhibit excellent skin conformation, resistance to deformation and good biocompatibility with the human skin [START_REF] Yeo | Multifunctional Epidermal Electronics Printed Directly Onto the Skin[END_REF][START_REF] Huang | Materials and Designs for Wireless Epidermal Sensors of Hydration and Strain[END_REF][START_REF] Kim | Epidermal Electronics with Advanced Capabilities in Near-Field Communication[END_REF][START_REF] Miyamoto | Inflammation-free, gaspermeable, lightweight, stretchable on-skin electronics with nanomeshes[END_REF].

Organic electronic materials have come to the fore as promising materials in interfacing the tissue due to the mechanical softness and biocompatibility as well as the low impedance of their films [START_REF] Inal | Conjugated polymers in bioelectronics[END_REF]. Amongst A c c e p t e d M a n u s c r i p t these materials, poly (3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) has been the standard organic electronic coating, owing its popularity mostly to its electrochemical stability (no degradation in aqueous environments rich in oxidative species) and being readily available as a dispersion in water through commercial vendors [START_REF] Elschner | PEDOT: principles and applications of an intrinsically conductive polymer[END_REF]. Another reason for the current popularity of PEDOT:PSS is that its dispersion can be processed through various means, allowing for integration into a wide range of existing fabrication procedures. Co-solvents, secondary dopants, and crosslinkers are often added to the dispersion to modulate the film conductivity, mechanical properties as well as viscosity so that it is ideal for printing and coating over large areas [START_REF] Pingree | The Changing Face of PEDOT:PSS Films: Substrate, Bias, and Processing Effects on Vertical Charge Transport[END_REF][START_REF] Mannerbro | Inkjet printed electrochemical organic electronics[END_REF][START_REF] Vuorinen | Inkjet-Printed Graphene/PEDOT:PSS Temperature Sensors on a Skin-Conformable[END_REF][START_REF] Bihar | Fully Printed Electrodes on Stretchable Textiles for Long-Term Electrophysiology[END_REF][START_REF] Elmahmoudy | Tailoring the electrochemical and mechanical properties of PEDOT: PSS films for bioelectronics[END_REF][START_REF] Bihar | Inkjet-Printed PEDOT:PSS Electrodes on Paper for Electrocardiography[END_REF][START_REF] Håkansson | Effect of (3-glycidyloxypropyl) trimethoxysilane (GOPS) on the electrical properties of PEDOT: PSS films[END_REF]. All these properties render PEDOT:PSS an ideal material to cast on unconventional substrates such as the tattoo paper. Zucca et al reported different techniques for the fabrication PEDOT:PSS based tattoo nanosheet electrodes. These involve spin casting the polymer on a commercial tattoo paper and inkjet printing a NaClO solution for the subtractive patterning of the tattoo [START_REF] Zucca | Tattoo Conductive Polymer Nanosheets for Skin-Contact Applications[END_REF], as well as coating poly(D,L-lactic acid) and PEDOT:PSS on a polypropylene substrate with a gravure roll [START_REF] Zucca | Roll to roll processing of ultraconformable conducting polymer nanosheets[END_REF]. In both cases, after transferring the tattoo electrode on the skin, EMG signals could be successfully recorded. The integration of interconnects is however challenging for tattoo electronics and needs to be considered for stability and use for long term measurements. Bareket et al developed tattoo electronics based on screen printed carbon electrodes and plasma polymerized PEDOT:PSS [START_REF] Bareket | Temporary-tattoo for long-term high fidelity biopotential recordings[END_REF]. The device recorded EMG signal for 3 hours, however the overall thickness of the electrodes (ca. 100µm) is a concern for the conformal contact between the skin and the electrode.

For these disposable and organic electronics-based tattoo electrodes to be ever commercialized, the fabrication should be cost and time effective. Solution processing technologies such as spraying techniques [START_REF] Pitsalidis | Electrospray-Processed Soluble Acenes toward the Realization of High-Performance Field-Effect Transistors[END_REF], screen printing [START_REF] Scheiblin | Screen-printed organic electrochemical transistors for metabolite sensing[END_REF][START_REF] Ersman | Screen printed digital circuits based on vertical organic electrochemical transistors Flex[END_REF], or inkjet printing [START_REF] Bihar | Inkjet-Printed PEDOT:PSS Electrodes on Paper for Electrocardiography[END_REF][START_REF] Roberts | Flexible Inkjet-Printed Multielectrode Arrays for Neuromuscular Cartography[END_REF] not only involve a small number of fabrication steps, but are also compatible with large-area flexible substrates. Inkjet printing, in particular, allows for the digital fabrication of customizable designs at atmospheric conditions, with patterns that can be easily modified on demand, ideal for fabricating diagnostic devices on flexible substrates [START_REF] Bihar | A Disposable paper breathalyzer with an alcohol sensing organic electrochemical transistor[END_REF][START_REF] Batchelor | Inkjet printed ECG electrodes for long term biosignal monitoring in personalized and ubiquitous healthcare[END_REF]. A recent work of Ferrari et al demonstrated PEDOT:PSS electrodes printed on a tattoo paper which could A c c e p t e d M a n u s c r i p t measure muscular activity with high precision on limbs and face [START_REF] Ferrari | Ultraconformable Temporary Tattoo Electrodes for Electrophysiology[END_REF]. The electrode allows for hair growth underneath, a great advantage over conventional systems promising for long-term recordings on areas with high hair density. For practical applications, however, where the measurements are intended to be carried on the skin for longer periods of time, the integration of the tattoo with the acquisition system remains as a challenge that needs to be addressed. Design solutions should exclude adhesives, insulating layers as well as metallic interconnects in order to reduce fabrication costs and involve a user-friendly interface for these disposable systems. It is also important that all the fabrication procedure is accomplished in one printing step.

In this work, we present a simple process for the fabrication and integration of a fully inkjet-printed e-skin device based on tattoo technology. Using PEDOT:PSS both as the active electrode as well as contact material, we record high quality EMG signals from the arm's biceps over 7 hours. Replacing conventional electrical wires or any metal contact fabricated via evaporation which limit the feasibility of e-skin technologies, we utilize PEDOT:PSS contacts printed on a conformable textile wristband. We characterize this "tattoo/textile electronics" system and investigate the performance of the devices compared to commercial electrodes used for EMG recordings. The thickness and roughness of the tattoo electrodes and electrical properties of the tattoo/textile electronics are evaluated. The tattoo/textile electronics-skin contact impedance at the relevant frequency range are measured. With the textile wristband conforming on the skin and the intimate adhesion of the tattoo, we are able to record stable EMG signals with comparable performance to conventional wet (gelled) Ag/AgCl electrodes. The proposed platform paves the way towards easy-to-fabricate and use, conformable, all-polymer biopotential electrodes that can be utilized in a wide range of diagnostics and as well as therapy applications.

Experimental

Fabrication of PEDOT:PSS based electronics on tattoo and textile: PEDOT:PSS ink was prepared using a mixture containing PEDOT:PSS dispersion (conducting polymer, Heraeus, Clevios PH1000), 20 wt% ethylene glycol (EG, co-solvent, Sigma Aldrich) and 0.8 wt% glycidoxypropyltrimethoxysilane (GOPS, A c c e p t e d M a n u s c r i p t crosslinker, Sigma Aldrich). While EG increases the conductivity of the films deposited, GOPS prevents their delamination during the transfer of the tattoo onto the skin. In this mixture, we also included 0.3wt% of commercially available surfactants and a combination of organic solvents to render rheological properties of the ink suitable for inkjet process without compromising softness and electrical conductivity.

A Dimatix DMP-2800 inkjet printer was used to fabricate the tattoo/textile electronics. 1cm 2 square pattern of PEDOT:PSS (single layer) was printed on a commercially available temporary tattoo sheet (Tattoo 2.1, the Magic Touch Ltd., 123 Applications). The sheet is composed of a single decal transfer paper (including 3 layers : a paper carrier, a starch-dextrin water soluble sacrificial layer, and a layer of ethylcellulose (ca. 400nm in thickness). For the wristband, we printed up to 8 layers of PEDOT:PSS ink on a commercially available satin ribbon (Ideatiss). The metallic snaps buttons were installed on the ribbon via a snap plier. The drop spacing for all printing procedures was 15 µm. Once the ink was printed, the samples were placed in a conventional oven to cure the films at 110°C for 60 min. The tattoo electrodes were transferred to the skin (no pre-treatment) by dissolving the water-soluble sacrificial layer and removed by washing the exposed skin with soap and water.

Electrical characterization of the tattoo electrodes:

A four-point probe system (Jandel) was used to determine the sheet resistance of PEDOT:PSS films. We used a mechanical profilometer (Ambios Technology) to estimate the thickness of a single PEDOT:PSS layer printed on the substrates. The electrical resistance of the PEDOT:PSS contact on the textile as well as the one between the tattoo electrode and the textile contact was measured using a digital Multimeter (Amprobe 37XR-A Tru-rms).

Electrode-skin contact impedance measurements were performed as shown in Figure 2. The working electrode (e.g. the tattoo/textile electronics) and the counter electrode (wet Ag/AgCl, Ambu blue sensor) were placed on the forearm of a volunteer, keeping 3 cm distance from each other. The reference electrode (wet Ag/AgCl, Ambu blue sensor) was placed on the elbow. A sinusoidal potential of 10 mV was applied at the working electrode using a potentiostat (Metrohm Autolab B.V.) with no constant differential voltage on the skin. The impedance was screened for a frequency range between 0.1 and 1000 Hz and the data A c c e p t e d M a n u s c r i p t were analyzed using NOVA software. We evaluated the electrical stability of the tattoo electrode upon mechanical deformation. We transferred the tattoo across the distal phalange of the index finger that is exposed to multiple flexions/extensions. Two copper tapes adhering to the printed PEDOT:PSS electrode on opposite edges were used to establish electrical connection with the acquisition system. The flexion/extension was repeated 250 times at 30° and at 90° angle of closure. The resistance of the electrode was measured at every 10 repetitions.

EMG recordings and data acquisition: Three able-bodied volunteers (2 males, 1 female, ages between 25 and 40 years old) participated in this study with informed consent acquired before conducting the experiments. All participants took part for the long-term EMG recordings and one volunteered for the measurement of the electrode-skin impedance. All protocols and procedures were approved by the direction of research of the Ecole des Mines de St. Etienne. Two tattoo electrodes were placed 2 cm apart from each other at the center of the biceps of the left arm. The contacts printed on the textile wristband were placed on top of the tattooed areas of the skin in order to connect the recording and counter electrodes to the acquisition system. 2 wet Ag/AgCl electrodes (Metrohm Autolab B.V.) were placed on the biceps of the right arm. In order to have comparable signals, the volunteer was asked to lift 1 kg of a standard weight with each arm simultaneously. A ground electrode was placed on the right leg. For each recording, the experiment has been repeated at least 6 times, and the data were collected once in every 3 hours over 7 hours (T0=1h, T0+3h and T0+6h). EMG data were acquired using A RHD2216 chip from Intan Technologies. The data were recorded with 16-bit resolution and a sampling rate of 1.1kHz per channel and then transferred to a PC through an USB cable. More information can be found in the datasheet of the RHD2216 chip from Intan Technologies. The on-chip filters were set as follows: the first order high pass was set at 10 Hz and the third order low pass filter was set at 300 Hz.

Processing of EMG data:

We used a customized Labview software to filter the signal and calculate the signal-to-noise-ratio (SNR). All recordings were treated with a first order Butterworth band-pass (10-300 Hz). The SNR value of each recording was then calculated based on the following equation:

             i i i i Sr Sc Sr SNR 2 2 log * 20 ) (
, where Sc is a portion of 5 s of the signal during a contraction and Sr is a portion of 5 s of the signal during a resting position.

Results and Discussion

Figure 1a shows a layer schematic of the tattoo electronics integrated with the textile contacts. The tattoo includes one layer of PEDOT:PSS ink inkjet-printed onto the temporary tattoo paper. PEDOT:PSS coating increases the roughness (RMS) of the substrate from 1.35nm to 3.77nm (Figure S1). The electrical conductivity of a 95±5 nm thick printed PEDOT:PSS electrode (single layer) is 240±10 S/cm. The thickness and the associated sheet resistance can be typically controlled by varying the number of printed layers of PEDOT:PSS. However, we find the tattoo substrate not compatible with printing multiple layers of aqueous ink due to instability of the sacrificial layer in aqueous. We estimated the conductivity of a single layer of the film to be adequate to acquire EMG signals. Following the fabrication, the electronics printed on the paper was transferred onto the skin by simply wetting the tattoo transfer paper and gently pressing the tattoo against the skin. As the transfer paper is wetted, it dissolves and the conducting polymer pattern remains on the skin. As inkjet offers freedom of form and design, different electrode patterns can be achieved (Figure 1b). The electrode conforms well to the skin and is in fact reported to enter deeply inside the creases of the skin [START_REF] Ferrari | Ultraconformable Temporary Tattoo Electrodes for Electrophysiology[END_REF] (Figure 1c). As the skin is compressed or stretched due to muscle movements, the electrodes move with the skin, acting as its natural component (Figure 1c). The electrodes tattooed on the skin have an intimate contact with the tissue and by using the conducting polymer which has a Young's modulus of ca. 150MPa in wet state [START_REF] Elmahmoudy | Tailoring the electrochemical and mechanical properties of PEDOT: PSS films for bioelectronics[END_REF], similar to that of skin ca. 2kPa-500 MPa [START_REF] Kalra | An Overview of Factors Affecting the Skins Youngs Modulus[END_REF],

we overcome the inherent mechanical mismatch (Figure 1d). The conducting polymer was also printed on the textile wristband made of knitted soft fabric (i.e. satin) using the same printing protocol as for the tattoo paper (Figure 1f). In order to optimize the electrical performance of the contacts on the textile, we varied the number of the printed PEDOT:PSS layers A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

(1.5x1.5 cm 2 ). The electrical resistance of the contacts decreased with the number of printed layers and reached a plateau with 6 layers of PEDOT:PSS (Figure 1g). The saturation is due to complete impregnation of the fabric with the polymer. Following the fabrication of electrodes, we inserted metal snap buttons to the textile on top of the conducting pattern. With the buttons attached, the textile was in the form of a wristband which could comfortably be wrapped around the target skin area where the tattoo electrodes were transferred and fixed using the snap button closure (Figure 1f). As such, we directly interface the tattooed electrodes with the contacts on the textile. The metal snaps enable to connect the tattoo electrodes to the acquisition system in order to record the electrophysiological activity. The electrical resistance between the tattooed electrodes on the skin and the metal snap on the textile is measured to be ca. 52±5 kΩ (Figure S2). This configuration obviates the need to use external wires in contrast to other studies with EMG electrodes relying on external connectors such as silver cotton wires [START_REF] Zucca | Tattoo Conductive Polymer Nanosheets for Skin-Contact Applications[END_REF], gold coated plastic foils [START_REF] Zucca | Roll to roll processing of ultraconformable conducting polymer nanosheets[END_REF], or polyimide films [START_REF] Bareket | Temporary-tattoo for long-term high fidelity biopotential recordings[END_REF]. These contacts tend to cause unstable measurements due to motion artefacts and discomfort to the user, as well as their integration requires additional fabrication steps. In the tattoo/textile electronics, there is only one main electronic material, PEDOT:PSS, and it is deposited in a single step using one fabrication protocol.

We compared the electrode-skin contact impedance of the tattoo/textile electronics with the conventional wet (gelled) Ag/AgCl electrodes, one hour after they have been in contact with skin (Figure 2a). Figure 2b shows the configuration of the measurements on the forearm of a volunteer. The commercial electrodes exhibited impedance values two orders of magnitude lower than the tattoo/textile electronics at low frequency range (from 0.1 Hz to 100 Hz). For frequencies above 100 Hz, the impedance of the tattoo/textile electronics was one order of magnitude higher. The lower impedance of the commercial electrodes is due to the presence of the gel electrolyte applied between the Ag/AgCl and the skin [START_REF] Searle | A direct comparison of wet, dry and insulating bioelectric recording electrodes[END_REF]. The tattoo/textile system, on the other hand, shows reduced impedance values for all the frequencies explored, as well as a smaller sample-to-sample deviation compared to the printed textile electrode only. At the frequency range where the significant EMG signals occur (5-450 Hz) [START_REF] Van Boxtel | Optimal signal bandwidth for the recording of surface EMG activity of facial, jaw, oral, and neck muscles[END_REF], the tattoo/textile electronics In contrast to the rigid conventional electrodes, tattoo electrodes conform with the skin. In order to demonstrate the stability of the tattoo electrode upon bending, we measured its electrical resistance under deformation. The tattoo was deposited between the distal and intermediate phalanges of the index finger of the volunteer who was asked to perform repetitive flexions/extensions (Figure 3a andb). We recorded the electrical resistance of the electrode as the finger was subject to different bending angles (30° and 90°) up to 250 cycles. While we do not observe a significant variation in the electrical resistance at a flexion to 30°, resistance increases slightly for flexion to 90.° This is attributed to the failure of the thick metallic adhesives used to contact the tattoo, proposed also by Zucca and colleagues [START_REF] Zucca | Roll to roll processing of ultraconformable conducting polymer nanosheets[END_REF]. In fact, we observe such cracks observed at the adhesive-tattoo interface after the experiments (Figure S3). In parallel, we examined the electrical stability of the contact printed on the textile electrode worn for a period of 7 hours on the Figure 4b shows the EMG signals recorded with tattoo/textile electronics as well as Ag/AgCl electrodes during the biceps activity for both short (1 s) and long (10 s) voluntary muscle contractions over 7 hours.

The recordings used to extract the signal to noise ratio (SNR) consisted of 10 seconds-long contractions during which the volunteer was asked to lift a 1 kg of standard weight with an elbow angle of ca. 90°, with a resting period of 10 seconds. As the electrodes were placed on the skin (T0=1h), the tattoo/textile electronics exhibited a mean SNR of 11±2 dB compared to a mean SNR of 29±1 dB for the wet Ag/AgCl A c c e p t e d M a n u s c r i p t

electrodes. The lower quality of the recordings with the dry tattoo after its adherence with the skin is attributed to the high electrode-skin contact impedance of these electrodes (Figure 2a). Despite the lower SNR, the tattoo/textiles electronics could measure the EMG activity. Interestingly, after 4 hours of contact with the skin (t=T0+3h), the performance of the electronics was substantially improved: SNR increased to 26±2 dB while the SNR of Ag/AgCl electrodes decreased to 25±1 dB. Note that the signal-to-noise ratio is expected to be slightly higher at the non-dominant side (i.e., left arm for the right-handed volunteers), validated by the tattoo/textile electronics upon t=T0+3h. We postulate that the quality of the contact of the tattoo/textile electronics with the skin increased due to the sweat-gland activity. The sweat produced by the body wets the tattoo, thus enhances the ionic conductivity at the skin-electrode interface and leads to a decrease of the contact impedance between the skin and PEDOT:PSS [START_REF] Gruetzmann | Novel dry electrodes for ECG monitoring[END_REF]. Another factor that contributed to the reduced impedance is the improved quality of contact between the tattoo and the textile.

As shown in Figure 1a, there is a thin layer of the ionic conductor, ethyl cellulose, between the two PEDOT:PSS layers (the tattoo and the textile). As the skin gets wet, this layer becomes more permeable to ions allowing the current to pass easier form the skin to the textile contact. Finally, 7 hours after the first recordings (t=T0+6h), the quality of the recordings are similar, i.e., SNR corresponds to 21±1 dB and 20±5 dB, for tattoo/textile electronics and Ag/AgCl electrodes, respectively. To demonstrate the reproducibility of the platform, we recorded similar type of EMG signals from 2 other volunteers using new pairs of printed electrodes as well as at different contraction levels. At t=T0+6h, the SNR values of tattoo/textile electronics and wet Ag/AgCl electrodes were 31±6dB vs 30±4 dB for one volunteer and 23±1dB vs 21±1dB for the other, respectively (Figure S4). We thus conclude that for recording cutaneous EMG signals, the tattoo/textile electronics show similar performance to clinical skin-contact electrodes.

In order to decouple the effect of the tattoo electrodes from the textile, the EMG signals were recorded using the textile electrodes (Figure S5). Although at T0, the SNR of the textile electrodes was higher than the tattoo/textile electronics, and over time, the textile recordings became less stable (e.g. 18±2dB at T0

A c c e p t e d M a n u s c r i p t and 4±3dB at T0+6h). Note that to be able to acquire these signals, the textile band has to be strongly tied around the arm to assure contact with the skin. We, therefore attribute the decrease in the SNR (or its instability) to the low skin-electrode contact quality rather than a degradation of the electrode. These results demonstrate the ability of the tattoo/textile electronics to record stable EMG signals over time alongside the simplicity of device fabrication/integration and gel electrolyte-free use.

Conclusions

We designed an inkjet-printed tattoo/textile electronics system utilizing the biocompatible conducting polymer PEDOT:PSS both as the transducer and the contact to the acquisition system. The polymer printed on the tattoo showed excellent adherence to the skin with no evident side effects such as skin irritation or allergies. The electrodes exhibited sufficient conductivity for acquiring muscle activities from the skin.

The same conducting polymer was printed on a textile used as a wristband around the skin and acted as a bridge between the tattoo electrode and the external electronics. We show that over 7 hours of EMG recordings (biceps contraction), the tattoo/textile electronics showed similar performance to conventional wet Ag/AgCl electrodes without the use of gel electrolytes. The integration of the conducting polymer both as the electrode and contact material minimizes the cost while improving the feasibility of such skinadhering electronics envisaged to be applied for diagnostics and/or therapy. The intimate contact of the tattooed electronics with the skin, its conformability and bending with the movements of the limbs is a great advantage over other cutaneous systems. Our approach for interfacing the external electronics avoids the use of inorganic rigid wires and presents a wearable solution. Due to the versatility of the materials and the processes employed, these systems can encompass a variety of applications such as myoelectric prosthesis or detection of muscle functioning.

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Figure 1 .

 1 Figure 1. The architecture of tattoo-textile electronics (a) Schematic of the tattoo/textile electronics. The PEDOT:PSS electrode printed on the tattoo paper is transferred to the skin. The textile containing the printed conducting polymer contacts is wrapped around the tattooed electrode on the skin, connecting the tattoo to the acquisition system. (b) Photograph of customized printed tattoo electrodes deposited on the skin. (c, d, and e) Photographs of printed tattoo electrodes on the skin: undeformed (c), compressed several times (d), and after compression (e). The tattoo electrode shows a good resistance to mechanical deformation, with no cracks observed at the surface of the electrode. (f) Photograph of the PEDOT:PSS printed on the textile (6 layers) with the snap buttons integrated. (g) Electrical resistance of the printed PEDOT:PSS patterns on the textile (1.5x1.5cm 2 ).

A c c e p t e d M a n u s c r i p t display an impedance of ca. 1 .

 1 83±0.01 MΩ at 5 Hz and 225±10 kΩ at 450 Hz. Albeit with impedance higher than conventional electrodes, we and others postulate that the dry electrodes have the advantage of being more stable[START_REF] Leleux | Conducting Polymer Electrodes for Electroencephalography[END_REF].

Figure 2 .

 2 Figure 2. Electrical characterization of the electrodes (a) Electrochemical impedance spectra of the textile, tattoo/textile electronics and wet Ag/AgCl electrodes after one hour of contact on the skin. The evaluation includes three devices and the shaded areas represent the standard deviation. (b) Schematic representation of the experimental set-up showing electrode positions on the forearm: working electrode is the textile, tattoo/textile electronics or wet Ag/AgCl electrodes, while counter and reference electrodes are wet Ag/AgCl.

A c c e

  p t e d M a n u s c r i p t arm wrist. We observed no changes in the polymer conductivity for this period, evidencing the stability of the textile contact.

Figure 3 .Figure 4 .

 34 Figure 3. Evaluation of the resistance of the tattoo electronics on the skin towards bending (a) The change in the electrical resistance of the printed tattoo electrode upon repeated contractions of the index finger at 30° and 90° bending angle, (b) Photograph of the tattoo on the index finger of the volunteer and the bending experiment. The tattoo electrode is connected to the acquisition system via copper adhesives which were attached underneath the edges of the conducting pattern.

Acknowledgments:

The authors thank Dr. Anna Maria Pappa (University of Cambridge) for fruitful discussions and Sky MacPhee and Dr. Alexandra Paterson (KAUST) for proof reading. This work was supported though grants by the ANR and MicroVitae Technologies.