Louise Rolland-Guillard
email: louise.rollandguillard@gmail.com

Elise De

La Rochebrochard

Wasna Sirirungsi
email: wasna.s@cmu.ac.th

Cheeraya Kanabkaew

Didier Breton
email: dbreton@unistra.fr

S Le Coeur
email: lecoeur@ined.fr

Reproductive health, social life and plans for the future of adolescents growing-up with HIV: a case

Keywords: adolescents, Thailand, HIV/AIDS, Perinatal transmission, Reproductive Health, sexuality

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

With antiretroviral therapy (ART), HIV infection has become a chronic disease and perinatally HIV-infected children are now able to survive and reach adolescence. Yet, they are entering this troubled period, often after a traumatic childhood: family history frequently disrupted by the illness or death of one or both parents; periods of serious illness; disclosure of their HIV status. Also, at a time when they initiate their sexuality, they have to learn how to manage their social, romantic and sexual life as an HIV-infected individual. Adolescence is a period of risk taking behaviours, and sexual behaviours such as intercourse without condom in this population could increase sexual transmission of HIV.

At the end of the 1980s, Thailand has been the country most affected by HIV in Southeast Asia (UNDP, 2004). As a result, of the National program for the prevention of mother-tochild transmission of HIV launched in 2000 [START_REF] Thisyakorn | Elimination of mother-to-child transmission of HIV: lessons learned from success in Thailand[END_REF], perinatal transmission has affected only one generation of children in Thailand, mostly those born from the beginning of the 1990s until the beginning of the 2000s. With generalized access to ART, since 2003, these perinatally HIV-infected children are now entering adolescence.

In resource-limited settings, only a few studies have describe the reproductive health, social life and plan for the future of perinatally HIV-infected adolescents (PHIVAs) taken all together, and included a comparison group of HIV-negative youth [START_REF] Bauermeister | Sexual behavior and perceived peer norms: Comparing perinatally HIV-infected and HIV-affected youth[END_REF][START_REF] Mu | Knowledge and perceptions of sexual and reproductive health and HIV among perinatally HIV-infected adolescents in rural China[END_REF][START_REF] Mutumba | Psychosocial challenges and strategies for coping with HIV among adolescents in Uganda: a qualitative study[END_REF]. In this national case-control survey, we aimed to compare some aspects of the reproductive health, risky behaviours, social life, family environment and plans for the future of PHIVAs, with uninfected adolescents from the general population.

Material and methods

TEEWA survey

The TEEns Living With ARV (TEEWA) survey was carried out from 2011 to 2012 in 20 hospitals throughout Thailand among PHIVAs (12-19 years) receiving ART. A similar survey was carried out from 2012 to 2013 among a control group in the general population matched (one control per case) on sex, age and place of residence. Controls were randomly selected from the population register of the community.

The study was approved by Ethics Committee, Faculty of Associate Medical Sciences, Chiang Mai University. All participants provided written informed consent/assent. They were assured that the information provided was kept confidential.

The whole survey procedure has been described in details elsewhere [START_REF] Le Coeur | Une enquête auprès d'adolescents nés avec le VIH: le projet TEEWA en Thaïlande[END_REF].

Population study selection

Among the 941 PHIVAs receiving ART in the study hospitals, 709 (75%) adolescents aged 12-19 years could be interviewed. For this analysis, PHIVAs living in orphanages (n=136) were excluded because their life style mostly depends on the orphanage's rules. Three additional PHIVAs who had no appropriate control in the general population were also excluded leading to a total of 571 PHIVAs and 571 uninfected controls.

Data collection

Self-administered questionnaires were completed by the cases and the controls, coupled with face-to-face interviews of their caregiver. Since some PHIVAs were not aware of their HIV status there was no reference to HIV, AIDS or ART in the self-administered questionnaire in order to prevent unintended disclosure.

The adolescent self-administered questionnaire was documenting aspects of their everyday life including school, work, self-reported happiness. Adolescents were interviewed on their puberty experiencevoice change for boys and first menstruation for girlssex education, romantic relationships (girl/boyfriend), and sexual activity. They were also interviewed on their relationship with their peers (hang-out with peers), with parents/caregivers (conflicts in the last year), risk behaviours (smoking and alcohol drinking), and plans for the future in terms of high education, family formation and children. Caregivers were interviewed on the household's financial situation and the vital status of the adolescent's parents.

Analysis

The analysis was stratified by gender and age: boys, aged 12-14 years and 15-19 years; girls, aged 12-14 years and 15-19 years. All variables were dichotomized. For this case-control study, the McNemar test was used to compare matched cases and controls. Data were analysed using SAS® version 9.4 software.

Results

The study population was composed of 571 PHIVAs matched with 571 controls. Both groups (cases and controls) were composed of 42% of boys and 58% of girls, 48% aged 12-14 years and 52% aged 15-19 years. Fifty nine percent of the adolescents were living in Northern Thailand, and 41% in the North-east, Centre and South.

Tables compare the characteristics of the adolescents in each age group, table 1 for boys and table 2 for girls. PHIVAs were significantly more often orphan-loss of one or both parents-(p<0.01), out of the education system (p<0.01), and lone children than the controls (p<0.01). Also, except for boys aged 15-19 years, the financial situation of their household was significantly more often perceived as "difficult" than the controls (p<0.01).

PHIVAs had experienced puberty later than the controls as indicated by the significantly lower percentage of boys and girls aged 12-14 years who had been through puberty (p<0.01).

There was no difference between PHIVAs and controls in terms of frequency of sexual education and sexual initiation. Among PHIVAs aged 15-19 years, 16% of boys and 8% of girls reported sexual debut versus 19% of boys and 10% of girls in controls. Among PHIVAs and controls, the median age at first sex was 15 years. Condom use at first sex was higher in PHIVAs than in controls (84% vs. 74%). There was no difference between PHIVAs and controls for tobacco use and alcohol drinking.

The social life of PHIVAs was not different from that of the controls as indicated by the percentage of those who declared that they hanged out with friends or who had a girl/boy friend.

However in PHIVAs aged 15-19 years, only 74% of boys and 79% of girls perceived themselves as happy versus 89% and 94%, respectively in the controls (p<0.01).

Finally, PHIVAs declared less frequently planning for high education, marrying or having a life-partner and children. Among female PHIVAs aged 15-19 and still attending school, only 72% reported planning for high education compared to 87% in the general population (p<0.01). The difference was not significant for boys in the same age group. Only 17% of female PHIVAs aged 15-19 planned to marry and 12% to have children compare to 39% and 35% in the controls (p<0.01). In male PHIVAs aged 15-19, 18% planned to marry and 14% to have children compare to 60% and 58% in the controls (p<0.01).

Discussion

Our results indicate that in Thailand, the sexual, reproductive, and social life of PHIVAs does not differ substantially from that of adolescents in the general population. However, PHIVAs appear to have lower expectations in terms of education and family formation.

As in other studies, we found that PHIVAs experienced puberty later than adolescents from the general population [START_REF] Buchacz | Delayed Onset of Pubertal Development in Children and Adolescents With Perinatally Acquired HIV Infection[END_REF][START_REF] Szubert | Pubertal development in HIV-infected African children on firstline antiretroviral therapy[END_REF] [START_REF] Bellavia | Delay in sexual maturation in perinatally HIV-infected youths is mediated by poor growth[END_REF][START_REF] De Martino | Puberty in perinatal HIV-1 infection: a multicentre longitudinal study of 212 children[END_REF]Paige L. Williams et al., 2013). Puberty delay in PHIVAs has been found to be associated with older age at ART initiation, growth retardation and HIV disease severity [START_REF] Bellavia | Delay in sexual maturation in perinatally HIV-infected youths is mediated by poor growth[END_REF][START_REF] Szubert | Pubertal development in HIV-infected African children on firstline antiretroviral therapy[END_REF]Paige L Williams et al., 2013).

After age 15, the proportion of adolescents -either cases or controls -reporting having experienced sex was low: less than 10% for girls and 20% for boys. However, there was a lack of power to ascertain a possible statistical difference between cases and controls. Similarly low rates of sexual initiation had been found in Thailand [START_REF] Lee | Risk-taking behaviors among vertically HIV-infected adolescents in northern Thailand[END_REF]) [START_REF] Ruangkanchanasetr | Youth risk behavior survey: Bangkok, Thailand[END_REF].

When comparing PHIVAs with controls, the most striking difference was their lower aspirations with regards to education, family formation and parenthood. Planning the future can be difficult for adolescents with life-long disease. These self-restrictions could be related to depression, more frequent in AIDS-orphans [START_REF] Cluver | Persisting mental health problems among AIDS-orphaned children in South Africa[END_REF]. Indeed, in our study, PHIVAs reported a lower level of happiness than the controls.

PHIVAs could be reluctant to continue their studies because of their disadvantaged socio-economic situation. Indeed, our study indicates that their households, lacking the income of the deceased parents, are facing more financial hardship compared to controls. Heavier pressures to become breadwinners for their household may explain the boys' lower willingness for high-education. Other reasons for school dropout could be neurocognitive impairments related to HIV, school absenteeism due to severe illnesses [START_REF] Mu | Knowledge and perceptions of sexual and reproductive health and HIV among perinatally HIV-infected adolescents in rural China[END_REF] or discrimination from their peers or from the school staff [START_REF] Mutumba | Psychosocial challenges and strategies for coping with HIV among adolescents in Uganda: a qualitative study[END_REF].

Desire for family formation was lower than in a study in northern Thailand, in which 30% of PHIVAs reported desiring a marital life (17% in our study) and 24% a willingness to have children in the future (11% in our study) [START_REF] Lee | Risk-taking behaviors among vertically HIV-infected adolescents in northern Thailand[END_REF]. Several qualitative studies among HIV-infected patients reported reluctance to building a family, following the difficult experience of one's disrupted family or fear of HIV status disclosure to partner [START_REF] Hosek | Psychological and social difficulties of adolescents living with HIV: A qualitative analysis[END_REF][START_REF] Troussier | La qualite de la vie sexuelle et affective favorise la prevention chez les personnes vivant avec le VIH[END_REF].

The disinclination of PHIVAs with regards to parenthood could be related to their own difficult experience as a perinatally HIV-infected child or the strong family planning counselling from the medical staff [START_REF] Fair | It's Like Losing a Part of My Family": Transition Expectations of Adolescents Living with Perinatally Acquired HIV and Their Guardians[END_REF]. In a low-fertility country [START_REF] Tu | Ultra-low fertility, gender equity and policy considerations[END_REF], social desirability is against having children when HIVinfected, though the risk of mother-to-child transmission is quasi-eliminated [START_REF] Lolekha | Elimination of mother-to-child transmission of HIV -Thailand[END_REF].

One limitation of our study is that despite its case-control design, we did not control for socio-economic background. Also, in the self-administered questionnaires, answers might have been affected by cultural bias, but there is no reason to believe that these biases affected the cases and controls differently.

Despite the constraint plans for their future and their disadvantaged socio-economic situation, PHIVAs in Thailand adopted similar social, sexual and risky behaviours than their peers from the general population. Social science studies indicate that Thai youth are prone to adopt behaviours from the Western teenagers' culture [START_REF] Dubus | Thaïlande : Histoire, société, culture[END_REF][START_REF] Siriyuvasak | Popular culture and youth consumption: Modernity, identity and social transformation[END_REF]. However, they are also influenced by Buddhism that encourages self-acceptance and acceptance of the destiny (karma) [START_REF] Gabaude | Approche du bouddhisme thaï. la Thailande contemporaine[END_REF] and emphasizes gender differences [START_REF] Pongsapich | La famille thaïlandaise, passé et présent. la Thailande contemporaine[END_REF].

Acknowledgements

We would like to thank the adolescents and their families who participated in the study. We are also thankful to the staff from the participating centres. We want to specially thank the staff from PHPT, in particular Nirattiya Jaisieng for the data management and Kunnatee Saopang for the data-entry, as well as the interviewers Parinya Jongpaijitsakul and Surush Suwunta.

Funding

This work was supported by the SIDACTION under Grant [AI22-1/01855]; and Oxfam GB under Grant [THAA51].

ORCID

L. Rolland-Guillard, http://orcid.org/0000-0001-9601-4025

Planning for the future is a vital aspect of identity development relying on socialization factors such as family and school [START_REF] Nurmi | How Do Adolescents See Their Future? A Review of the Development of Future Orientation and Planning[END_REF]. Although PHIVAs behave like any other adolescents of their age, they might fear that their prospect will change in their adult life. It may be advisable that policy makers provide affirmative action policies to help counterbalance PHIVAs' educational handicap. Psychosocial and medical services may also deliver comprehensive information to ensure that PHIVAs are able to make informed decisions with regards to family formation.

Disclosure statement

No potential conflict of interest was reported by the authors. (j) Khi-2 test was used because the matching could not be maintained since some individuals had already dropped of school.


Hassen Aziza
email: hassen.aziza@univ-mrs.fr

Christian Dufaza
email: christian.dufaza@univ-mrs.fr

Annie Perez
email: annie.perez@univ-mrs.fr

Said Hamdioui
email: s.hamdioui@tudelft.nl

Configurable Operational Amplifier Architectures based on Oxide Resistive RAMs

Keywords: Memristor, Opamps, RRAM, Analog calibration

This paper introduces memristor-based operational amplifiers in which semiconductor resistors are suppressed and replaced by memristors. The ability of the memristive elements to hold several resistance states is exploited to design programmable closed-loop operational amplifiers. An inverting operational amplifier, an integrator and a differentiator are studied. Such designs are developed based on a calibrated memristor model, and offer dynamic configurability to realize different gains and corner frequencies at reduced chip area.

Introduction

Emerging technologies such as Resistive RAMs (RRAMs) are attracting significant attention, due to their interesting characteristics such as high integration density, CMOS compatibility and non-volatility to replace the current conventional memories [START_REF] Shahrabi | Towards chip-level reram-cmos co-integration[END_REF] [2] [START_REF]An overview of non-volatile flip-flops based on emerging memory technologies[END_REF]. A typical RRAM device consists of two metallic electrodes that sandwich a thin dielectric layer used for permanent storage. Oxide-based Resistive Random Access Memory (so-called OxRAM) uses transition metal oxides as a dielectric layer. Metal oxides such as HfO 2 , NiO, TiO 2 and TaO 2 are promising candidates due to their compatibility with CMOS processes. In this study, an HfO 2 oxide-based RAM stack is considered. A recent advance in the field of OxRAM memories is related to Multi Level Cell operation (MLC) [START_REF] Aziza | Multilevel operation in oxide based resistive RAM with SET voltage modulation[END_REF] [START_REF] Aziza | Resistive RAMs as analog trimming elements[END_REF]. According to this approach, more than two data states are made possible for each memory cell, simply by finely controlling the programming of the cell. Proposed memristor-based operational amplifier (OpAmp) topologies use OxRAM cells as analog programmable devices by exploiting all the resistance states of the memory cell (i.e. extension of the MLC approach). High voltages are used to program the OxRAM states during the OpAmp calibration and low voltages are applied across the OxRAM cell during the OpAmp operation. Thus, the state of the OxRAM does not change during the OpAmp operation. It is worth noting that a series of short pulses are applied across the OxRAM cell during the calibration process. Pulse switching in 10 ns is reported in the literature [START_REF] Myoung-Jae | A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O5-x/TaO2-bilayer structures[END_REF]. Narrow programming pulses are possible due to the excellent high speed switching characteristic of the OxRAM cell. This programming strategy allows an optimal control of the programmed resistance despite the intrinsic cell variability [START_REF] Aziza | Evaluation of OxRAM cell variability impact on memory performances through electrical simulations[END_REF] [START_REF] Canet | Impact of resistive paths on NVM array reliability: Application to Flash & ReRAM memories[END_REF]. The approach to use memristors in programmable analog circuits was suggested in [START_REF] Pershin | Practical approach to programmable analog circuits with memristors[END_REF]. However, the memristor emulator used for simulations was far from actual memristive devices and the memristor programming interface within the analog circuits was not discussed. In [START_REF] Ibrayev | Memristive Operational Amplifiers[END_REF], an introductory idea of using memristive devices in an open-loop OpAmp for neuromorphic applications is presented. Simulations were performed based on a SPICE memristor model. Also in this case, the approach was too conceptual and far from actual memristive device implementations. In [START_REF] Jahromi | Ultra-low power Op-Amp design with memristor-based compensation[END_REF], an ultra-low power open-loop OpAmp was designed in a 0.18µm technology using a memristor-based compensation technique and the VTEAM model introduced in [START_REF] Kvatinsky | VTEAM: A general model for voltage controlled memristors[END_REF]. But no references to actual memristive devices was provided. In [START_REF] Olumodeji | Behavioural modelling of memristive devices targeted to sensor interfaces[END_REF], the adoption of memristive elements to customize a sensor interface is proposed. However, the technical implementation of the memristive element as a trimming device is absent and the trimming methodology is not developed. Alternatively, in [START_REF] Sackinger | An Analog Trimming Circuit Based On A Floating-Gate Device[END_REF] Floating Gate (FG) non volatile devices are used as analog trim elements by controlling the amount of charges transferred onto the FG. However, this technique needs a specific fabrication process to implement FG devices. In this work, configurable closed-loop operational amplifiers are proposed based on a 130nm technology from ST-Microelectronics and an OxRAM model calibrated on silicon. The analog storage feature of the OxRAM cell is evaluated based on actual data before its integration in operational amplifiers. A full design scheme targeting the implementation of OxRAMs as programmable analog devices within OpAmps is presented, including the OxRAM programming interface. This work is motivated by the greatest advantage of OxRAMs to be fabricated with low thermal budget, allowing their implementation into the Back-End-Of-the-Line (i.e. on top of CMOS transistors) with high integration density [START_REF] Sandrini | Heterogeneous integration of ReRAM crossbars in 180 nm CMOS BEoL process[END_REF]. Moreover, as OxRAMs resistance can be programmed, closed-loop OpAmp parameters can be set dynamically. The paper is organized as follows: section 2 presents the OxRAM technology, section 3 focuses on three configurable OpAmp architectures (inverting amplifier, integrator and differentiator) and provides simulation results and analysis. Finally, section 4 gives some concluding remarks.

Resistive RAM technology

OxRAM memories

In memory devices relying on a resistance change, complex physical mechanisms are responsible for reversible switching of the electrical conductivity between high and low resistance states. This resistivity change is generally attributed to the formation/dissolution of conductive paths between metallic electrodes [START_REF] Larentis | Resistive switching by voltage-driven ion migration in bipolar RRAM Part II: Modeling[END_REF]. OxRAM cell operation is depicted in Figure 1a. The resistance change is triggered when the voltage difference across the TOP and BOTTOM electrodes of the cell (labeled respectively "T" et "B" in Figure 1b) reaches specific voltages (namely V SET, V RESET and V FORM ). SET operation is achieved by applying a voltage difference across the cell greater than the threshold V SET . RESET operation is achieved by applying a voltage difference across the cell lower than the threshold V RESET . To obtain intermediate ON and OFF states, programming can be done gradually by applying an increasing number of identical voltage pulses across the cell [START_REF] Huang | Using binary resistors to achieve multilevel resistive switching in multilayer NiO/Pt nanowire arrays[END_REF].

OxRAM Model

The proposed OxRAM modeling approach relies on electric field-induced creation/destruction of oxygen vacancies within the switching layer. The model enables continuously accounting for both SET and RESET operations into a single master equation in which the resistance is controlled by the radius of the conduction pathways (r CF ) [START_REF] Bocquet | Robust compact model for bipolar oxide-based resistive switching memories[END_REF]:

( ) T k V q Ea CF T k V q Ea CF CF CF b cell ox dOx b cell red dOx e r e r r dt dr ⋅ ⋅ ⋅ + - ⋅ ⋅ ⋅ - - ⋅ ⋅ - ⋅ ⋅ - = α β α β Re Re 10 10 max (1)
where β RedOx is the nominal oxide reduction rate, E a is the activation energy, α red and α ox are the transfer coefficients (ranging between 0 and 1), k b is the Boltzmann constant, r CFmax is the maximal size of the conductive filament radius, T is the temperature and V cell the voltage across the cell. Moreover, the model makes assumptions of a uniform radius of the conduction pathways, a uniform electric field in the cell and temperature triggered acceleration of the oxide reduction reactions ("redox"). Finally, the total current in the OxRAM includes two components, i.e. one is related to the conductive species (I CF ) and the other to the conduction through the oxide (I OX ).

( ) ( )

OX CF OX CF CF x Cell CF r r L V I σ π σ σ π ⋅ ⋅ + - ⋅ ⋅ ⋅ = 2 max 2 (2) HRS x Cell Cell HRS OX L V S A I β ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⋅ = (3)
where L x is the oxide thickness, S Cell is the total area of the device, σ Ox the oxidation rate and σ CF the reduction rate. To take into account I OX trap assisted current (Poole-Frenkel, Schottky emission, Space Charge Limited Current (SCLC)), a power law between the cell current and the applied bias is considered with two parameters A HRS and β HRS . Finally, the total current flowing through the cell is given is by:

OX CF Cell I I I + = (4)
I CF is the main contributor to LRS current (I LRS ) and I OX is the main contributor to HRS current (I HRS ). The memory cell compact model is calibrated on silicon. The model was confronted to quasi-static and dynamic experimental data before its implementation in electrical circuit simulators. As presented in Figure 2a, after calibration, the model satisfactorily matches quasi-static and dynamic experimental data measured on actual HfO 2 -based memory elements (TiN/Ti/HfO x /TiN stack [START_REF] Nail | Understanding RRAM endurance, retention and window margin trade-off using experimental results and simulations[END_REF]). In Figure 2b, the evolution of SET voltages (V app ) as a function of the programming ramp speed is presented. The model implementation focused on this dependence which is crucial for the model to be confidently implemented in circuit simulators.

OxRAM operation as a programmable resistance

To monitor the variation of the analog resistance, an analysis of the cell resistance variation after FORMING is performed. The memory cell is formed gradually by applying a series of short programming pulses across the OxRAM cell. The number of pulses is controlled by a programmable pulse generator. The pulse width is set to 50 ns within a period of 100 ns and the pulse level is set to 3 V. As a result, the resistance of cell decreases. Figure 3a presents the evolution of the HRS resistance versus the number of pulses. Figure 3b change of the resistance value from a low impedance state to a high impedance state) allowing a restart of the calibration process. A third observation is related to the resistance step variation which can be decreased or increased by modulating the programming pulse width and level, allowing a fine tune of the cell resistance. Moreover, as the OxRAM memory state is comparable to a simple resistance, no refresh of the cell is needed (i.e. the cell resistance maintains its value).

Configurable Operational Amplifier Topologies

Open-loop amplifier

Figure 4 shows the circuit diagram of a two-stage CMOS amplifier [START_REF] Razavi | Design of analog CMOS integrated circuits[END_REF]. The first stage consists of a n-channel differential pair M1-M2 associated with a p-channel current mirror load M3-M4 and a n-channel tail current source M5. The second stage consists of a p-channel amplifier M6 associated with a n-channel current-source load M7. The high output resistances of these two transistors provide a relatively large gain for this stage and an overall moderate gain for the complete amplifier. Since multi-stage amplifiers have more poles and zeroes than single-stage amplifiers, all multi-stage amplifiers suffer closed-loop stability problems. One common compensation technique requires placing a compensation capacitor C C between the input and the output nodes of the second stage and a low value resistor represented by M C placed in series with the capacitor. The main amplifier parameters obtained after simulations are presented in Table 1.

+ + + -

Configurable closed-loop inverting amplifier

Based on the amplifier presented in Figure 4, a closed-loop inverting OpAmp configuration is built and presented in Figure 5a. In Figure 5a the feedback resistance R2 and the input resistance R1 of the classical inverting OpAmp are replaced by OxRAM cells. The OpAmp including its programming interface is presented in Figure 5b. In OpAmp calibration mode, the programming interface is activated when the EN signal is set High. In the same time transmission gates S1 and S2 disconnect the OpAmp output and isolate the OpAmp input from the rest of the circuit. Then, SET and RESET pulses are applied across the cell through the 3-state buffers in order to program dynamically and independently each OxRAM cell. Table 2 presents the different voltage levels to apply across R1 and R2 to perform "soft" SET/RESET operations. Note that the OxRAM bottom electrode B is shared between R1 and R2 allowing the use of only one RESET a Evaluated with a 5pF load signal. Regarding the calibration speed, the calibration process is not time consuming as a series of short pulses are applied across the cell.

In OpAmp normal operation mode, OxRAM cells behave as conventional resistances, and voltages across the cells are always below OxRAM programming voltages. Indeed, the input voltage is meant to be low and the OpAmp output swing voltage is limited to 1.2V as presented in Table 1.

-

+

. SET/RESET voltage levels

OxRAM input signals

SET1 RESET SET2 OxRAM States # Configuration L L L no effect 1 L L H R2 SET 2 L H L R1 & R2 RESET 3 L H H R1 RESET 4 H L L R1 SET 5 H L H R1 & R2 SET 6 H H L R2 RESET 7 H H H no effect 8
According to Figure 3 and as mentioned in section 2.3, HRS resistance variation is in the MΩ range [15MΩ -156kΩ] and LRS resistance variation is in the range [955Ω -500Ω].

The gain of the open-loop OpAmp presented in Figure 5 is equal to -R2/R1. Thereby, the gain of the OpAmp can be adjusted dynamically after changing R1 and R2 resistance states.

As an application example, let's consider configuration 3 of Table 2. In this configuration, R1 and R2 are reset to 250kΩ after applying a series of 39 programming pulses across both OxRAM cells (HRS resistance variation versus the number of pulses is given in Figure 3a). As expected, a unity gain is provided by the OpAmp. Then, only R1 is set to 662Ω after 50 programming cycles according to configuration 2 of Table 2 (LRS resistance variation versus the number of pulses is given in Figure 3b). A new gain equals to 377 is now provided by the OpAmp. This gain can be increased dynamically by increasing the number of pulses applied across R1. Figure 6 presents the evolution of the OpAmp gain versus the number of programming pulses. The gain variation is in the range [377 -490].

Configurable integrator and differentiator

Figure 7a presents the classical OpAmp integrator circuit where resistances are replaced by OxRAM cells. The input resistance of the integrator is replaced by the OxRAM cell R1 and the feedback resistance, connected in parallel with capacitor C1, is replaced by the OxRAM cell R2. The feedback resistor R2 gives the circuit the characteristics of an inverting amplifier with a finite closed-loop gain of R2/R1. At very low frequencies the circuit acts as a standard integrator, while at higher frequencies the capacitor shorts out R2 due to the effects of capacitive reactance, reducing the amplifier gain and making the circuit to behave as a Low Pass Filter (LPF). The OpAmp including its programming interface is presented in Figure 7b. In order to reduce the overall closed-loop gain at high frequencies, the extra input resistor R1 was added to the input, limiting the differentiator increase in gain at a ratio of R2/R1. At low frequencies, the circuit acts as a differentiator amplifier making the circuit to behave as a High Pass Filter (HPF). The OpAmp including its programming interface is presented in Figure 8b.

For both integrator and differentiator, capacitor C1 value is set to 159nF and R1 and R2 are programmed in parallel (i.e. R1 and R2 are programmed to the same value). Figure 9 presents the frequency response (Bode plot) of the integrator and differentiator after a RESET operation for an HRS resistance variation in the range [0.5MΩ -5MΩ]. The evolution of the corner frequency versus the resistance variation is extracted from Figure 9 and presented in Figure 10 for the integrator and differentiator. The theoretical corner frequency is also added for comparison purposes, demonstrating the proper operation of the configurable OpAmps. It is worth noting that the corner frequency variation is in the range [0.24kHz -2.4kHz] with respect to R1 and R2 variations. Figure 12 presents the differentiator transient responses. When applying a constantly changing signal such as a sine-wave (Figure 12a) or a triangular wave (Figure 12b) to the input of the differentiator circuit, the resultant output signals are respectively a cosine wave and a rectangular wave, demonstrating the differentiation capability of the circuit.

Discussion

In the presented OpAmp methodology, the memristive device is programmed to a desired resistance prior to the operation of the amplifier. To achieve a proper programming of the OxRAM cells, 3 additional three-state buffers and 2 pass-gates are added to the conventional closed-loop OpAmp designs. These additional components impact performances of the closed-loop OpAmps by introducing a slight voltage drop (< 0.1mV) at the OpAmp inputs. This voltage drop can be reduced by oversizing the pass gates or by modulating R1/R2 resistance ratio. Benefits of the presented methodology are related to the suppression of semiconductor resistances usually implemented in closed-loop OpAmp topologies. Moreover, an incircuit resistance calibration feature is provided. Another important aspect of the presented work is the use of actual memristive devices (HfO 2 OxRAM stack model calibrated on silicon) combined with a programming interface designed in a 130 nm High Voltage (HV) CMOS technology. The HV technology is chosen to allow the use of thickgate-oxide transistors able to sustain OxRAM programming voltages.

Conclusion

Memristive devices provide an inspiring variety of opportunities for circuit designers. In the presented approach, OxRAM based memristive devices are programmed to a desired resistance value prior to their operation within closed-loop OpAmps (i.e. inverting amplifiers, integrators and differentiators). The OxRAM cells operate in such a way that in the analog mode of operation (when the OxRAM performs a useful function as an analog circuit element) only voltages of small magnitude (0-1.2V) are applied to the device, while higher-amplitude voltages are used only for programming (~3V). The final closed-loop OpAmps feature a programmable gain and corner frequency while suppressing the use of semiconductor resistances.

Figure 1 .

 1 Figure 1. (a) OxRAM I-V hysteresis and (b) OxRAM cell symbol

Figure 2 .

 2 Figure 2. (a) I-V characteristic measured on HfO2-based devices [20] and corresponding simulation using a bipolar OxRAM physical model. (b) SET voltage as a function of the programming ramp

Figure 3 .

 3 Figure 3. (a) HRS resistance variation and (b) LRS resistance variation versus the number of pulse cycles

Figure 4 .

 4 Figure 4. Two-stage CMOS amplifier

Figure 5 .

 5 Figure 5. (a) OxRAM-based inverting OpAmp (b) OxRAM-based inverting OpAmp with programming interface

Figure 6 .

 6 Figure 6. Absolute value of the OpAmp gain (|-R2/R1|) versus the number of pulses applied across R1

Figure 7 .Figure 8 .

 78 Figure 7. (a) OxRAM-based integrator (b) OxRAM-based integrator with the programming interface

Figure 9 .

 9 Figure 9. Frequency response of the integrator and differentiator versus R1 and R2 resistance variation in the range [0.5MΩ -5MΩ].

Figure 10 .

 10 Figure 10. Differentiator, integrator and theorical corner frequency fc versus resistance variation (R1 and R2)

Figure 11 .Figure 12 .

 1112 Figure 11. Transient simulation results of the integrator for (a) sine wave and (b) triangular wave inputs

  Before any SET/RESET operation, a FORMING operation is needed. The FORMING operation, achieved one time in the device life is a voltage-induced resistance switching from an initial virgin state with a very high resistance to a conductive state[18]. After FORMING, the OxRAM element can be reversibly switched between a High Resistance State (HRS or OFF state) and Low Resistance State (LRS or ON state). It is important to note that the FORMING stage is the first and most critical step as it determines the switching characteristics during the future operation of the memory cell. Also, a FORMING operation needs more important voltage values.

Table 1 .

 1 CMOS operational amplifier parameters

	Parameter	Value a
	DC supply (V)	1.8
	Technology (nm)	130
	Open loop gain (dB)	72
	Output swing (V)	1.2
	Slew-rate (V/µs)	10
	Phase margin (Degree)	75
	CMRR (dB)	80
	Power consumption (µW)	150


Omar Kassem

Mohamed Saadaoui

Mathilde Rieu

Jean-Paul Viricelle

A novel approach of a fully inkjet printed SnO2-based gas sensor on a flexible foil

Keywords: inkjet printing, flexible support, gas sensor, SnO2, platinum, gold, polyimide

In recent years, printed and flexible gas sensors have quickly emerged as an innovative area of great interest because of their lightness and low cost. These flexible sensors can be easily integrated into autonomous systems for many applications such as smart food packaging and premature disease detection. In this paper, a novel approach was applied to manufacture a fully inkjet-printed gas sensor on a flexible polymeric foil. Platinum heater and gold electrodes were printed on the top side of the substrate, separated by a thin insulating layer of printed polyimide. An aqueous sol-gel process was adopted to synthesize nanosized SnO2-based sol that guaranty a crystallization at 350 °C, which is entirely consistent with the polyimide foil. Then, the sol was transformed into a stable ink and inkjet printed over the gold electrodes. The printability of different inks was optimized to ensure flawless ejection of droplets, and the complex physico-chemical interactions between the inks and different interfaces were controlled to get well-defined patterns with high resolution. Finally, electrical measurements of the printed sensor were performed to characterize the response and the sensitivity to different concentrations of ethanol, ammonia and carbon monoxide gases, at working temperature of 300 °C, in dry and wet air.

Introduction

The field of flexible and printed electronics is a burgeoning area in electronics development at academic and industrial level, because of potential applications in everyday life by printing electronic devices on large area soft substrates like plastic and paper [START_REF] Katsuaki | Introduction to Printed Electronics[END_REF]. Printing techniques offer wide advantages like rapidity, simplicity, eco-compatibility, and are the viable alternatives to the clean-room patterning processes like vacuum deposition subtractive technologies and lithography which involve multiple steps and high costs [START_REF] Raut | [END_REF].

Among the different printing technologies, inkjet printing has gained a significant attention for its interesting characteristics. It is a digital and maskless patterning technique used to print well-defined patterns with less material wastage, on a wide range of substrates including textile, paper and plastic. Using inkjet printing, a picoliter volume of ink can be ejected in the form of droplets from the reservoir and through nozzles of a printhead. Nevertheless, challenges remain associated with the accurate control and stability of the jetting. Inkjet printing is a technology where several parameters interact, such as ink viscosity, surface tension, particles size and surface wettability [START_REF] Raut | [END_REF]. Thus, it is essential to optimize inks compositions, printing parameters and annealing conditions for functional high resolution printed patterns.

A vast variety of materials can be deposited by inkjet printing, including metallic nanoparticles and nanowires [START_REF] Balliu | Scientific Reports[END_REF][4][5][6], polymers [7,8], semiconductors [9,10], quantum dots [11] and carbon nanotubes [12]. These materials are used to produce different types of electronic components such as lightemitting diodes [13], biosensors [14,15], supercapacitors [16], transistors [START_REF] Grubb | Scientific Reports[END_REF]18], and solar cells [19]. Recently, considerable attention has been given to inkjet printing of metal oxides based inks for gas sensing applications [20]. The fabrication of flexible gas sensors presents an interesting perspective for progress in many fields including premature medical diagnosis [21], detection of hazardous gases [22] and smart food packaging [23]. The commonly sensitive elements used for gas sensors are the semiconductor metal oxides like SnO2, ZnO, WO3, CuO, NiO, TiO2, CeO2 etc. [24][25][26][27]. They are low cost, highly sensitive and have good thermal and structural stability. In addition, they are able to detect different type of environmental gases including oxidizing gases (NO2, O3) and reducing gases (ethanol, acetone, H2S, CO, NH3, CH4) [28,29]. However, crystallization of metal oxides requires annealing at high temperature which can be problematic when polymers are used as a substrate.

Among the various types of metal oxides, tin dioxide (SnO2) is one of the most used material as an active layer for gas sensor [30] because of its high sensitivity, chemical and structural stability, low cost, rapid response and its ability to detect many toxic and inflammable gases. SnO2 crystal structure contains oxygen vacancies that make the material an oxygen-deficient n-type semiconductor. SnO2 is synthesized in different forms (thin film, powder, nanowire, etc.) using various soft chemical synthesis methods, including polyol process [31], Okazaki method [32], chemical precipitation [33], organic-free synthesis method [34] and surfactant mediated method [35]. However, in comparison with other chemical synthesis processes, sol-gel is a simple and cost-effective technique, and appears to be very promising for deposition by inkjet printing. Wenfeng Shen [36] has formulate a SnO2-based ink by alcoholic sol-gel process. The ink was printed on silicon and alumina substrates, and X-ray diffraction analysis shows that the rutile structure of SnO2 was obtained at an annealing temperature of 550 °C.

More recently, Rieu et al. [20] have successfully developed an alcoholic sol-gel method to prepare a SnO2-based ink, which was jetted on polyimide foil and annealed at 400 °C.

In our work, a new stable SnO2-based ink is formulated by aqueous sol-gel. The synthesized sol contains highly dispersed nanocolloids smaller than 10 nm in size, which confers good stability to the ink and enable SnO2 to crystallize at a temperature below 400 °C, compatible with our flexible support. Afterwards, inkjet printing technique is adopted as a deposition technology in order to fully print a flexible gas sensor based on tin oxide. The sensor is designed and printed in a novel multi-layer configuration to include all sensor elements on the top side of the plastic, in contrast to what is usually used where the heater is often integrated on the back-side of the substrate [20,22,37]. The sensor is made up of the following elements:  A platinum-based heater that activates the sensitive layer  Gold-based electrodes to connect the sensor to the electrical characterization bench  A gas-sensitive layer based on SnO2  An insulating layer of polyimide that electrically insulates the platinum heater from the electrodes and the sensing SnO2 layer.

This original configuration leads to an easier integration of the sensor into an electronic data acquisition card. In addition, it reduces significantly the interface between the heater and the electrodes, which minimizes energy consumption.

We print platinum and polyimide inks that are newly commercialized and have never been reported, excepting a commercial gold ink which was reported in the literature [4,20]. We study the printability of these inks and perform the physical and electrical characterizations.

Finally, we present the sensor responses and sensitivity towards three gases: ethanol, ammonia and carbon monoxide, under dry and wet air, at 300 °C as operating temperature.

Experimental section

Inkjet printer specifications

For inkjet printing, we use a commercial lab-scale Dimatix inkjet-printer (DMP-2800, FujiFilm) equipped with a 10 pL cartridge. This printer requires an ink with surface tension in the range of 28 -33 mN/m, viscosity in the range of 10 -12 mPa.s, pH between 4 and 9, and particles size smaller than 200 nm to avoid nozzles clogging.

Formulation of SnO2-based ink by aqueous sol-gel process

SnO2-based sol was firstly prepared by aqueous sol-gel method as described in the literature for the preparation of SnO2 nanopowders [38]. The synthesis method consists in dissolving tin chloride (SnCl2 99.99% -Sigma-Aldrich) in deionized water. Then, a 25% ammonia solution (NH4OH Sigma-Aldrich) was added dropwise until the pH becomes slightly basic, for complete removal of chloride ions (Cl -). Afterwards, tin hydroxide precipitate was recovered and dissolved in glacial acetic acid (CH3COOH 99% -Sigma-Aldrich) and ethylene glycol (EG, HO-CH2-CH2-OH 99.8% -Sigma-Aldrich) was added in order to form a tin complex. The mixture was heated at 90 °C and a pale yellow sol containing a complex of tin cations was obtained after 1 h. In our previous work [39] an analysis carried out on the synthesized sol showed that the average diameter of the colloids was around 6 nm, and also confirmed the absence of aggregates in solution. Moreover, we successfully investigated the thermal behavior of the sol and we found that a crystallized structure of SnO2, corresponding to a rutile structure, was obtained at 350 °C, which is entirely compatible with the use of polyimide foil.

Synthesized sol was transformed into ink by evaporating the majority of the acetic acid to increase the ink pH up to 4 -5, and by adding three solvents to adjust its rheology. For homogeneous and stable ink preparation, we used solvents with different boiling temperature (Tb) and vapor pressure (Pvap) as ethylene glycol (Tb = 197.3°C, Pvap = 8 Pa), glycerol (Tb = 290°C, Pvap = 0.4 Pa) and ethanol (Tb = 78°C, Pvap = 5800 Pa). Finally, 2-Isopropoxyethanol surfactant was used for fine-tuning of the surface tension [39]. The final SnO2 ink, shown in Figure 1, has a viscosity of 10 mPa.s and a surface tension of 32 mN/m, and exhibits a very good stability over time.

Designing and printing of gas sensor A 50 µm thick polyimide foil was selected as a substrate (Upilex® S-Goodfellow). This plastic exhibits high thermal and chemical resistance and surface smoothness (Rz < 60 nm). Prior to processing, the substrate was cleaned by acetone, isopropanol and deionized water then it was dried in an oven at 120 °C for 30 minutes. Commercial gold (Au) ink (NPG-J from Harima, Japan) was used to print electrodes with an interspace of 300 µm. Platinum (Pt) ink, recently developed and marketed (Akko Lab LCC, Russia) was used to print the heating element. Electrodes and heater were printed on one side of Upilex foil and electrically separated each other by an insulating layer of polyimide which is printed using a commercial and high thermal resistance polyimide (PI) ink (PI 6643-001 JNC corporation, Japan). Finally, SnO2 ink prepared by sol-gel process was deposited onto the polyimide and over gold electrodes. The shape of the sensing element was a square of 1 mm 2 . Rheological properties of commercial inks provided by suppliers and SnO2 ink prepared in our previous work are summarized in Table 1, and the multilayer configuration used to manufacture a fully inkjet flexible gas sensor is shown in Figure 2. Optical microscopy, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM). Morphology of printed patterns was firstly observed by binocular microscope (Nikon SMZ1500) and optical microscope (Nikon Eclipse LV150). Observations of platinum film at nano-scale were performed using a Zeiss Ultra 55 SEM equipped with an energy dispersive spectroscopy (EDS), and by AFM Nanowizard III (JPK Instruments AG, Germany) in the tapping mode. The AFM image was obtained from a 1 µm 2 surface, scanning with 512 by 512 pixels. A cross-section lamella of all printed layers was prepared using Focus Ion Beam (FIB) (Equipex MANUTECH-USD). Carbon and platinum were deposited on the top of the sample during the preparation of the lamella. Thickness of printed layers was determined by SEM.

Surface energy measurement. Surface energy and contact angle measurements were characterized using a goniometer (Apollo Instrument OCA200, Dataphysics). We use Owens-Wendt-Rabel-Kaelble (OWRK) method to calculate the surface energy of the substrate [39].

Characterization of electric resistivity of printed metal thin films. Kelvin structures were used to characterize the electrical resistivity of printed Au and Pt thin films. The resistance (R) was measured using a 4-point probe system (SÜSS MicroTec PM5) with a Keithley 2010 source-meter. The electrical resistivity was calculated using the following formula:

ρ (Ω. m) = R(Ω). S(m 2 ) L (m)
Where L is length of the printed patterns measured by optical microscope (Nikon Eclipse LV150) and S is the section determined by mechanical profilometer (Ambios XP-2), taking into account the effect of material accumulation at the edges, known as the "Coffee-ring".

Electro-thermal simulation and calibration of the heater. Comsol Multiphysics 5.3 software was used to optimize the design of the heater using electro-thermal simulation. Platinum and polyimide properties presented in table 2 were used. Then, the temperature coefficient of resistance (α) of Pt printed and sintered at 350 °C was characterized experimentally using the Kelvin structure described above. The experiment was performed in a clean room 1000 with controlled temperature and humidity conditions. The sample placed on a Peltier thermoelectric module (European Thermodynamics, Ref: GM250-127-14-16) was polarized under 4 points to accurately measure its electrical resistance at different temperatures. A thermometer based on fiber optic gallium arsenide (AsGa) with an accuracy of ± 1 °C was used to determine the temperature (Optocon, Fotemp1). The temperature coefficient of resistance (α) was extracted from a linear regression of the following formula:

R(T) = R 0 (1 + α∆T)
Where R0 is the resistance at room temperature (T0) and ΔT = T -T0 Electrical characterizations of the sensor. Electrical characterizations of the fully-printed sensor were performed using three different gases: Ethanol (C2H5OH), ammonia (NH3) and carbon monoxide (CO), in dry and wet synthetic air. The sensor was installed in an 80 cm 3 glass chamber under constant gas flow of 10 L.h -1 . Cylinders of Ethanol (555 ppm, diluted in nitrogen), NH3 (297 ppm diluted in nitrogen) and CO (300 ppm diluted in air) purchased from Air Product and Chemicals Company were used. Before injecting the target gas in the test chamber, the sensor was kept for one night at operating temperature to desorb organic species and to achieve good conductance stability. By applying a supply voltage, the platinum heater printed under the SnO2 sensitive layer enabled to heat up the sensor between 100 and 300 °C. Gas injections were done using gas flow meters controlled through LabView software.

Conductance measurements were performed by electronic unit equipped with voltage divider circuit, and data acquisition was done with LabView software also. After stabilization of the sensor at 300 °C under dry air, a thermogram showing the evolution of the sensor conductance during a slow temperature descent to 100 °C, with a ramp of 1°C/min, was recorded. The conductance of the sensor was then stabilized under 20 ppm ethanol for 1 h at 300 °C, then the variation of the conductance under C2H5OH as a function of temperature was studied between 300 °C and 100 °C with the same 1°C/min ramp. The samples undergo the same thermal cycle with the same conductivity measurements under 20 ppm CO and 20 ppm NH3.

In the next step, sensor response measurements, 20 min exposure to gas/air mixture and 30 min recovery in air, were recorded for different gas concentrations. We define relative response of the sensor as: (G-G0)/G0

Where G0 is the initial conductance of SnO2 measured under air just before injection of target gas, and G is the conductance after 20 minutes of gas injection. Relative response was plotted versus gas concentrations to monitor the sensor response and to calculate its sensitivity. Sensor sensitivity is a characteristic parameter. It expresses how large is the change in the sensor signal upon a certain change in gas concentration, in other words, it is defined as the slope of the relative response curve versus target gas concentrations.

Results and discussion

Sensor design

The sensor was designed and printed in multilayer configuration to embed all the elements of the sensor on the plastic top face. The process steps are illustrated in Fig. 3.

Following the cleaning and drying of the substrate, a single layer of polyimide was printed on Upilex foil (Fig. 3a). This step was integrated into the fabrication process owing to the difficulties faced when Pt ink was printed directly on Upilex, and it will be discussed later in this paper. Then, Pt ink was printed on polyimide layer to prepare the heating structure (Fig. 3b). Next, a second polyimide layer was printed over the first one and on platinum layer simultaneously (Fig. 3c). This new polyimide layer provides electrical insulation between the heater and electrodes that were printed on the second polyimide layer using Au ink (Fig. 3d). Finally, a sensitive layer based on SnO2 was deposited over the gold electrodes (Fig. 3e). This configuration enables the sensor to be easily interfaced with the electrical characterization test bench of the sensor or in electronic acquisition card. In addition, the thickness of the printed polyimide is 10 times lower than that of Upilex substrate given at 50 µm. Therefore, the integration of the heater and the sensitive layer on the top surface of the plastic foil will reduce the interface between these two elements, which can limit heat dissipation and consequently minimize the energy consumption of the sensor.

Preparation of an electrical insulating layer of polyimide by inkjet PI 6643-001 polyimide ink has a surface tension of 30 mN/m and a viscosity of 20 mPa.s at room temperature (Table 1). Printing an ink with such high viscosity requires high nozzles temperature and high excitation voltage to compensate for the increase in elongational viscosity during ejection [42].

Raising up nozzles temperature decreases the fluid viscosity, but a large increase in nozzles temperature can destabilize the ink by promoting solvents evaporation, which causes clogging nozzles [43]. Therefore, it is essential to find a good compromise of nozzles temperature to facilitate the ink ejection without clogging the nozzles. A value of 32 °C was retained in this case. In addition, a waveform adapted to the rheological behavior of the polymer ink also had to be defined as depicted in Fig. 4.

The waveform has two main phases that essentially influence the piezoelectric element deformation and the droplet ejection. The first phase setting allows defining the amount of ink that will be drawn from the reservoir to the micro-chamber. Phase 2 setting is performed to define the quantity and speed of ink ejection via the nozzles. These two consecutive phases can be adjusted in terms of "slew rate" and "duration", which indicate how fast the piezoelectric element is deformed and how long it remains in this position, respectively.

In the case of polyimide printing, adjustments were made on phase 2 while observing individual jet with the built-in drop watcher camera of the Dimatix printer. In comparison with the standard waveform [39] the duration was extended from 4 to 14 µs and the slew rate was increased from 0.9 to 1.3 V/µs. This optimization step aims to prevent the ink deceleration caused by the ink wettability on the nozzle during the ejection phase and to overcome the viscoelastic behavior of the ink that opposes its detachment from the nozzles. Afterwards, ejected droplets were observed as function of the voltage applied on piezoelectric membrane, called "firing voltage". We found that the application of firing voltage between 20 and 25 V is the best compromise that ensure the formation of a stable jet.

For lower ejection voltages, the drop may not be easily ejected, while the application of very high excitation voltage may result in the formation of satellites droplet, because the surface tension of the fluid can not sustain the fluid in one droplet. Therefore, a main droplet is separated from the tailing part which forms satellites [44].

Fig. 5 shows sequential images of drop formation, 30 μsec after ejection, at different firing voltage values between 20 and 25 V, captured with stroboscopic flashes. The imposition of an increasing electrical potential on the piezoelectric membrane results in a higher drop ejection rate, leading to a straight ejections with longer ligament behind the spherical portion of the drop. This behavior is characteristic of the inkjet printing of a low molecular weight viscoelastic polymer [45].

In parallel to the firing voltage adjustment, the drop spacing has to be optimized to ensure a good surface coverage when printing a continuous polyimide film on Upilex foil. The surface energy value of Upilex measured was 47 mN/m. Polyimide droplets were first printed at different operating voltages and their diameters are measured using an optical microscope. Fig. 6 shows the obtained results. Uniform spherical droplets were observed, which confirms the compatibility of the polyimide ink surface tension (30 mN/m) with the Upilex surface energy (47 mN/m). The increase in firing voltage increases the velocity and the printed volume, resulting in larger diameter droplets [6]. An average width of 72 ± 1.4 μm, obtained for an applied firing voltage of 20.8 V, increases to 86.8 ± 1 μm for a firing voltage of 25 V. Therefore, the drop spacing was set at 30 -35 µm, depending on the selected voltage, to achieve the overlap of the deposited drops necessary for a homogeneous and continuous film.

Then, a square-shaped polyimide layer was inkjet printed on Upilex foil and heated at 80 °C for 5 minutes and then at 350 °C for 30 minutes for solvents evaporations and complete imidization of polyimide precursor, according to the ink supplier. The surface energy measurement of the polyimide printed film by OWRK method gave a value of 40 mN/m. This value is higher than the surface tension of PI ink (30 mN/m), Au ink (25 mN/m), Pt ink (30 -40 mN/m) and SnO2 ink (32 mN/m) (Table 1) and will ensure good inks spreading without the need for a surface treatment.

In the proposed fabrication process, the polyimide ink (PI -6643-001) must be printed on a polyimide layer pre-printed and polymerized on Upilex (Fig. 3c). Therefore, a drop test was carried out partly on the polyimide film to determine the optimal drop spacing to get a continuous layer. Using an average value of the firing voltage (23 V), single droplets of polyimide ink deposited on the pre-printed polyimide film have a mean diameter of 62 ± 1 μm, and are smaller than the droplets deposited on Upilex foil using the same firing voltage (diameter ≈ 78 ± 1 μm, Fig. S1 ESI †). This is due to the surface energy of printed polyimide film (40 mN/m) which is lower than that of Upilex (47 mN/m), resulting in a lower spread of the deposited droplets. The drop spacing of printed polyimide ink was set at 30 μm in this case.

Inkjet printing of Au ink

Au and Pt metal inks have very different rheological properties, which necessitates specific optimizations of the ejection conditions for each ink. In comparison with PI and Pt inks, Au ink exhibits low viscosity and surface tension values (Table 1), which requires less time to form the drop. Thus, a controlled printing of Au ink was achieved by applying a low firing voltage of 17 ± 1 V and by decreasing the slew rate of the waveform ejection phase to around 0.4 V/µs (Fig. S2, ESI †). Furthermore, nozzles temperature was set at ambient temperature and a 5 inch H2O vacuum was applied, to prevent the ink from flowing by gravity through the cartridge nozzles. Now the ink ejection conditions are optimized, it is possible to proceed to the evaluation of the printed patterns quality. The Au ink surface tension given by the supplier at 25 mN/m is much lower than the surface energy of both Upilex foil (47 mN/m) and printed polyimide layer (40 mN/m), which leads to a significant spreading of the ink once it is deposited on the substrate. To overcome this problem, the drop spacing was set to 45 µm and the platen temperature was maintained at 50 °C to promote the ink solvents evaporation and consequently the spreading of the ink would be reduced [46,47]. Once the gold ink was printed, the resulting film was sintered at 200 °C for 1 h to evaporate solvents and organic residues and to coalesce the gold nanoparticles on which it is possible to print the SnO2 sensitive layer. In order to characterize Au layer, a final annealing step was performed at 350 °C for 2 h. This temperature was dictated by the crystallization temperature of SnO2 (Fig. 3e). Annealing steps were carried out in standard oven using a ramp of 5 °C/min. The electrical resistivity of Au printed pattern annealed at 350 °C (2 h) was 13 µΩ.cm, which is 5 times higher than the resistivity of bulk gold (ρ = 2.44 µΩ.cm at 25 °C).

Preparation of platinum heater by inkjet

Inkjet printing of Pt ink

Pt ink present a relatively high viscosity (15 -25 mPa.s). Thus, drop ejection was optimized by increasing the nozzles temperature to 30 °C to decrease the viscosity using the standard waveform provided by the printer and a firing voltage of 21 ± 1 V (Fig. S2, ESI †). In addition, Pt ink contains low weight percentage (5% wt.) of metal nanoparticles. It is therefore essential to print multiple layers to reduce the electrical resistance of the resulting film. In our case, we printed 4 layers of Pt by heating up the printer platen at 40 °C during the printing process to ensure fast evaporation of solvents and limit the loss in resolution of the printed pattern.

The final annealing of Pt patterns was dictated by the cumulative annealing temperature of the polyimide ink, gold and SnO2 inks which was done at 350 °C for 3 h. Our first results show that thin films of platinum ink, printed directly on Upilex and sintered at 350 °C were not electrically conductive. An elemental analysis by EDS reveals a discontinuity in the platinum film (Fig. S3, ESI †). We assumed that this phenomenon could be attributed to the viscoelastic mechanical deformation of Upilex foil, characterized by a high out-of-plane thermal expansion coefficient (αz from 100 ppm/°C to 400 ppm/°C between 20 °C and 400 °C) compared to the in-plane one (from 5 ppm/°C to 10 ppm/°C) and the manifestation of the glass transition temperature around 300 °C [48,49]. Thus, the thin platinum layer is mixed with a polymer from Upilex substrate when the annealing temperature exceeds 300 °C as confirmed by the increase of the carbon peak in the EDS spectrum, thus leading to local electrical and morphological discontinuity.

However, printing platinum on the pre-printed polyimide layer, and annealing it at 350 °C for 3 h results in the formation of a conductive film (Fig. S4, ESI †), and confirms that the high electrical resistance observed at 350 °C is directly related to the use of Upilex as a substrate and not to platinum ink. The difference in surface roughness and surface energy between the two polyimide (substrate and layer) can influence on Pt deposition, but we assumed that the main reason for Pt film discontinuity is the high thermal expansion of Upilex substrate and its viscoelastic deformation. This problem was overcome by the introduction of an intermediate polyimide layer that may also act as a buffer. Observation of the platinum film printed on polyimide by SEM and AFM (Fig. S5, ESI †) shows a compact microstructure with significant coalescence where the particles size was estimated at 30 -40 nm, compared to the initial size of the Pt nanoparticles of about 6 nm (table 1). We therefore used the printed polyimide as an interface between Upilex and the rest of the gas sensor layers.

Calibration of printed platinum heater

To establish the correlation between electrical resistivity, printed lines widths and annealing conditions, 3 Kelvin structures of platinum with a length of 700 µm, and widths of 100 µm, 200 µm and 300 µm were printed and sintered at several combinations of sintering temperature and time, as shown in Fig. 7. It can be observed firstly that for a fixed temperature, it is possible to achieve low resistivity values by heating for a longer period. The sintering time has a significant influence on the film resistivity, but when temperatures above 150 °C were applied, this influence becomes less important on the final resistivity. The lowest resistivity values are those obtained after annealing the samples at 350 °C for 3 h.

Moreover, we notice that the bars associated with the resistivity of structures with line widths of 200 µm (red) and 300 µm (blue) are almost the same height, indicating that the resistivity is similar under different annealing conditions and thus, the annealing mechanism is similar for both structures. On the other hand, the resistivity bar of the structure with a 100 µm wide (green) shows higher overall resistivity values, which can be associated with the coffee-ring effect promoted by heating the printer platen up to 40 °C during printing, encountered more easily when small width structures are printed by inkjet [50]. Therefore, a line width of 200 µm was chosen to design and manufacture the heater because of its small dimension, and its consistent resistivity values, in the same order of magnitude as those obtained with the 300 µm line.

A platinum-based heater was prepared by inkjet and used to rise the temperature around the sensitive area made by SnO2. The variation of the resistance versus temperature is given in Fig. 8 and a linear fitting of the resulting curve is used to extract the temperature coefficient (α) of printed platinum.

The obtained coefficient is 0.0029°C -1 which is lower than the one of the bulk platinum given at 0.0039°C -1 . This difference can be attributed to several parameters: the nanometric size of platinum particles [51], the substrate nature [51] and the complex microstructure of the metallic thin film which will bring more barriers during the electron-transfer process [52].

Electro-thermal simulation of heater

In order to obtain both a homogeneous temperature distribution on a surface corresponding to that of the SnO2 layer (1 mm²) together with a low power consumption, heater simulation was developed thanks to Comsol software. The heater structure including Upilex, platinum and printed polyimide was modeled in 3D (Fig. 9a) considering thermal and electrical properties of these materials (Table 2). A serpentine structure has been studied with geometrical parameters (a, L, n, described in Fig. 9b) that are going to be optimized. In this study, printed polyimide was considered to have the same characteristics as Upilex, because the thermal properties of the printed polyimide are unknown. The thickness of the Pt film found at 130 nm (Table 2) was extracted from the section of printed Pt pattern using a mechanical profilometer (Ambios -XP2), taking into account the important coffee-ring effect of the printed pattern (Fig. S6, ESI †). The "free triangular" mesh type was used with a maximum element size of 100 µm (Fig. S7, ESI †). Simulations were subsequently carried out to study the influence of the number of fingers (n) and their length (L) on the temperature distribution and the power output. Distance "a" has been set at 900 µm and the width of the interconnection between heating resistor and pads was extended at 1300 µm to limit heat diffusion to the external connections. The track width and inter-line heater spacing were set at 200 µm and 160 µm, respectively. Fig. 10 shows the dependence of maximum temperature and electrical power on geometric dimensions. To reach a temperature of 300 °C, the power output must be between 360 and 400 mW. This power range can be obtained by optimizing the couple (n, L). Therefore, the number of resistance fingers was set at 3 (corresponding to a total width of 2480 µm) and their lengths at 4.3 mm to minimize the total size of the heater while promoting a uniform temperature distribution over a millimetric surface as shown in Fig. 11.

To compare the simulation results with the experiment, a heater with n = 3 and L = 4.3 mm was manufactured by inkjet printing and annealing at 350 °C for 3 h. Electrical voltages between 4 and 12 V were applied to determine the resistance and the corresponding electrical power. The temperature was deduced using the temperature coefficient (α) of platinum at 0.0029 °C-1 . The obtained results show a very good agreement between the thermal response experimentally achieved and the simulation (Fig. 12). At 300 °C, the error percentage between the applied and simulated electrical power was about 3.5 %. The power output (360 -400 mW) can be reduced by printing more Pt layers to decrease the initial electric resistance, or by reducing the active surface area, to get a miniaturization of the structure and a gain on the applied power.

Gas sensing characterizations

After the optimization of the printing and annealing conditions of polyimide, platinum and gold inks, the preparation of a fully inkjet printed gas sensor was carried out using the process explained in the Fig. 3. Two transparent layers of SnO2 (1 mm 2 ) were printed over the electrodes by introducing an intermediate annealing step at 350 °C between each printing step to avoid cracks formation in the printed film [39]. The sensor is illustrated in Fig. 13. SEM cross-sectional images presented in Fig. 14 show that the thicknesses of the Pt, PI, Au and SnO2 printed films were around 55 nm, 4 µm, 180 nm and 100 nm, respectively.

A study was performed to determine the temperature at which SnO2 sensor exhibits a maximum response to 3 different gases: ethanol, ammonia and carbon monoxide (Fig. 15). The results show that SnO2 conductance in dry air increases with temperature until it reaches a maximum between 150 °C and 200 °C. This is a classic behavior observed in SnO2-based sensors studies [53]. In addition, the conductance of SnO2 under ethanol shows a continuous increase until 300 °C.

Fig. 15 shows that the maximum response of SnO2 under NH3 is also increasing up to 300 °C. In addition, a particular phenomenon was observed in the case of ammonia detection. Indeed, we notice that at temperatures below 225 °C, the conductance of SnO2 exposed to NH3 was lower than the one obtained under dry air. This phenomenon has been reported previously by Kamalpreet Khun et al. [54] and by Hijazi et al. [26]. They explained this phenomenon by different reaction mechanisms at low and high temperatures. Consequently, NH3 has a reducing behavior at temperatures above around 225 °C, and oxidizing at lower temperatures.

Finally, the maximum conductance of SnO2 under CO was at 250 °C, but the difference between conductance under dry air and conductance under CO was slightly higher at 300 °C. Thus, in the next experiments, sensors temperature will be fixed at 300 °C.

Fig. 16 shows the response of SnO2 flexible sensor at 300 °C, upon exposure to different concentrations of C2H5OH, NH3 and CO (from 5 to 100 ppm under dry air and from 10 to 100 ppm under wet air). The sensor was swept under dry air for 1 night at 300 °C to stabilize its electrical conductance. First, it can be seen from Fig. 16 that the conductance of SnO2 increases when exposed to the target gas and it drops rapidly and returns its original state when the injection was stopped in the testing chamber in each cycle, indicating that the SnO2 gas sensor has reversible response for different C2H5OH, NH3 and CO concentrations under dry air. These gases exhibit a reducing character. They react with the ionosorbed oxygen species and release electrons to the conduction band according to equation [START_REF] Katsuaki | Introduction to Printed Electronics[END_REF], which leads to an increase of conductance [55]. Test bench configuration did not allow estimating any response time due to significant dead volume.

R + O -→ RO + e - (1)
In order to evaluate the sensor sensitivity under dry air, the relative response (G-G0) / G0 was plotted as a function of the gases concentration injected into the cell. Fig. 16 shows that the relative response increases with increasing gases concentration, and the sensor is quite sensitive in low range [0 -20 ppm] but rapid signal saturation occurs for higher gases concentrations. Indeed, the sensor exposed to ethanol exhibit a sensitivity around 1.36 ± 0.17 ppm -1 between 0 and 20 ppm. After this concentration, the sensitivity starts to decrease and reach approximately 0.25 ± 0.1 ppm -1 between 40 and 100 ppm. The same phenomenon was observed when the sensor was exposed to ammonia and to carbon monoxide under dry air. For NH3, the sensitivity of the sensor was 0.14 ± 0.03 ppm -1 between 0 and 20 ppm and it decreased to 0.005 ± 0.001 ppm -1 for higher concentrations. Finally, the sensitivity of the sensor was 0.1 ± 0.02 in the range [0 -20 ppm], and it decreased to 0.01 ± 0.002 in the range of [40 -100 ppm] when the sensor was exposed to CO gas under dry air. We notice that the fully inkjet sensor is able to detect gases at low concentrations, which is very interesting, especially for environmental and medical applications. For instance, the lower limit of human ammonia perception by smell is close to 50 ppm. However, even below this limit, ammonia causes serious problems to the health like irritation of the respiratory system, skin and eyes [56]. Moreover, measuring ammonia levels in exhaled air can be a fast diagnostic method for patients with Helicobacter pylori bacterial stomach infection [57].

Environmental humidity is an important factor that influences the performance of metal oxide gas sensors. We tested the effect of moisture on the conductance and response of fully inkjet printed SnO2 sensor. The water vapor behaves like a reducing gas and causes an increase in the conductivity of the sensor [58]. The effect of humidity was studied on the conductance and response of the sensor when exposed to different concentrations between 10 and 100 ppm of C2H5OH, NH3 and CO, as shown in Fig. 16. Although the sensor responses to the 3 gases under wet air are low compared to the responses found under dry air, these responses remain reversible and dependent on the concentrations of the injected gas. It should also be noted that sensors tested under wet air are less sensitive than those tested under dry air. Indeed, the adsorption of water on the surface of SnO2 causes an increase in the conductance baseline of the sensor and thus leads to a decrease in response and sensitivity. In addition, the competitive adsorption of water molecules leads to a decrease in the chemisorption of oxygen species on the SnO2 surface due to the decrease in the active sites responsible for gas sensing phenomena [59].

Like other metal oxides, SnO2 sensors exhibit lack of selectivity. In order to improve the selectivity, two strategies are considered:

 The first involves a surface functionalization of the SnO2 sensitive layer using an organic functional groups having different polarities, which leads to a change of the sensor response toward specific gases like ammonia [26].  The second implies the incorporation or the doping of atoms into the SnO2 structure [60]. This method is consistent with the inkjet printing as it can be done during the sol preparation by sol-gel method.

Conclusion

In summary, a flexible SnO2 gas sensor was successfully deviced using inkjet printing technology. SnO2based sol was synthetized by aqueous sol-gel method, and then transformed into ink with suitable rheological properties. SnO2 could be crystallized at 350 °C, which is entirely consistent with the use of polyimide substrate. Platinum was chosen as the base material for the sensor heater. Platinum printing was optimized and Kelvin structures were printed on the polyimide to perform electrical calibrations for the heater simulation and design. Platinum heater and gold electrodes were printed on the top side of Upilex substrate, separated by an insulating layer of printed polyimide. Then, 2 layers of SnO2 were printed over the gold electrodes and annealed at 350 °C for 1 h. The proper operation of the prepared gas sensor was validated under exposure to different concentrations of C2H5OH, NH3 and CO gases, under dry and wet air, at 300 °C.

Table 1

 1 Properties of commercial inks at 20 °C

		Pt ink	Au ink	PI ink	SnO2 ink
		Akko Lab, LCC	NPG-J	PI 6643-001	Kassem et al. [39]
	Viscosity [mPa.s]	15 -25	7	20	10
	Surface tension [mN/m]	30 -40	25	30	32
	Solid cotenant [% wt.]	5	53	30	5
	Particles size [nm]	6	7	-	6
					Ethylene glycol
	Solvents	Ethylene glycol	Petroleum hydrocarbon	γ-Butyrolactone Diethyleneglycol ethylmethyl ether	Ethanol Glycerol 2isopropoxyethano
					l

Table 2

 2 Properties of polyimide and platinum

	Properties	Polyimide [40]	Platinum [41]
	Heat capacity [J/kg.K]	1130	133
	Thermal conductivity [W/m.K]	0,29	71
	Density [kg/m 3 ]	1300	21450
	Electrical resistivity [µΩ.cm]	n.a.	35. (1 + 0,0029. ∆𝑇)
	Thickness	50 µm for Upilex, and 4 µm for printed polyimide	130 nm

Supporting Information ESI †