Kristian Neovius

MD/PhD Finn Rasmussen
email: finn.rasmussen@ki.se

Johan Sundström

Martin Neovius

Forecast of future premature mortality as a result of trends in obesity and smoking: nationwide cohort simulation study

Keywords: mortality, obesity, overweight, smoking Abbreviations BMI Body mass index HR Hazard ratio CI Confidence interval SD Standard deviation

Introduction

Obesity has increased dramatically in recent decades and is today highly prevalent, with an estimated worldwide prevalence in 2005 of 20.3% in economically developed countries [START_REF] Kelly | Global burden of obesity in 2005 and projections to 2030[END_REF]. In Swedish adolescents, overweight has tripled, obesity quintupled and morbid obesity increased ten-fold from the period 1969-1974 compared to 2000-2005 [START_REF] Neovius | Shift in the composition of obesity in young adult men in Sweden over a third of a century[END_REF]. With the strong association between obesity status and subsequent cardiovascular disease [START_REF] Falkstedt | Body mass index in late adolescence and its association with coronary heart disease and stroke in middle age among Swedish men[END_REF], disability pension [4 ,5] and premature death [6 ,7 ,8], it will likely be several decades until the full burden of recent obesity trends on healthcare and society is manifested.

In contrast to the trends of increasing obesity, the prevalence of smoking, a factor also strongly associated with cardiovascular disease, disability pension [START_REF] Neovius | The combined effects of overweight and smoking in late adolescence on subsequent disability pension: a nationwide cohort study[END_REF] and premature death [START_REF] Doll | Mortality in relation to smoking: 50 years' observations on male British doctors[END_REF], has decreased substantially during the last decades in several industrialized countries [11 ,12 ,13].

Smoking and obesity in adolescents, arguably the two most important modifiable risk factors in this age group, independently increase premature mortality risk, but the combined effects of these simultaneous opposite trends on life expectancy is unknown.

The aim of this study was to estimate the net effect on future premature deaths due to trends in obesity and smoking in adolescent Swedish men.

Material and methods

A Markov model [START_REF] Sonnenberg | Markov models in medical decision making: a practical guide[END_REF] with Monte Carlo simulation was developed to simulate the effects on expected number of premature deaths between age 19 and 56y based on inputs of obesity and smoking status from a nationwide cohort study of Swedish men. The time horizon of the model was chosen in order to avoid extrapolation outside the observed data. The model was constituted by two health states ("Alive" and "Dead") with possible annual transitions from "Alive" to "Dead". Cycles of one year until cohort participants had reached age 56y or died were utilized. In each cycle, individuals of the cohort were subjected to time-dependent risks of death, based on obesity and smoking status at age 19y, and age at cycle (i.e. 19-56y, with increasing risk of death with older age). Simulations were run for 10 000 iterations, and empirical 95% confidence intervals were retrieved from the sorted simulation results as the 250 th and 9 750 th values.

Input data

The model input data consisted of prevalence of smoking and obesity, as well as obesity-, smoking-, and age-specific annual risk of death. The data were retrieved from the nationwide Swedish Military Service Conscription Register, which has been presented in greater detail elsewhere [START_REF] Andreasson | Alcohol and mortality among young men: longitudinal study of Swedish conscripts[END_REF]. Briefly, military conscription was mandatory for all Swedish men during the study period, and is normally completed during the last year of high-school (age 18-19y). At conscription the young men go through a two-day test including medical examinations.

Based on the unique personal identification number assigned to each Swedish citizen, a dataset was created by register linkage between the Military Service Conscription Register, Register of the Total Population, and Population and Housing Censuses. In order to be included in the current study, subjects were required to have performed their military conscription tests between 16-20y of age, and to have data recorded on measured weight and height as well as self-reported smoking habits at conscription (i.e. complete case analysis was used). The age restriction was used to keep the sample homogenous by age.

Risk equation input data

The risk equations employed in the model were derived from the cohort performing military conscription tests in 1969-70. A proportional hazards regression model using the parametric Weibull distribution [START_REF] Carroll | On the use and utility of the Weibull model in the analysis of survival data[END_REF] was used to model absolute annual risks of premature death for each one-year cycle in the simulation model. Participants were censored at emigration, death or end of follow-up, whichever came first. Except for the predictors smoking and obesity status, the models were adjusted for household socio-economic index (white collar, blue collar, selfemployed, other). The information on socio-economic index was extracted from the Population and Housing Census 1970 and thus represents household socio-economic index during childhood. Data on alcohol consumption and highest attained education were also available, but due to high degree of collinearity with socio-economic position these variables were not used simultaneously. The estimates for BMI-and smoking categories were unaffected when these covariates were also used in the regression model.

Body mass index (BMI) was used as proxy for fatness. It was defined as weight in light clothing (kg) by height (m) squared, without shoes, and categorized into the World Health Organizationdefined BMI categories underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9) and obesity (≥30) [START_REF] Who | Obesity: preventing and managing the global epidemic. Report of a WHO consultation[END_REF]. To minimize errors of misclassifications, exclusion limits were applied for height (≤150 or ≥210 cm), weight (≤40 or ≥150 kg) and BMI (≤15 or ≥60 kg/m 2 ). Smoking status was categorized into three levels (non-smoker/1-10/>10 daily cigarettes) using a questionnaire.

Risk factor prevalence input data

Prevalence of obesity and smoking were retrieved from the 1969-70 (n=45 920) and 2004-05 (n=56 411) conscription cohorts. To predict distribution over obesity status categories for future cohorts, data from each individual year from 1969 to 2005 (n = 1.6 million) were used. The predictions were performed with logistic regression using year of military conscription testing as predictor. The future prevalence was then calculated according to the following formula:

prevalence = ) * ( ) * ( 1 year year e e
where α is the intercept on the log odds scale and β is the log odds ratio for the obesity status category, as has been done previously [START_REF] Neovius | Shift in the composition of obesity in young adult men in Sweden over a third of a century[END_REF].

Regarding smoking, individual participant data were obtained from the Military Service Conscription Register for the 1969-70 cohort. Thereafter information on smoking at the time of military conscription was registered and reported by the Swedish Federation of Alcohol and Drug Awareness [START_REF] Can | Drug habits among conscripts 2005 (Mönstrandes drogvanor[END_REF], but not made available for register linkage for research purposes. Therefore smoking prevalence data for 2004-05 were taken from the Swedish Federation of Alcohol and Drug Awareness. "Daily smoking" was set as ">10 cigarettes/day", and "Practically daily smoking" and "More seldom smoking" as "1-10 cigarettes/day".

Simulations

The model was run for hypothetical cohorts of 50 000 men in order to enable direct comparisons between cohorts using several input scenarios. Firstly, the expected number of premature deaths from 19 to 56y was estimated for the cohort from 2004-05 and compared with the 1969-70 cohort. Secondly, the isolated effects of changing trends in smoking (smoking=2004-05, BMI distribution=1969-70) or BMI distribution (smoking=1969-70, BMI distribution=2004-05) were estimated. Thirdly, a cohort consisting exclusively of normal weight non-smokers was simulated.

Finally, several future cohorts were simulated under the assumption of unaltered smoking habits since 2004-05 but a hypothetical continuing increase of overweight and obesity according to the prevalence projections.

Results

Of 50 398 men in the cohort performing military conscription in 1969-70, 45 920 met the inclusion criteria [START_REF] Neovius | Combined effects of overweight and smoking in late adolescence on subsequent mortality: nationwide cohort study[END_REF]. The participant characteristics are shown in Table 1. During 1.7 million person-years of observation (median 37.7y) 2 897 individuals died. Overweight and obesity compared to normal weight, and smoking compared to non-smoking, were significantly associated with increased risk of premature death, while underweight was not (Table 2). The relative risks estimated by use of the parametric survival models employed in the simulation were almost identical to the estimates from the semi-parametric Cox models used in a previous study on the same cohort (data not shown) [START_REF] Neovius | Combined effects of overweight and smoking in late adolescence on subsequent mortality: nationwide cohort study[END_REF]. The data followed a Weibull distribution and no violations of the proportional hazards assumption were detected when investigated graphically.

During the period from 1969-70 to 2004-05, overweight almost tripled and obesity increased seven-fold among young Swedish men, while smoking more than halved from 59% to 23% (Table 2).

Model fit

Using the smoking and obesity prevalence in 1969-70 as inputs for the risk equations, the simulation model was found to fit well to the observed data, producing only small discrepancies in number of deaths during a time horizon of 37 years and a cohort size of 45 920 individuals, as in the dataset (Appendix, Table A1). The model underestimated the total number of deaths by 1.2% (2 861 vs. 2 897) compared to reality.

Expected outcome for 2004-05 and 1969-70 cohorts

When using the model to simulate cohorts of 50 000 men from 19 to 56y, a total of 3 116 (CI 95% 2 941, 3 301) deaths were expected to occur for the 1969-70 cohort (Figure 1). Despite higher prevalence of overweight and obesity in the 2004-05 cohort, 14% (6, 21%) fewer deaths were expected over the same time horizon due to decreases in smoking.

Isolated effects of smoking and BMI trends

Keeping all other parameters constant and only altering the BMI distribution in the 1969-70 cohort to that of 2004-05 resulted in a 7% (-1, 16%) increase in the number of expected deaths (Figure 1, "Effect of altered obesity"). The corresponding effect of only altering smoking status patterns predicted 20% (15, 25%) fewer deaths (Figure 1, "Effect of altered smoking").

Potential for preventive efforts

If all 19y old Swedish men had been normal weight non-smokers, and all other risk factors being similar, 33% (28, 38%) fewer deaths would be expected until age 56y compared with the cohort of 1969-70 (Figure 1, "Normal weight non-smokers"). Compared to the observed cohort characteristics in 2004-05, 22% (17, 28%) fewer deaths would be expected.

Future mortality

For the hypothetical case of an unabated obesity trend, the resulting number of deaths from age 19y to 56y for future cohorts is shown in Figure 2. The calculations were based on the assumption of unaltered smoking prevalence at conscription from 2004-05, but increasing overweight and obesity according to extrapolation of the trends observed in the Swedish military conscription data from 1969 to 2005. It was estimated that the survival benefits associated with the successful decrease in smoking during the last four decades would be offset by the effect of increasing overweight and obesity within three decades. In 2030, a young Swedish male cohort was predicted to consist of 2.6% underweight, 48.6% normal weight, 29.7% overweight and 19.1% obese individuals, and to experience 3 140 (2 860, 3 440) deaths from age 19y to 56y, compared to 3 116 (2 940, 3 300) for the lean but heavily smoking cohort of 1969-70 (Figure 2).

Discussion

By employing a simulation model, we estimated the effects of changes in obesity and smoking prevalence on premature death during the main part of productive age in Swedish men. Male cohorts today were predicted to enjoy reductions in premature mortality from age 19y to 56y compared to male cohorts born 35 years earlier, independent of health care improvements.

Despite substantial increases in adolescent overweight and obesity the past 35 years, their effect on future premature death was predicted to be offset by health benefits from concurrent decreases in smoking. However, approximately one third of the survival gains from reduced smoking was estimated to be counteracted by the increases in overweight and obesity.

Based on observed declining mortality, some public health officials in Sweden have argued that either the "alarms of an obesity bomb" are grossly exaggerated, or that the detrimental health effects of obesity have been counteracted by a simultaneous improvement of another risk factor [START_REF] Asplund | Alarms of obesity bomb grossly exaggerated (Larmen om fetmabomb är kraftigt överdrivna)[END_REF]. The present study supports the latter. Although obesity was modeled with an increased risk for premature death similar to that of smoking > 10 cigarettes/day, male Swedish cohorts of today were predicted to experience fewer premature deaths compared to a cohort from the early 1970's, despite a trebled and seven-fold increased prevalence of overweight and obesity, respectively. This is explained by the more than halved smoking prevalence during the last decades, and by the fact that the prevalence of obesity was very low to begin with. If the obesity trend does not level off but continues along the same trajectory as between 1969 and 2005, and smoking prevalence remains constant, the survival benefits of the last four decades" successful anti-smoking campaigns were estimated to be offset for young adults in 2030. By then about 30% of the young men were predicted to be overweight, and 20% obese. However, this projection is purely hypothetical and should be interpreted with caution, especially since three recent studies indicate that increases in childhood obesity are leveling off in Sweden [20 ,21 ,22]. If the prevalence rise of overweight and obesity will be less steep in reality, it will take longer time to attain the projected prevalences and corresponding number of deaths predicted by the model.

A recent study by Stewart et al. [START_REF] Stewart | Forecasting the effects of obesity and smoking on u.s. Life expectancy[END_REF] investigated the effects of extrapolated increases in obesity and decreases in smoking, as hitherto observed in the US, on life expectancy. They concluded that in the future in the US, the negative effects on health of increasing obesity will outweigh the positive effects of decreasing smoking. Although similar to the secondary analysis of the present study, a comparison is not straightforward since different questions were addressed.

Forecasts generated by simulation models are often afflicted by a high degree of uncertainty, partly due to unknown future factors and partly due to the mix of sources providing input data.

Having all input data from one single data source covering practically the whole Swedish male population was an advantage of the present study.

This study also used data with a long follow-up period and BMI measured early in life, when chronic diseases associated with weight loss are rare, thus minimizing the effect of reverse causation impacting the risk equations [START_REF] Lawlor | Reverse causality and confounding and the associations of overweight and obesity with mortality[END_REF]. Unfortunately, the data provided information on obesity and smoking status only at the beginning of the follow-up period, with no repeated measurements. Obesity in childhood has been found to be associated with obesity in adulthood [25 ,26], but smoking continuation, cessation or initiation could be harder to predict from smoking habits in late adolescence. It is, however, likely that smoking cessation during follow-up was more common in this cohort than smoking initiation, resulting in some decline in smoking prevalence over time [START_REF] Rasmussen | Importance of smoking habits for longitudinal and age-matched changes in body mass index: a cohort study of Swedish men and women[END_REF]. If this is the case, the association between smoking and mortality would be somewhat underestimated. Also, this study analyzed premature mortality, defined as deaths before age 56y. A large proportion of deaths occur after this age, and if the relation between the risk factors smoking and obesity in terms of mortality risk changes with age, this could have an impact on the results of this study. Smoking is likely to incur increased mortality risk also in higher ages, but there are indications that this is not always the case for overweight [START_REF] Walter | Mortality and disability: the effect of overweight and obesity[END_REF]. It is therefore possible that the truncation of the analysis at age 56y could imply an overestimation of the obesity related deaths in relation to smoking related deaths.

Another limitation of the present study was the assumption that each predictor"s associated risk for death has remained unaltered over time. A heavily smoking, obese individual at conscription today was assumed to be subjected to the same risk of death as in the early 1970"s. This assumption is not totally valid as risks have changed over time, e.g. due to more effective treatments, which would lead to an overestimated number of deaths for present cohorts. It is also possible that risks have changed as the excess weight of obese individuals today may be attributable to obesogenic environmental influences to a greater extent. However, it is not clear how it would affect the obesity related mortality risk whether an individual"s obesity has its origin primarily in genetic or environmental factors. On the other hand, the mortality in the later cohorts could be underestimated if the observed decrease in prevalence of smoking is in fact due to a shift towards usage of moist snuff (snus) rather than becoming tobacco-free [START_REF] Boffetta | Use of smokeless tobacco and risk of myocardial infarction and stroke: systematic review with meta-analysis[END_REF], although it has been suggested that health gains can still be realized by current smokers switching to snus [START_REF] Gartner | Assessment of Swedish snus for tobacco harm reduction: an epidemiological modelling study[END_REF]. It is also probable that current cohorts quit smoking at a lower rate than older cohorts, while gaining weight at a higher rate. Lacking longitudinal data on these parameters, we assumed these rates to be unchanged over time.

In conclusion, despite large increases in overweight and obesity during the last four decades, a decline in premature deaths among Swedish males is expected due to health gains from reduced smoking. However, approximately a third of the survival gains from reduced smoking were estimated to be counteracted by increases in overweight and obesity. Data are mean (standard deviation) for continuous variables. Underweight (BMI<18.5), normal weight (BMI 18.5-24.9), overweight (BMI 25-29.9) obesity (BMI≥30).

Missing data: socio-economic index n=18.

  Statistical analyses were conducted in Stata (version 10.0, College Station, Texas, USA), while the simulation model was built in Microsoft Excel 2003 with Visual Basic for Applications.

Figure 1 Figure 2

 12 Figure 1 Total number of expected deaths in simulated cohorts of 50 000 men during

Figure 2

 2 Figure 2

Table 1

 1 Baseline characteristics of the cohort performing conscription tests in 1969-70.

		Underweight Normal weight Overweight Obesity	Total
	n (%)	6 325 (13.8%) 36 605 (79.7%) 2 623 (5.7%) 367 (0.8%)	45 920 (100%)
	Age (years)	18.7 (0.5)	18.7 (0.5)	18.7 (0.5)	18.7 (0.5)	18.7 (0.5)
	Anthropometry
	Height (cm)	178.6 (6.5)	178.1 (6.3)	177.7 (6.4) 177.2 (6.7)	178.1 (6.3)
	Weight (kg)	56.3 (4.6)	66.7 (6.8)	84.1 (7.4) 101.1 (10.0)	66.6 (9.2)
	BMI (kg/m 2 )	17.6 (0.7)	21.0 (1.6)	26.6 (1.3)	32.2 (2.0)	21.0 (2.6)
	Smoking
	Non-smoker	2 325 (37%)	15 430 (42%) 1 107 (42%) 145 (40%)	19 007 (41%)
	1-10 cig/day	2 286 (36%)	11 771 (32%)	711 (27%) 84 (23%)	14 852 (32%)
	>10 cig/day	1 714 (27%)	9 404 (26%)	805 (31%) 138 (38%)	12 061 (26%)
	Socio-economic index
	White collar	2 953 (47%)	15 549 (42%)	913 (35%) 99 (27%)	19 514 (43%)
	Blue collar	2 308 (37%)	13 723 (38%) 1 198 (46%) 198 (54%)	17 427 (38%)
	Self-employed	842 (13%)	6 229 (17%)	448 (17%) 63 (17%)	7 582 (17%)
	Other	220 (3%)	1 088 (3%)	64 (2%)	7 (2%)	1 379 (3%)

Conflict of interest

None declared

Appendix

Table A1. Mean number of deaths according to the simulation model during 37 years of followup in a hypothetical cohort of 45 920 participants compared to observed data (n=45 920, median follow-up = 37.7 years). a Estimated by use of a proportional hazards regression model employing a Weibull distribution b Models included obesity and smoking status as main predictors, and were further adjusted for household socio-economic index

BMI/Smoking


Daniel Pape

Pascal Perrier

Susanne Fuchs

Sonia Kandel

Does physical realism of articulatory modeling improve the perception of synthetic speech?

INTRODUCTION

The role of the precise time variation of formants (henceforth formant trajectories) in the perception of speech has been at the center of many recurrent debates, essentially along two opposing theories. For the first theory, formant trajectories do not contain any relevant phonetic information by themselves. In the best case, when properties other than the starting and ending points matter [START_REF] Sussman | Linear correlates in the speech signal: The orderly output constraint[END_REF], the trajectories would only be the vector of information related to the intended target [START_REF] Lindblom | On the role of formant transitions in vowel recognition[END_REF]. For the second theory, the temporal characteristics of the transition would be, independently of any target, the relevant perceptual information. Thus, formant trajectories in all their details would be the object of perception ( [START_REF] Strange | Dynamic specification of coarticulated wovels[END_REF]). Evidence supporting this theory has been recently provided by the perturbation study carried out by Cai et al [START_REF] Cai | A system for online dynamic perturbation of formant trajectories and results from perturbations of the Mandarin triphthong /iau[END_REF]. In presence of a real-time perturbation of their perceived formant trajectory, Chinese speakers of Mandarin producing the triphthong /iau/ tended to react by modifying their articulation in order to generate the usual trajectory. A possible way to understand the different findings made in the context of both main theoretical streams is inspired by the work of Viviani and colleagues (i.e. [START_REF] Viviani | Biological movements look uniform: evidence of motorperceptual interactions[END_REF]) on visual perception and the identification of hand gestures. These authors have found evidence that this perceptual task could be strongly determined by the knowledge that human beings have of the physical characteristics of their own movements. In this context, it can be assumed that formant transitions that are not compatible with the physical mechanisms underlying speech production should be perceived as being incorrect. This could explain compensation strategies such as those observed by Cai et al. In normal speech, the physical mechanisms of speech production could determine the patterns of the spectro-temporal variation between discrete targets, and these patterns would in turn become "the" physical objects associated with the perception of sound sequences. This article presents the first step in a process of evaluating the potential impact of the physical properties of the articulators on the perception of speech. Perceptual tests of synthetic stimuli generated with different models incorporating various degrees of physical complexity of these articulators have been run and analyzed.

SUBJECTS AND METHOD Subjects

Twenty-three subjects (16 men, 7 women, aged between 25 and 50 years) participated in the experiment. None of them was aware of the methods used to generate synthetic speech. All are native speakers of French. None of them has reported any speech or hearing problem. Method Vowel1-Vowel2-Vowel1 (V1V2V1) and Vowel1-/g/-Vowel1 (V1CV1) acoustic stimuli were synthesized from sagittal vocal tract shapes, via the generation of an area function and the use of a Kelly-Lochbaum acoustic model ( [START_REF] Story | A parametric model of the vocal tract area function for vowel and consonant simulation[END_REF]). In all cases, the fundamental frequency was kept at 110Hz. The three different models used to generate the stimuli differed in how the transitions between target vocal tract shapes are generated. In the first class of stimuli (Mod1) transitions were obtained with a two-dimensional biomechanical model of the vocal tract [START_REF] Perrier | Influences of tongue biomechanics on speech movements during the production of velar stop consonants: a modeling study[END_REF] controlled on a target basis. Motor commands are specified for each elementary sound and movements are generated by a time shift of the target commands at a constant rate [START_REF] Perrier | Influences of tongue biomechanics on speech movements during the production of velar stop consonants: a modeling study[END_REF]. For Mod2 and Mod3 stimuli, the target vocal tract shapes are the ones actually reached in Mod1 stimuli for V1, V2 and C. The timing was the same for all classes of stimuli. For Mod2 and Mod3 the transitions between the target shapes were computed along straight paths. These two models differ in the time course of the displacements along these path. In model 2 (Mod2), the displacement has a constant speed. In model 3 (Mod3), speed is an arc of sinusoid in line of the kinematic properties of an undamped second order system. In sum, Mod1 stimuli correspond to the most realistic physical model, Mod2 stimuli are less realistic and Mod3 are physically the least realistic ones. For another perception test, degraded stimuli V1V2V1 were also generated by replacing the central part of V2 by a silence with the same duration according to the paradigm of the silent centers [START_REF] Strange | Dynamic specification of coarticulated wovels[END_REF]. All perception tests were conducted in an anechoic chamber at GIPSA-lab. The subjects were first asked to assess the silent center stimuli. The task of the listeners was to identify V2. A choice was given between 5 possible vowel answers. Then subjects were asked to assess the naturalness of the non-degraded stimuli (V1V2V1 first and V1CV1 afterwards). All experiments were forced choice. The reaction time (RT henceforth) was measured. The purpose of the statistical analysis is to see if, for all subjects taken together, there are differences between the Mod1, Mod2 and Mod3 stimuli. We used the generalized linear model with mixed effects. The class of stimuli (Mod1, Mod2, Mod3) was chosen as the fixed factor.

RESULTS

These tests assessing the potential link between the degree of realism of our models and the perceived naturalness of the synthesis did not show any significant results. As concerns the silent centers stimuli, with respect to the percentage of correct responses given for all the subjects, no difference between the models can be shown. However, various studies have shown that the listeners' Reaction Time should be considered when analyzing these data. Indeed, long Reaction Times may be related to the involvement of high-level cognitive processing, which are beyond the scope of our study, namely the auditory perception itself. Hence, in a second analysis, only the responses (all subjects taken together) corresponding to short Reaction Times in the [1000ms 2000ms] interval were taken into account. Under this condition, the identification of the target vowel V2 in the silent center experiment was significantly better for the Mod1 stimuli than for the Mod2 stimuli. A trend was also observed suggesting a better identification for Mod3 stimuli as compared to Mod2 stimuli. The differences between Mod1 and Mod3 are not significant.

CONCLUSIONS

As concerns the perceptual rating of the naturalness of the three classes of stimuli, our study did not show any influence of the degree of realism of the underlying articulatory models. This could be due to the fact, that even in the best cases the acoustic synthesis is still far from being equivalent to natural speech (among other because of the constant F0). The silent centers experiments have shown that the stimuli generated with a pure kinematic model, integrating no biomechanical and no dynamic influences allowed a significantly worse identification of the missing central vowel V2. It can be interpreted as evidence for the fact that information related to the dynamics of the vocal tract articulators is used by listeners to recover the intended but not uttered vowel target.


Ralf Winkler

Liang Ma

Pascal Perrier

A model of optimal speech production planning integrating dynamical constraints to achieve appropriate articulatory timing

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

It has been often suggested that similarly to skilled arm or limb movements in humans, the production of speech gestures could be based on an optimal planning in the central nervous system. This planning would use internal representations of the speech production apparatus [START_REF] Bailly | Learning to speak. Sensori-motor control of speech movements[END_REF][START_REF] Guenther | A theoretical investigation of reference frames for the planning of speech movements[END_REF] to determine the motor command patterns allowing to achieve the desired communicative goals with a minimum of effort. In this context major issues are related to the nature of the speech communication goals (targets or spatiotemporal trajectories) and to the definition of minimum of effort (minimum of changes in motor commands, minimum velocity peak, trajectory smoothness, or minimum of jerk...). GEPPETO 1 , the speech production model presented in this paper, has been designed within this general theoretical framework [START_REF] Perrier | Speech planning for V1 C V2 sequences: Influence of the planned sequence[END_REF]. It is based on a motor control model involving optimal planning to shape a biomechanical model of the vocal tract coupled with a harmonic acoustical model of speech production. It will be shown how force constraints can be taken into account in order to achieve appropriate articulatory timing.

THE GEPPETO MODEL

In GEPPETO, speech goals are linked to phonemes and they are specified as 3D ellipsoids in the acoustic space of the first three formants. These ellipsoids are determined by their centres, and by standard deviations along the directions F1, F2, F3. These standard deviations are assumed to account for the acoustic variability that is tolerated around each canonical realisation without any consequence on the auditory perception. The motor control model includes a muscle force generation mechanism based on the Equilibrium-Point hypothesis (Feldman, 1986). The motor control variable is the muscle length threshold above which active muscle force is produced. Movements are generated by shifting the motor control variables at a constant rate of shift between motor targets. In GEPPETO, the motor targets are associated with the speech goals in the acoustic domain. These motor commands are sent to the seven muscles of a 2D biomechanical model of the tongue [START_REF] Perrier | Influences of tongue biomechanics on speech movements during the production of velar stop consonants: A modeling study[END_REF] that is embedded in a 2D description of the vocal tract boundaries. The tongue model deforms and moves as the result of the combination of the influence of the target motor commands, of their timing and of the dynamical properties of the model (muscle forces, tissues elasticity, friction and contacts with external structures). For a given speech sequence, the target motor commands are selected thanks to an optimal planning in which all the targets commands are kept within the closest possible neighbourhood compatible with the perceptual goals and with constraints in terms of a global level of force. More specifically, in GEPPETO, optimal planning is constrained by the double necessity to select motor commands appropriate to the achievement of target spectral patterns inside the target ellipsoids and to ensure that the global level of force remains within a given range during the whole movement. The choice of this range is guided by the intended speaking rate and by perceptual accuracy requirements: for slow speaking rates or low accuracy requirements, a low level of force can be used, but for fast speaking rates and great accuracy a strong level of force is required. To include these constraints in the optimal planning process, two internal forward models have been learned. The first one called "static forward model", accounts for the relation between motor commands and formants. The second one called "dynamical forward model" associates the motor commands with the corresponding global muscle force level. Optimisation is achieved by sequential quadratic programming (SQP), which allows closely mimicking Newton's method (Quasi-Newton) for constrained optimization. In the current state of the model three ranges of global force levels have been defined: low, normal and high. The corresponding force values were derived from a database of 8293 tongue movements simulated with the 2D biomechanical model from the rest position to a randomly chosen target. The value of total muscle force was computed for all targets. Then, the minimum and maximum forces were extracted together with an intermediate value, and they were used as levels for the three force ranges.

RESULTS

This optimal planning process has been tested so far on VCV sequences, starting from the tongue rest position. Optimal motor command patterns were found for each segment of the sequences under the three force constraints. Then, the 2D biomechanical tongue model was used to simulate tongue movements for these 3 force levels and for three timings of the motor commands: slow, normal and fast. Simulations are currently in progress. The first results show that the global muscle force can have a significant impact on the articulatory trajectories, in terms of curvature and in terms of positions actually reached at targets. At a slow speaking rate, changing the global level of force tends to have less consequence on the articulatory positions reached at targets than at high speaking rates, but it seems to have an impact on the trajectory shape.

CONCLUSION

The results support the original hypothesis that applying the appropriate dynamical constraints in the optimal planning process helps dealing with articulatory timing and perceptual accuracy expressed in terms of articulatory positions at targets. In this perspective, contrary to the intrinsic timing theories, time control could be seen as a combination of centrally specified and physically constrained characteristics.

GEPPETO holds for "GEstures shaped by the Physics and by a PErceptually oriented Targets Optimization"


Christophe Perrard
email: christophe.perrard@femto-st.fr

Keywords: robots, external control unit, micro-robot with minimalist computing power, noninvasive medical processing

This paper deals with the control and the synchronisation of a team of micro-robots that performs a non-invasive surgical act into a human body. These micro-robots are very small sized (from ten to some hundred microns). A single unit embeds the minimum of computing power and memory (minimalist electronics) to run a very light program, such as a finite-state machine. The mass effect of the joined micro-robots will allow to achieve the mission through a satisfactory way. The originality of this work being to consider a micro-robot as a disposable unit, which has to be as simple as possible, we need to combine reduced computing power with recent architectures. Some kinds of mission are described and the most promising is detailed. Preliminary simulations support our approach.

Introduction

Today, many efforts are made to improve non-invasive surgery. Mini-robots and micro-robots are one of the answers. On the first hand, mini-robots are centimetric sized. They are powered by batteries for some hours and are able to proceed inside the body. They are already in use in hospitals for example to inspect and to send images of the small bowel [olympus 08] or they are still prototypes under experiments like the biopsy and endoscopic capsule of the VECTOR project ([vector], [ McCaffrey 08], [Duffait 03],…). Unfortunately, these mini-robots can not reach narrow areas, due to their large (!) dimensions. On the other hand, micro-robots (about ten to some hundred microns sized) are still research objects. They promise to self displace ([ Shoham 11], [ Kosa 08], … ) or to act on tumorous cells in an autonomous way [Pouponneau 09], or to treat coronary artery occlusion [Park 08]… The powering of such a kind of robot is still largely discussed by researchers (external energy fields [Martel 07] or boarded micro-storages like micro-fuelcells, micro super-capacitors, or micro-radio-sources [Bogue 10a & b]). However, in most cases, one single micro-robot will not be able to fully achieve most of the missions to perform (tumor marking, drug delivery, tumorous cells killing, 3D imaging,…). Therefore, a team of micro-robots will be necessary.

Technical constraints

Bio-medical environment needs

Constraints related to the use of mini-robots inside a patient's body (surgery, for instance) will have to be taken under consideration. Some of them are :

-Non toxicity and non injury: No physical damage to the body and no chemical combination between the micro-robot and the cells of the body are allowed. Then, non aggressive elements are required and biocompatible materials must be chosen -Eliminability: Guarantee of the retrieval of the whole units of the team after processing, or guarantee of their safe destruction inside the body (by using absorbable materials for instance) -Safety: No wrong operations are allowed -Disposability: No re-use of the units of the team for another mission into another body

Safety needs : monitoring and control of a mission allocated to a team

Due to safety needs, we postulate the largest restriction to the decision autonomy of each unit of the team. Because of the need to monitor the progress of a mission, a man-machine interface has to take place. This device will transmit acknowledgments of progress inside the mission planned by the physician. It will return to the physician the sate of progress of the mission and some other information, like a number of still active microrobots.

Microbotics constraints

Due to their very little dimensions, and due to their single use particularity, micro-robots must be minimalist in terms of mechatronics and in terms of computing capacity (costs savings through design and manufacturing). Therefore, only a few sensors and actuators can be embedded.

Embedded energy constraints

Once more, due to the very little dimensions of a micro-robot , the amount of embedded energy is very low. Then, each energy consumption has to be limited as much as possible.

Propositions to control a team of minimalist micro-robots for medical applications

The proposed work is the summary of preliminary research to control a team of micro-robots and to define their synchronization, as well as their control architecture. The originality of this work being:

to consider a micro-robot as a disposable unit, which has to be as simple as possible ; then we need to combine reduced computing power with recent architectures, -to consider the achievement of the mission through a satisfactory way due to the mass effect of the joined micro-robots.

Design choices

Architecture choices

We propose to coordinate the team of micro-robots by using a centralized controller. However, such a centralized controller will not be able to run each program of each micro-robot of the team. The communication link between the centralized computer and the units of the team would be overloaded. Then, each unit will have to run its own program. The control architecture must thus be hybrid between fully centralized and fully distributed.

Due to these constraints, a very simple data exchange system with the man-machine interface has to be used, in order to authorize the progress through each important step of the embedded program.

Micro-robots design choices

On the first hand, a micro-robot is a disposable unit due to single-use considerations and due to its loss eventuality, during the mission execution. On the second hand, a micro-robot has very small size. Then microrobots have limited robotic resources.

Mechanically, each unit is unable to displace or direct itself (displacement is the result of body fluidic moves, in order to save motion actuators, save control surfaces actuators and save energy). It can perceive one particular characteristic (i.e. one single sensor) of the environment in the human body, and it can execute one or two single actions (like delivering a medicine, grip on a surface in order to stop its move). These actions don't need a continuous flow of energy to run. A single pulse is enough.

From the micro-robot controller point of view, a single unit embeds the minimum of computing power and memory (minimalist electronics) to run a very light program, such as a finite-state machine (however, this program will be able to run different parallel tasks).

From the communications point of view, due to extreme simplicity requirements and due to energy consumption savings of each unit of the team, we forbid units to specifically communicate with each other. Then, microrobots aren't distinguished by a specific address and no addressed message will be sent. We propose to favour general diffusion of the messages. Then, a unit is able to receive and to distinguish one or two external signals, and to send one unique signal. This former one would be a continuous one if needed.

External controller choices and relationship between the main controller and the team of microrobots

We propose to use a centralized computer to synchronize the actions of the micro-robot of the team and to provide the man-machine interface. However, due to the possible great number of units of the team, full centralized control of the team isn't considered as relevant [Mataric 94].

When the received signal from the micro-robots is above a given threshold, it means that enough micro-robots are in the situation to perform the mission (the controller does not know which unit is ready or not, but the only number of them). Then the controller sends another signal to synchronize the execution. Only the micro-robots that are ready to perform it execute the mission. The other micro-robots are considered as lost. This loss can be temporary.

When the mission is done, all the micro-robots are assumed to be eliminated through natural ways. Some units can be lost through this way before they are ready to perform the mission.

Position of the proposed work

In this proposal, we are dealing with a kind of smart medicine, that is able to process only where it is needed, and when it is needed, instead to a common one, that processes the whole body (including the sane parts of the body) without coordination.

A typology of the different kind of possible control architectures is given by Jérémi Gancet [ Gancet 05]. The architecture we propose fits his level 2 of decisional autonomy (Supervision and execution of a task is decentralized while coordination, task planning and task allocation are centralized).

Lounis Adouane [Adouane 05] proposed a work using the same designation (the control of a swarm of minimalist micro-robots), but it was based on intelligent, self-directed and high level communicative mini (and not micro) robots. More present works from the same domain [ Benzerrouk 11] does not correspond to ours proposal for the same reasons. Despite the possible great number of micro-robots of the proposed team, we don't deal with swarm robotics because no particular behaviour will emerge and since we do not look for emergent intelligence [ Sahin 05].

Swarms robotics works compares their improvements to a reference of a non-communicating swarm of robots [Hsieh 09]. Then, our proposal can be considered as such a kind of basis.

Some mission examples

In order to perform different missions, the team of micro-robot has to be adapted in terms of design. However, It is not a good solution to modularize the parts of a micro-robot, in order to let the physician to configure each unit of the team because of their very little dimensions. In such a case, micro-assembly will occur, and will be extremely difficult to achieve. Then, if modularization occurs, it will only be useful for the manufacturer. However, the team of micro-robots can be an heterogeneous one. Some elements of the team can be specialized, in order to minimize the complexity of each unit while increasing the functionalities of the whole team.

The surveillance mission

This kind of mission is the simplest one. Indeed, a team of micro-robots is wandering inside the human body. Its mission is to signal the discovery of a particular kind of cell. Each time this happens, the concerned micro-robot emits a brief signal. This allows to know if a particular kind of cell is present inside the body. The frequency of the received signals is an indication of the number of detected cells. This mission can last as long as some embedded energy is left and as long as micro-robots are present inside the body (elimination). Of course, the frequency of the received signal depends on the the number of remaining units too.

If the team can be resident into the body, this kind of mission can be useful to keep a watch on people at risk.

The spotting mission

This second kind of mission is based on the previous one. A team of micro-robots is wandering inside the human body. Its mission is to continuously signal the discovery of a particular kind of cell and to locate it. This last point is an improvement of the previous mission. If this occurs, the micro-robot clamps itself onto an obstacle (intestine or artery wall) as soon as possible and emits the signal. The main external controller has the capacity to determine where the emission of a micro-robot comes from. This allows to know if a particular kind of cell is present into the body, where these cells are and how much they are. This kind of mission has the same purpose as the scintigraphy but avoids radio-emissions.

The drug delivery mission

This kind of mission is based on the previous one. However, the micro-robots have the additional ability to deliver a load of medicine. This is described in details in the next paragraph.

A mission study: the drug delivery case

The drug delivery mission is a typical case. A team of micro-robots is injected inside the body by the physician. This team will spread inside the body according to the different fluidic moves. The body contains obstacles, onto which a micro-robot can bounce or clamp on as needed. Some elimination zones catch every micro-robot that reaches them. Such a micro-robot is definitively lost. An interest zone bounds the target, such as a tumor. area, where a particular obstacle lies (this obstacle is the target, like a tumor). This interest zone can be spotted by extra-body means, such as ultra-sound. After a sufficient waiting time, some units of the team are clamped on the obstacle to process, inside the interest zone, while other units are lost and some others are still moving.

The physician knows how many units are ready to process to the drug delivery through the power of the added emitted signal of the micro-robots already in place (ready units).

If the power of this signal is enough, the mission can be performed (the physician allows the next steps of the mission to the ready units).

If the power of the returned signal is too low, two options can be taken under consideration:

the drug delivery can be delayed, in order to allow other micro-robots to reach the interest zone, and improve the number of units able to deliver the drug another team of micro-robot can be injected, in order to improve the number of ready units probability Figure 1 describes the main controller and its attached devices, in order to perform the drug delivery mission, according to the above description. Interactions between the physician and the swarm (in a successful mission case) Figure 3 describes the sensors and actuators that are required by each unit of the team, in order to perform the drug delivery mission. Inputs of each micro-robot controller are:

• a sensor to inform the controller if the micro-robot is inside an interest zone (inside an interest zone)

• a sensor to detect the bump onto an obstacle (obstacle detection)

• a sensor to detect if the micro-robot is into an elimination zone (inside an elimination zone)

• a sensor to receive the drug delivery authorization (drug delivery signal)

• a sensor to receive the end of task signal (end of task signal) Outputs of each micro-robot controller are:

• an actuator to enable and disable the clamping system (activate clamps)

• an actuator to drop the medicine load (deliver drug)

• an emitter to send the ready signal to the main controller (signal ready) Each input and output information is considered as a digital one. Each output has one single state of stability. The actuator that drops the load of medicine is a "one shot" system. 1. when activated, the controller waits the detection of an interest zone (step 0) 2. the controller waits an obstacle into the interest zone (step 1) 3. the controller commands the micro-robot to clamp onto the obstacle, signals the micro-robot is ready and waits the drug delivery authorization from the main controller (step 2) 4. still ordering the clamping, the controller orders the delivery of the drug load of the micro-robot , and waits the signal of end of the task (step 3). Then, each micro-robot in such situation had enough time to accomplish its part of the mission. 5. the controller allows to de-clamp from the obstacle, in order the micro-robot restarts wandering into the body (step 4). The controller waits the elimination zone detection while reporting to the main controller the mission was successful (reactivation of the ready signal) 6. the micro-robot is eliminated (step 5) This textual description concerns the case the mission of the micro-robot is successful. Any other case is • The green curve represents the number of micro-robots that reach the target, without elimination consideration. A maximum of n 1 <n 0 units succeed, because some of them are definitely lost (they are wandering in the wrong direction, for instance) • The red curve represents the number of micro-robots still wandering when including an elimination zone, without target consideration. A minimum of n 2 > 0 may be reached, due to the loose phenomenon. • The blue curve represents the number of micro-robots that reach the target when considering an elimination zone. It isn't the mere sum of the two previous curves. This is due to the clamping action that fixes the micro-robots and then avoids the elimination phenomenon. Of course, the real evolution of these curves depends on the real conditions of the experiments (distance of injection point to the target, number of micro-robots of the team, direction and intensity of the body flows). However, our simulations validate the main shape of these curves.

Conclusions

We propose to investigate the control of a team of micro-robots that operates inside a human body. This approach deals with a kind of smart medicine that only acts where and when it is needed. This is done under the acknowledgement of the physician itself, who supervises and allows the different steps of the processing of a particular kind of mission. Today, such micro-robots do not exist. Energy batteries, communication modules, among others, are not ready and the main controller itself is not reality (only coordinated programs exist into the simulator). However, in order to avoid most technical problems, the design of such micro-robots is chosen as simple as possible. In the same way, the embedded controller of each micro-robot is very simple, in order to use micro-electronic batch processes. However, micro-electronic will not be the only investigated way. Maybe, biological substrates will be useful.

Our further works will concern:

• First, some improvements of our simulator have to be developed. For instance, we need to use a more accurate model of the human body, with the fluidic moves that exist inside it, and the possible interactions between the body and the micro-robots. • Secondly, we will imagine some other missions that can be allocated to the team of micro-robots. May be, this team may be more efficient if it is a heterogeneous one, with some specialized units. Then, what are the functions? Whose micro-robots will support them? How many micro-robots do we need to perform efficiently the mission? Then, will the team be more efficient if micro-robots can aggregate together in order to share their different skills and functions?

Figure 1 .:

 1 Figure 1. : External main controller and its equipments (drug delivery mission case)Figure2describes the signal exchanges between the physician (through the main controller) and the swarm of micro-robots, in a successful drug delivery mission case.

Figure 2 .

 2 Figure2. Interactions between the physician and the swarm (in a successful mission case) Figure3describes the sensors and actuators that are required by each unit of the team, in order to perform the drug delivery mission. Inputs of each micro-robot controller are:• a sensor to inform the controller if the micro-robot is inside an interest zone (inside an interest zone)• a sensor to detect the bump onto an obstacle (obstacle detection)• a sensor to detect if the micro-robot is into an elimination zone (inside an elimination zone)• a sensor to receive the drug delivery authorization (drug delivery signal)• a sensor to receive the end of task signal (end of task signal) Outputs of each micro-robot controller are:• an actuator to enable and disable the clamping system (activate clamps)• an actuator to drop the medicine load (deliver drug)• an emitter to send the ready signal to the main controller (signal ready) Each input and output information is considered as a digital one. Each output has one single state of stability. The actuator that drops the load of medicine is a "one shot" system.

Figure 3 .

 3 Figure 3. Micro-robot controller input-output (drug delivery mission case) Each micro-robot embeds a simple finite-state machine (represented in Figure 4 with the Grafcet notation [David 92]) consisting of:1. when activated, the controller waits the detection of an interest zone (step 0) 2. the controller waits an obstacle into the interest zone (step 1) 3. the controller commands the micro-robot to clamp onto the obstacle, signals the micro-robot is ready and waits the drug delivery authorization from the main controller (step 2) 4. still ordering the clamping, the controller orders the delivery of the drug load of the micro-robot , and waits the signal of end of the task (step 3). Then, each micro-robot in such situation had enough time to accomplish its part of the mission. 5. the controller allows to de-clamp from the obstacle, in order the micro-robot restarts wandering into the body (step 4). The controller waits the elimination zone detection while reporting to the main controller the mission was successful (reactivation of the ready signal) 6. the micro-robot is eliminated (step 5) This textual description concerns the case the mission of the micro-robot is successful. Any other case is

Figure 4 .

 4 Figure 4. Micro-robot controller Grafcet program (drug delivery mission case)

Figure 5 .

 5 Figure 5. Modelling of the environment of a « drug delivery mission » issued of our C++ program

Figure 6 .

 6 Figure 6. Evolution of the number of micro-robots of the team


Jérôme Dejeu

Abdeslam Et Taouil

Patrick Rougeot

Sophie Lakard

Fabrice Lallemand

Boris Lakard
email: boris.lakard@univ-fcomte.fr

Morphological and adhesive properties of polypyrrole films synthesized by sonoelectrochemical technique

Keywords: Polypyrrole, Sonochemistry, Electrochemistry, Atomic Force Microscopy, Adhesion force or pull-off force

This paper presents sonochemically deposited polypyrrole films and their characteristics of adhesion on substrate with related to the surface roughness and morphology by comparing to the conventional films deposited electrochemically. In particular, the use of high frequency sonication (500 kHz) for sonoelectrochemical deposition of polypyrrole was reported for the first time. Chronocoulometry was used to electrosynthesize polypyrrole films on Si substrates of different thicknesses in order to determine the influence of the polymer thickness on its rugosity, its morphology and its adhesion force properties. The effects of high frequency sonication on the electropolymerized thin film's properties were investigated using Scanning Electron Microscopy and mechanical probe scanning showing that sonication led to surfaces having a smaller roughness and a better homogeneity than non sonicated ones. The adhesion force properties of polypyrrole films were also studied by means of force-distance curves obtained by Atomic Force Microscopy. For polymer films obtained in the absence of sonication, the measured adhesion forces decreased when the polymer thickness increased. On 2 the contrary, for polymer films obtained under sonication, the adhesion of the films decreased when the polymer thickness decreased.

Introduction

Since polyacetylene was shown to have high electrical conductivities when properly doped [1,2], -conjugated organic polymers have been studied extensively from both fundamental and practical points of view. Indeed, these conducting polymers can be used for many various applications including: energy storage and batteries [3][4][5], field effect transistors [6,7], electrochromic devices [8,9], protection against corrosion [10][11][12], sensing of liquids [13][14][15][16][17][18], gas [19][20][21][22][23] and biomolecules [24][25][26][27][28], separation of molecules by polymeric membranes [29,30], electrocatalysis [31,32], development of solar cells [33,34] and light-emitting diodes [35,36]… Among all the conducting polymers, polypyrrole is one of the most studied for its high electronic conductivity, good stability in air and aqueous media and ease of preparation by electrochemical polymerization. However, the electropolymerized films properties of electropolymerized films are strongly influenced by the parameters chosen for the electrosynthesis: solvent [15,[37][38][39], temperature [40,41], potential or current density [42,43], monomer concentration [44,45], nature of dopant [46][47][48][49] and thickness [15,49].

It must also be noticed that several techniques have been used to modify and improve the polymer's properties such as the use of a magnetic [50] or centrifugation fields [51] during electropolymerization. In order to modify and improve polymer film properties, an original technique consists in applying an sonication to the electrolyte solution during the electrochemical deposition of polymer films. This technique has already showed interesting effects on electroorganic processes by playing a role in the electropolymerization of aniline for example [52], as well as by modifying the properties of electrodeposited polymer films through the microagitation and the cavitation phenomena that it generates in liquid media [53,54]. Indeed, sonoelectrochemical procedure provides higher mechanical resistance as well as electrical conductivity as the film characteristics as compared to the film without sonication, as shown in previous works using 20 kHz frequency sonication [54]. On the contrary, at lower frequencies, cavitation phenomena are very violent and cavitation bubbles can implode so violently that this can lead to partial destruction of the electrodeposited polymer film [55] which is definitely unwanted.

In the area for micromanipulation research, the surface modification of micromanipulator is one of the large issues to improve the catching and releasing functionality precisely.

Modification of the adhesive characteristics of the manipulator surface with functional polymers is one of the important approaches to resolve the problem. That is why we want to test the ability of sonicated polypyrrole films to functionalize manipulator surface to improve its adhesive properties.

Considering the fact that at high frequencies the behaviour of cavitation bubbles is completely different and the destructive effect of sonication towards the surfaces is reduced compared to at low frequencies, and due to possible deposition of polypyrrole with a high frequency sonication, we use 500kHz sonication for sonoelectrochemical PPy deposition. This sonication overcomes the problem to destroy the PPy film electrodeposited on substrate under 4 20 kHz sonication as generally reported in the literature [54]. The knowledge of the adhesion force is thus essential to enable the advent of reliable micromanipulation techniques. Current approaches are based on experimental measurements performed with atomic force microscope [56,57,[58][59][60], interferometric surface force apparatus [61,62], measurement platform utilizing the contact mechanics theory of Johnson, Kendall, and Roberts [63].

Consequently, in the present work, the effects of polymer thickness and high frequency sonication (500 kHz) on the electropolymerized polypyrrole (PPy) films were studied.

Coulometric techniques were used to deeply study PPy films. Important film parameters such as thickness, roughness, morphology and adhesion were explored to observe the effects of sonication. Thus, the determination of the properties of the polymer films allowed us to evaluate the possibility to use them for applications in micromanipulation.

Experimental details

Materials

Pyrrole (ACROS, article 15771 1000, 99% pure) was distilled under reduced pressure before use. Lithium perchlorate LiClO 4 (Acros,  99%, 13453-78-6) was used as received without further purification. De-ionized water (solvent) was distilled twice before use (Milli-Q, resistivity 18 M.cm). The substrates used for polymer deposition were 1 0 0-oriented standard 3'' type Si wafers (thickness: 380 µm, p type, dopant: B, resistivity: 0.015 .cm, from Institute of Electronic Materials Technology).

Electrochemistry

A PGZ-301 potentiostat was employed. The three-electrode setup used consisted of a saturated calomel reference electrode (SCE), a platinum counter-electrode and a Si working electrode (1 cm 2 ). For sonoelectrochemical experiments, a reactor filled with 700 mL of electrolyte solution is used. The working electrode is immersed in the electrolyte solution and located in front of the ultrasonic transducer (Fig. 1). Lithium perchlorate is used as the electrolytic salt. The solvent is water and the concentrations for the studied solution are: 0.

Sonochemistry

All experiments were performed at a constant ultrasonic frequency of 500 kHz, in a cell filled with 700 ml bath. Sonication is produced parallel to the liquid surface, face to face with the working electrode surface (Fig. 1). The circular transducers consist of a piezoelectric ceramic fixed on a circular plate (56 mm) made of glass. The multifrequency generator is supplied by Electronic Service (St. Alban Leysse, France). An electronic circuit consisting of a variable inductance and capacitor allows the adjustment between the generator impedance and the strongly capacitive transducer impedance.

The sonoreactor was characterized using calorimetric characterization. In this aim, it was filled with distilled water (700 ml) and the ultrasound generator was switched on at an electrical power of 25 W. Thus, the absorbed acoustic power, P ac was calculated using:

P ac = m.Cp.(dT/dt).
where m is the mass of water (kg), C p the specific heat capacity of water (4180 J kg -1 K -1 ) and (dT/dt) is the temperature rise per second (K s -1 ). This transmitted acoustic power P ac was found to be equal to 7.8 W. This characterization was completed by sonochemiluminescence observations. Indeed, acoustic distribution is not homogeneous along the cylindrical sonoreactor [64], and the observation is possible with the help of luminol oxidation. This technique is based on the creation of radicals in the solution due to ultrasound wave propagation. Luminol becomes luminescent in the presence of radical species, which indicates ultrasound activity. The intensity of this activity increases with the intensity of luminescence. It is worth noticing that pictures of light distribution in a horizontal flat transducer are distorted by reflections and/or refractions of the light on the walls of the reactor, which is cylindrical in shape. However the vertical distribution display remains unaffected and it is important to see that ultrasound activity is the most concentrated at the interface solution/air. The working electrode surface is thus placed at this location to take benefit as much as possible from ultrasound activity.

Surface characterization

X-ray Photoelectron Spectroscopy (XPS, Thermo VG spectrometer) was used to determine elemental composition and to determine the oxidation state of elements. All spectra were recorded at a 45° take-off angle, relative to the substrate with a spectrometer using monochromatized Al K radiation (1486.6 eV). The binding energies of the core-levels were calibrated against the C1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian-Lorentzian curves (80% of Gaussian character).

The roughness of the polymer films was determined by stylus-based mechanical probe profiler (Alpha-Step IQ, KLA Tencor). Both average roughness (R a ) and peak to peak roughness (R q ) were estimated on a scan length of 1000 µm at a scan speed of 20 µm/s. Polymer morphologies examinations were performed using a high-resolution scanning electron microscope. Once synthesized and dried, polymer samples were examined in a LEO microscope (Scanning Electron Microscopy LEO stereoscan 440, manufactured by Zeiss-Leica, Köln, Germany) with a electron beam energy of 15 keV.

Adhesion

The sample characterization was performed with a commercial atomic force microscope (stand-alone SMENA scanning probe microscope NT-MDT, Russian). The experiments were done under controlled environment with a laminar flow (humidity 30 % and 25°C). The forces measurement performed on this Atomic Force Microscope (AFM) was done in the "Nanorol platform" whose aim is to measure the micromanipulation nanoforce and which is based on the measurement of the AFM cantilever deformation with a laser deflection sensor.

The Si rectangular AFM cantilever, whose stiffness is 0.3 N/m, was fixed and the substrate moved vertically. These work application objectives are to improve reliability of micro-object manipulation, so interactions have been studied between a micrometric sphere and a plane.

Measurements were in fact performed with a cantilever where a borosilicate sphere (r = 5 µm radius) was glued on the free extremity and below this one (company Novascan Technologies, Ames, USA). Ten measurements were done at different points on the same sample with a driving speed of 1 µm/s. Figure 2 is a typical distance-force curve that allows to measure an attractive force (pull-in force) when the sphere is coming close to the substrate (near -20 nN). Thus, Fig. 2 shows a pull-off force which represents the adhesion between the borosilicate sphere on the tip AFM cantilever and the substrate. In this example, the pull-off force is reaching -1.1 µN.

Results and discussion

(Sono)electrochemical synthesis of polypyrrole films

Polypyrrole films were synthesized, at a fixed potential of 1.2 V/SCE, from aqueous solutions containing 0.1 M pyrrole and 0.1 M LiClO 4 on Si substrates. Chronocoulometry (Fig. 3) was used to synthesize the polymer films since this technique allows to determine the charge quantity brought to the electrode in order to control the amount of deposited polymer and the thickness of the deposited films. To study the effects of high frequency sonication on the electrochemical behaviour of PPy and on the polymer thickness, some PPy films were elaborated under sonication when others were prepared without sonication. Thus, ten films grown at different charge quantity have been studied: 5 were obtained without sonication and 5 with 500 kHz frequency irradiation. The chosen charge densities for the electrosyntheses were 50.1 mC.cm -2 , 145.5 mC.cm -2 , 243.2 mC.cm -2 , 486.4 mC.cm -2 and 1459.1 mC.cm -2 . More, the chronocoulometry curves (Fig. 3) show that under sonication, the wanted charge quantity is reached much faster than in silent conditions. The improvement of the mass transfer under sonication can be explained by the propagation of the mechanical vibration that creates a global convection within the liquid medium due to the energy losses, which increases mass transfer of the monomers in solution to the electrode. Moreover close to an interface, the collapse of cavitation bubbles becomes asymmetric and the generated micro-jets enhance mass transfer to the electrode surface, allowing its activation by impingement. The monomers are thus brought to the electrode interface more efficiently and a higher current density and better electropolymerization are obtained [65]. V/SCE, applied charge density: 486.4 mC.cm -2 .

Polypyrrole film thickness

Since the thickness of polymer films can influence their adhesion properties [15,49]. To obtain polypyrrole films of different thickness, the electrical charge associated with pyrrole oxidation was varied using chronocoulometry synthesis technique. To correlate polypyrrole thickness and electrical charge, we used Faraday's law assuming 100 % current efficiency for polypyrrole formation: x = qM / ρAzF where x is the film thickness, q is the electrical charge associated with polypyrrole formation, M is the molar mass of the polymer, F is the Faraday constant, A is the area of the working surface, ρ is the density of the polymer and z is the number of electrons involved which is in this case of 2.25 [66,67]. The nominal density for polypyrrole films (ρ) was taken as 1.5 g.cm -3 [68,69]. This considered, the required charge density to grow a film with an average thickness of 0.1 µm is 48.5 mC.cm -2 . Thus, Table 1 presents the mean film thickness of polypyrrole (x) estimated from electrical charge density (Q) associated with pyrrole oxidation by application of Faraday's law. associated with pyrrole oxidation. In fact, the comparison of these two carbon atoms areas showed that, following overoxidation, the β carbons in the film were less abundant than the α ones. This indicates, that the β positions were the ones involved in the polymer functionalization. The third peak at 286.3 eV is attributed to carbons of the polymer C-N or C-N + ; the fourth one at 287.6 eV to C-N + carbons and the peak much weaker at 289.1 eV to carbonyl C=O groups. The appearance of a C=O component may be associated with the overoxidation of PPy at the β carbon site in the pyrrole rings. The N1s spectra (Fig. 4c) indicate the presence of four peaks in the case of PPy.

Characterization of the polypyrrole films

3.3.a. Spectroscopic characterization

It contains a main signal at 399.6 eV which is characteristic of pyrrolylium nitrogens (-NHstructure) and a high BE tail (BE = 400.7 and 402.3 eV) attributable to the positively charged nitrogen (-NH + (polaron) and =NH + (bipolaron)). The spectra also show a small contribution at 397.3 eV that we associate with =N-structure. Fig. 4d

3.3.b. Surface morphology

Fig. 5 shows SEM micrographs of PPy films obtained with and without sonication and having a thickness of 0.1 µm and 3 µm. In all cases (Fig. 5), the same squared structure is observed.

Nevertheless, the film obtained under sonication appears to have a denser and less squared surface structure. This can be linked to the asymmetric collapse of cavitation bubbles, which lead to activation of the surface by impingement, and certainly therefore to a multiplication of nuclei sites. However, it seems necessary to do roughness measurements to confirm that sonicated PPy films are more homogeneous and have a smaller roughness than non sonicated PPy films.

3.3.c. Polypyrrole film roughness

To understand the values of the adhesion measurements obtained in the next section, it is useful to determine the roughness of each polymer film to correlate the roughness and the adhesion of the polymer films. The Si substrate and the different polypyrrole films roughnesses were investigated using a stylus-based mechanical probe profiler (Table 2).

These measurements show that the roughness of PPy films obtained without sonication increases when the thickness increases. However, the average roughness of the three thinnest 2: Average roughness (R a ) and peak to peak roughness (R q ) of polypyrrole films as a function of their thickness. For uncoated Si substrate: R a = 0.392 µm and R q = 0.504 µm

Polypyrrole films adhesion properties

The adhesion force, so called pull-off force, was measured for all polymer coatings versus their thickness and their electrodeposition condition (with or without sonication). The experimental curves are presented Figure 6. Figure 6a indicates a weak influence of the thickness on the pull-off force without sonication since the pull-off force evolves from 32 to 44 nN. The use of sonication decreases the adhesion force to 12 nN for the smallest thickness (0.1 µm). The average value of the different measurements at different thicknesses and electrodeposition condition are summarized in Table 3. From this table, it can also be noted that the adhesion force measured between the cantilever and the polyppyrole film was very inferior to that measured between the free polymer substrate and the cantilever (near hundred times less than for 0.1 µm sonicated PPy film). Consequently, when applied to micromanipulation, the PPy film electrodeposited under sonication on the gripper end-effector could be an interesting coating to decrease drastically the adhesion force. The unique problem is the film grip on the Si wafer. Indeed, sometimes during the cantilever retract after contact with PPy film, a polymer film fragment was attached on the sphere extremity. The film grip must be improved before the micromanipulation task test on robot. obtained for the film without and with ultrasonic irradiation.

Conclusion

It was shown, for the first time, that the electrochemical polymerization of polypyrrole under high frequency sonication was possible on Si substrates. More, thanks to a better mass transfer, the preparation of coatings having the same thickness is quicker in ultrasounds conditions than in silent conditions as proved by chronocoulometry. Then, it was demonstrated that sonoelectrodeposited films are more homogeneous and have a smaller roughness than other polymer films. Moreover, it was shown that the adhesion forces between sonicated polypyrrole films and Si substrates are very weak. This latter result indicates that sonicated polypyrrole films are good candidates for applications in robotics since the micromanipulation of robots needs the reduction and the minimization of the adhesion forces between the micro-objects and the gripper. The influence of the thickness has also been studied. It was demonstrated that the decrease of the thickness, for sonicated polypyrrole films, led to surfaces having smaller roughness and better homogeneity, and above all to a decrease of the adhesion forces. Consequently, the main perspectives of this work consist in the functionalization of micro-objects by thin sonicated polypyrrole films and their applications in micromanipulation task. The possibility to obtain conducting polymers with improved properties (better mass transfer, better homogeneity, small roughness) by the use of sonoelectrochemistry is another original result from this work and also opens new perspectives in the polymer science.

1 M

 1 in pyrrole and 0.1 M in LiClO 4 . The electrochemical polymerization of PPy films was done, by applying a potential of 1.2 V/SCE, using chronocoulometry technique. The solutions were kept under an argon flow during the experiments.

Fig. 1 .

 1 Fig. 1. Scheme of the sonoreactor equipment.

Fig. 2 .

 2 Fig. 2. Attractive and adhesive typical force-distance curves between substrate and cantilever

Fig. 3 .

 3 Fig. 3. Chronocoulometry curve of PPy electrosynthesis from an aqueous solution containing

Fig. 4a shows

  Fig.4ashows the XPS of the 3.0 µm sonicated polypyrrole film deposited onto a Si surface

Fig. 4 .

 4 Fig. 4. Survey-scan XPS of polypyrrole films electrosynthesized onto silicon surfaces by

Fig. 5 .

 5 Fig. 5. SEM images of PPy films elaborated under silent conditions (a: 0.1 µm, b: 3 µm) and

  films are not very different and close to 0.5 µm when the roughness of the two thickest film is far more important (> 1.5 µm). The same trend is observed for PPy films obtained with sonication since the thin films have a roughness close to 0.4 -0.6 µm when the thickest coatings have average roughness comprises between 0.8 and 1.3 µm. It is also important to note that sonicated PPy films have a smaller roughness than non sonicated ones, indicating that the sonication leads to more homogeneous and smoother surfaces whatever the

  Fig. 6. Force-distance curve for different PPy thicknesses on Si substrates (spring constant

Table 1 :

 1

Thicknesses (x) of polypyrrole films as a function of electrical charge density (Q)

Table 3 :

 3 Influence of the PPy film thickness on the Pull-off force (spring constant 0.3 N/m)

	PPy thickness	Without US	With US
	3 µm	32 nN	41 nN
	1 µm	34 nN	36 nN
	0,5 µm	37 nN	18 nN
	0,3 µm	41 nN	16 nN
	0,1 µm	44 nN	12 nN
	Uncoated Si	1 µN

6. Force-distance curve for different PPy thicknesses on Si substrates (spring constant 0.3 N/m). a: without, b: with ultrasonic irradiation.

Acknowledgment

This work was supported by the EU under HYDROMEL (contract NMP2-CT-2006-026622):

Hybrid ultra precision manufacturing process based on positional-and self-assembly for complex micro-products, and by the French National Agency (ANR) under NANOROL (contract ANR-07-ROBO-0003): Nanoanalyse for micromanipulate. The force measurement performed by Atomic Force Microscopy was done in the "Nanorol platform" (The "Nanorol platform" can be used by external person. The availability and the booking of the station are consultable via internet at: \url{http://nanorol.cnrs.fr/events.php).