MD Aiste Lengvenyte

Frédéric Coppola

Isabelle Jaussent

Philippe Courtet

Emilie Olié

Improved functioning following computerized working memory training (COGMED®) in euthymic patients with bipolar disorder and cognitive complaints: an exploratory study

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

Improved functioning following computerized working memory training (COGMED®) in euthymic patients with bipolar disorder and cognitive complaints: An exploratory study

1. Introduction Bipolar disorders (BD) are considered among the top 20 leading causes of disability worldwide. BD global burden is continually increasing [START_REF] Ferrari | The prevalence and burden of bipolar disorder: findings from the Global Burden of Disease Study 2013[END_REF], and has been ranked as having the second-highest effect on sick leave days according to a World Mental Health survey [START_REF] Alonso | Days out of role due to common physical and mental conditions: results from the WHO World Mental Health surveys[END_REF]. Sustained symptomatic remission is generally attainable with the current treatment strategies. However, treatment does not seem to have a major effect on the decline in general functioning, leading to poor quality of life and socioeconomic functioning [START_REF] Tse | Meta-analysis of predictors of favorable employment outcomes among individuals with bipolar disorder[END_REF].

Cognitive dysfunction and residual depressive symptoms have been identified as the main drivers of functional disability in euthymic patients with BD [START_REF] Roux | Associations between residual depressive symptoms, cognition, and functioning in patients with euthymic bipolar disorder: results from the FACE-BD cohort[END_REF].

Patients with cognitive impairment tend to report poorer quality of life, despite similar mood symptoms as patients without cognitive deficits [START_REF] Jensen | Discrete neurocognitive subgroups in fully or partially remitted bipolar disorder: Associations with functional abilities[END_REF]. Memory and executive functions are particularly strong contributors to the occupational functioning outcome in BD, with greater impact than residual mood symptoms [START_REF] Miskowiak | Assessing and addressing cognitive impairment in bipolar disorder: the International Society for Bipolar Disorders Targeting Cognition Task Force recommendations for clinicians[END_REF][START_REF] Tse | Meta-analysis of predictors of favorable employment outcomes among individuals with bipolar disorder[END_REF].

Moreover, cognitive impairment is reported by 50-70% of patients during periods of symptomatic remission [START_REF] Jensen | Discrete neurocognitive subgroups in fully or partially remitted bipolar disorder: Associations with functional abilities[END_REF], and could be objectified by neuropsychological tests in 12-30% of patients [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF]. Specifically, it has been hypothesized that working memory (WM) acts as a mediator for subthreshold depressive symptoms in predicting the functional outcome [START_REF] Ellard | Transdiagnostic treatment of bipolar disorder and comorbid anxiety using the Unified Protocol for Emotional Disorders: A pilot feasibility and acceptability trial[END_REF]. A meta-analysis had demonstrated that all cognitive functions together correlate with the global functioning with an effect size of 0.34, while the working memory alone has an effect size of 0.29 [START_REF] Depp | Meta-analysis of the association between cognitive abilities and everyday functioning in bipolar disorder[END_REF]. These results underline the critical role of working memory in the global functioning of patients with BD. Another study in community-dwelling older adults found that only the composite of executive function and working memory tasks was a significant predictor of medication adherence [START_REF] Insel | Executive function, working memory, and medication adherence among older adults[END_REF]. Working memory has also been demonstrated to be an independent predictor of the quality of life in BD patients [START_REF] Mackala | Cognitive performance and quality of life early in the course of bipolar disorder[END_REF]. However, targeted interventions are lacking.

Cognitive remediation (CR) [START_REF] Anaya | A systematic review of cognitive remediation for schizo-affective and affective disorders[END_REF] is a promising strategy to address this deficit. It aims to strengthen cognitive functions by increasing neuronal plasticity, with the final goal of improving everyday function [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF]. CR increases neuroplasticity in the hippocampus, as well as the resting state efficiency in the frontal lobe [START_REF] Valenzuela | Can cognitive exercise prevent the onset of dementia? Systematic review of randomized clinical trials with longitudinal follow-up[END_REF]. A recent study in patients with severe mental illnesses showed that CR significantly contributed to their occupational outcome [START_REF] Ikebuchi | Does improvement of cognitive functioning by cognitive remediation therapy effect work outcomes in severe mental illness? A secondary analysis of a randomized controlled trial[END_REF]. The few studies on CR benefits in BD found that they are comparable to those observed in schizophrenia [START_REF] Anaya | A systematic review of cognitive remediation for schizo-affective and affective disorders[END_REF], where much research has focused on this topic. Clinical trials in BD demonstrated a possible association between CR and improvement in cognition, reduction of depressive symptoms and better psychosocial functioning [START_REF] Deckersbach | RESEARCH: Cognitive rehabilitation for bipolar disorder: An open trial for employed patients with residual depressive symptoms[END_REF][START_REF] Solé | Cognitive Impairment in Bipolar Disorder: Treatment and Prevention Strategies[END_REF]. However, studies on CR in BD are characterized by considerable variability in the used software, treatment regimens, and outcome measures.

This might explain the inconsistent results among studies, ranging from no effect to significant improvement [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF].

The diversity in CR modalities may be addressed by using digital approaches [START_REF] Veeh | Cognitive remediation for bipolar patients with objective cognitive impairment: a naturalistic study[END_REF]. The number of studies on the digital transformation of CR is still insufficient [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF], limiting the applicability of this treatment modality. Interestingly, systematic reviews and meta-analyses found that digital tools for CR are as effective as faceto-face treatments when associated with support by a trained therapist via brief spoken or written guidance and feedback [START_REF] Andersson | Guided Internetbased vs. face-to-face cognitive behaviour therapy for psychiatric and somatic disorders: a systematic review and meta-analysis[END_REF]. Studies on CR in schizoaffective disorders showed positive results of computerized CR therapy, with an effect size for neurocognitive improvement ranging from 0.32 to 0.41 [START_REF] Anaya | A systematic review of cognitive remediation for schizo-affective and affective disorders[END_REF][START_REF] Lopez-Fernandez | Cognitive Remediation Interventions in Schizoaffective Disorder: A Systematic Review[END_REF]. The efficacy of digital approaches in BD has been demonstrated by several studies [START_REF] Andersson | Guided Internetbased vs. face-to-face cognitive behaviour therapy for psychiatric and somatic disorders: a systematic review and meta-analysis[END_REF][START_REF] Lewandowski | Treatment to Enhance Cognition in Bipolar Disorder (TREC-BD): Efficacy of a Randomized Controlled Trial of Cognitive Remediation Versus Active Control[END_REF][START_REF] Veeh | Cognitive remediation for bipolar patients with objective cognitive impairment: a naturalistic study[END_REF].

WM may represent a particularly attractive therapeutic target due to its strong links with other cognitive domains. WM deficits are present during syndromic remission and can play a crucial role in predicting the functional outcome in bipolar disorder [START_REF] Del Mar Bonnín | Verbal memory as a mediator in the relationship between subthreshold depressive symptoms and functional outcome in bipolar disorder[END_REF][START_REF] Lima | Cognitive deficits in bipolar disorders: Implications for emotion[END_REF]. Moreover, the efficacy of programs that specifically target WM has already been demonstrated in patients with head trauma, attention deficit and hyperactivity disorder [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF]. A randomized clinical trial in patients with head trauma found that WM training significantly improved the general functioning of the patients (Björkdahl et al., 2013). Moreover, it has been demonstrated that WM training has modulatory effects on brain dopaminergic system, decreasing the density of D1 receptors (Klingberg and McNab, 2009).

In a search for the accessible treatment options, it seems necessary to evaluate the efficacy of cognitive remediation programs that have a short duration, little constraints for the patients, are relatively cheap, and are very specific in their targets. The computerized WM remediation could be a good target due to the relatively low need for human interaction, has a limited timeframe and requires minimal restrictions. Therefore, we investigated the effect of computerized WM training using the COGMED® program [START_REF] Klingberg | Computerized Training of Working Memory in Children With ADHD-A Randomized, Controlled Trial[END_REF] in patients with BD and cognitive complaints. Our primary hypothesis was that this intervention would improve global functioning. We also investigated (secondary outcomes) whether this intervention can improve therapeutic compliance and subjective life quality, and explored the association between WM and global functioning.

Methods

This was a preliminary naturalistic, prospective, monocentric, open, non-controlled study on the effect of a supervised WM digital intervention in euthymic patients with BD and memory complaints. The study was approved by the Montpellier University Hospital ethics committee (CPP Sud Mediterranée IV) and was registered at clinicaltrials.gov (identifier NCT02988518). All participants gave their written informed consent.

Clinical assessment

Lifetime and current psychopathology were assessed using the Structured Clinical Interview for DSM disorders (SCID-1) [START_REF] First | Structured clinical interview for DSM-IV axis I disorders[END_REF]. Depression severity was evaluated using the MADRS [START_REF] Montgomery | A new depression scale designed to be sensitive to change[END_REF] and the Quick Inventory of Depressive Symptomatology self-report (QIDS-SR16) [START_REF] Rush | The 16-Item Quick Inventory of Depressive Symptomatology (QIDS), clinician rating (QIDS-C), and self-report (QIDS-SR): a psychometric evaluation in patients with chronic major depression[END_REF]. Manic symptomatology was assessed using the YMRS [START_REF] Young | A rating scale for mania: reliability, validity and sensitivity[END_REF] and the Altman Self-Rating Mania Scale (ASRM) [START_REF] Altman | The Altman Self-Rating Mania Scale[END_REF]. Additional self-questionnaires included the State-Trait Anxiety Inventory (STAI Form Y) [START_REF] Spielberger | Manual for the State-Trait Anxiety Inventory (Self-Evaluation Questionnaire)[END_REF] to evaluate anxiety, the Medical Adherence Rating Scale (MARS) [START_REF] Fialko | A large-scale validation study of the Medication Adherence Rating Scale (MARS)[END_REF] to determine treatment adherence, the SF-36 (McHorney et al., 1994) to assess the quality of life, and the Mac Nair questionnaire [START_REF] Mcnair | Self-assessment of cognitive deficits[END_REF] to evaluate memory and attention complaints.

Global functioning was assessed using the Functioning Assessment Short Test (FAST) [START_REF] Rosa | Validity and reliability of the Functioning Assessment Short Test (FAST) in bipolar disorder[END_REF] that is specific for patients with BD. FAST explores six domains of general functioning: autonomy, professional domain, cognitive domain, financial domain, interpersonal relationships, and leisure activities. The total score is the sum of the score of the 24 items that are given 0 to 3 points/each. FAST reliability and validity in patients with BD have already been demonstrated [START_REF] Rosa | Validity and reliability of the Functioning Assessment Short Test (FAST) in bipolar disorder[END_REF] corrected and uncorrected errors was counted. Three subtests ("codes", "symbols", and "digit span") of the Wechsler Adult Intelligence Scale, Fourth Edition (WAIS-IV) [START_REF] Wechsler | WAIS-IV: Wechsler Adult Intelligence Scale[END_REF] were used to evaluate the information processing speed ("codes" and "symbols" subtests), the attention capacity and associative memory ("codes" subtest), and WM verbal component ("digit span" subtest). The WM visuospatial component was assessed with the Corsi block-tapping test of the Wechsler Memory Scale, Third Edition (WMS-III) [START_REF] Wechsler | WAIS-III: Administration and Scoring Manual: Wechsler Adult Intelligence Scale[END_REF]. Mental flexibility was assessed using a verbal fluency test.

COGMED® program

After the baseline assessment, patients received information about the importance of At the end of each week, the psychiatrist phoned each patient. Before the conversation, a systematic analysis of each patient's progress was performed, and a personalized feedback content was prepared. At the beginning of the conversations, patients were asked if they had any technical problems with the program and were accordingly advised. Then, the feedback regarding the progress and was given, and an overview of each exercise was done. If patients were experiencing difficulties, additional explanations were provided. Patients were then encouraged to continue their progress and invited to find strategies to increase their efficacy, as well as give themselves small rewards. The conversation ended with an enquiry about the daily mood and attention performance of the patients. Each call lasted approximately ten minutes.

Statistical analysis

Statistical analysis was carried out using "IBM SPSS STATISTICS version 21". The primary outcome was the change of total FAST score between the baseline and posttreatment visit. Secondary outcomes were changes in the MARS, SF-36, and Mac Nair scores and in neuropsychological functions (executive functions, WM, information processing speed, and attention capacity). The convergent validity of the tests used to evaluate WM was analyzed using the Spearman's rank correlation coefficient (rho). The strength of the correlation was considered high (rho > 0.7), moderate (0.4 to 0.7), or low (<0.4). The assumption of normality was tested before all comparisons. The paired sample Student's t-test was used if data were normally distributed, and the non-parametrical Wilcoxon signedrank test in all other cases. Univariate analyses with Pearson's correlation coefficient (for continuous variables) and Student's t-test (for categorical variables) were used to assess the relationship between the change of FAST total score and sociodemographic, clinical and neuropsychological variables. Then, the repeated-measures ANOVA was performed to adjust for possible covariates for the FAST change. A post-hoc analysis was performed using the four groups of functional impairment according to the FAST score, as proposed by Bonnín and colleagues [START_REF] Bonnín | Journal of A ff ective Disorders Thresholds for severity , remission and recovery using the functioning assessment short test ( FAST ) in bipolar disorder[END_REF]. The proposed cut-off was used to evaluate the rate of improvement (from severe to moderate), remission (from moderate/severe to mild), and recovery (switch to no impairment).

The p-value threshold for statistical significance was set at <0.05.

Results

Patients' characteristics at baseline

In total, 40 patients with BD were included, and many reported a relatively high delay between the first symptoms and diagnosis. Their main baseline characteristics are presented in Table 1. All patients were taking mood stabilizers, with a high level of treatment adherence according to the MARS scale score. The most recent thymic episode was mainly a depressive episode, with residual depressive symptomatology. No residual manic symptomatology was detected, and the anxiety level was low. Twelve participants had current psychiatric comorbidity, mainly bulimic hyperphagia (N=8).

Functional impairment was severe in 6 patients, moderate in 23, mild in 6, and absent in 5 patients, according to the classification by Bonnín and colleagues [START_REF] Bonnín | Journal of A ff ective Disorders Thresholds for severity , remission and recovery using the functioning assessment short test ( FAST ) in bipolar disorder[END_REF].

Impairment in the occupational, cognitive, and interpersonal relationship domains was particularly pronounced, as shown by the FAST scores. Subjective memory and attention complaints were marked (mean score: 68.78, SD 14.77). The baseline neuropsychological evaluation showed that the participants' cognitive profile was relatively preserved. ), but the difference was not significant (p=0.06).

Comparison of clinical variables between baseline and post-treatment visits

The FAST total score was significantly different between baseline and post-treatment visit, with a relative change [(final visit -initial visit)/initial visit] of -0.28 (SD = 0.21), p<0.0001. This difference remained significant after exclusion of patients with head trauma (mean relative change: -0.27, SD = 0.21; p<0.0001). Three FAST sub-scores (cognitive functioning, autonomy and occupational functioning) were significantly different between visits (relative change: -0.49, SD = 0.28; -0.13, SD = 1.16; and -0.09, SD = 0.71, respectively), with the highest decrease in the cognition score (see table 2).

After the exclusion of patients without functional impairment during the post-hoc analysis, the change in FAST score further increased, with a mean change of 7.50 (SD=4.46), from 29.29 (SD 10.77) at inclusion to 20.14 (SD=9.15) at the post-treatment visit, p<0.01.

Among the 28 patients who had cognitive impairment at inclusion and completed the study, the general functioning improvement was observed in 3 (10.71%), remission in 9 (32.14%), and recovery in 5 (17.86%). In total, 17 patients (60.71%) showed a favourable clinical change. patient's evaluations. The difference was particularly significant for the question about concentration (concentration item in the MADRS and QIDS-SR16 scales). However, this change was no longer significant after excluding the concentration item of the MADRS but not of the QIDSR-SR16 scale. Treatment adherence and subjective quality of life did not change between visits.

Treatment (molecules and dosage) did not change between visits in 20 patients, while the changes did not concern the main mood-stabilizing drug in the other 12.

Comparison of neuropsychological variables between baseline and post-training visits

Verbal and visuospatial WM positively changed after the intervention, as shown by the WAIS-IV digit span and WMS-III Corsi block-tapping test scores (Table 3). A significant change in the WAIS-IV "codes" subtest (attention capacity, associative memory and information processing speed) was identified. The score of the Stroop test, which evaluates information processing speed, attention capacity and cognitive inhibition, also improved.

Conversely, the change in the "symbols" test, which reflects the information processing speed, was not significant, as well as the changes in the TMT test scores (visual attention and task switching capacities). Semantic, but not phonemic verbal fluency changed significantly.

Table 3. Comparison of the cognitive functioning tests before and after cognitive remediation

Global functioning change and working memory

The FAST change did not correlate with the working memory change. However, in repeated measures ANOVA, the difference between two FAST timepoints became not significant after the adjustment for the working memory change (F(1,29)=1.56, p=0.22). The repeated-measures ANOVA with adjustment for other possible covariates for the FAST change that correlated with the FAST change in the univariate analysis was also performed.

After controlling for the number of thymic episodes, history of head trauma, depressive symptomatology at baseline, and decrease of depressive symptomatology according to MADRS, the FAST change remained significant (F(1,13)=22.12, p<0.01).

The pharmacological treatment modality, regimen changes, and a number of used drugs were not associated with any of the primary or secondary outcomes.

Discussion

To our knowledge, this is the first study exploring the feasibility of a short computerised WM training program in euthymic patients with BD and memory complaints.

This is also the first study that applied the functional impairment severity grading to assess the clinical relevance of an intervention. Per our hypothesis, patients' functioning was significantly improved one month after completion of the intervention, with a mean change in the total FAST score of -6.78. These results are comparable with those of longer CR and functional remediation studies (from 21 weeks to 6 months), with an average FAST total score decrease between six and eight points [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF][START_REF] Torrent | Efficacy of Functional Remediation in Bipolar Disorder: A Multicenter Randomized Controlled Study[END_REF][START_REF] Veeh | Cognitive remediation for bipolar patients with objective cognitive impairment: a naturalistic study[END_REF]. After excluding patients without functional impairment at baseline, the change in FAST score increased to -7.5. Improvement, remission or recovery was observed in twothirds of completers. Specifically, the FAST cognition sub-score decreased significantly, suggesting that patients experienced fewer difficulties in everyday tasks that required concentration, problem-solving and memory abilities. Less pronounced, but still significant improvements in occupational functioning and autonomy were observed, suggesting a broad impact of the intervention. Conversely, no change in the financial, interpersonal and leisure domains was detected, possibly due to the short follow-up period, because these functions might take longer to restore. Taken together, our exploratory study demonstrated a functioning increase and provided the basis for future research.

Concerning the secondary outcomes, the subjective quality of life and treatment adherence did not significantly change, differently from some previous CR studies [START_REF] Deckersbach | RESEARCH: Cognitive rehabilitation for bipolar disorder: An open trial for employed patients with residual depressive symptoms[END_REF][START_REF] Torrent | Efficacy of Functional Remediation in Bipolar Disorder: A Multicenter Randomized Controlled Study[END_REF]. While the subjective quality of life increased, the change did not reach the level of significance. This could be explained by the insufficient power of the study sample and the short study duration. Indeed, the translation of better functioning to meaningful changes in life quality might take more time. The relative stability of treatment adherence may be due to its already high level at baseline, possibly due to a selection bias. Indeed, most of the patients in our sample had a high level of consciousness about their condition. Half (4/8) of the patients who abandoned the study had lifetime substance use disorder history, while this was the case for only 12.50% of completers (4/32).

While this difference was not statistically significant (p=0.06), this feature should be explored in future studies.

As expected, both verbal and visuospatial components of WM significantly increased according to the WAIS-IV digit span and Corsi block-tapping scores. Although it is challenging to measure WM precisely, the convergence between the chosen subtests was good. Patients also reported significantly fewer memory and attention complaints after the intervention. While no CR trial has specifically targeted WM, CR trials in BD have shown improvements in neurocognitive performances [START_REF] Preiss | Personalized cognitive training in unipolar and bipolar disorder: a study of cognitive functioning[END_REF], although the extent of this effect is debated and more evidence-based research is needed [START_REF] Demant | Effects of Short-Term Cognitive Remediation on Cognitive Dysfunction in Partially or Fully Remitted Individuals with Bipolar Disorder: Results of a Randomised Controlled Trial[END_REF].

Besides WM, various other cognitive domains significantly improved, specifically attention capacity, associative memory, cognitive inhibition, and cognitive flexibility, while changes in information processing speed, visual attention and task-switching were minimal and not significant. Previous studies reported no far-transfer effect of WM on other domains, or a small effect [START_REF] Lampit | Computerized cognitive training in cognitively healthy older adults: a systematic review and meta-analysis of effect modifiers[END_REF]. However, these studies included healthy subjects, and their results are not generalizable to patients with BD. Studies on structural brain changes during WM training revealed improvement in prefrontal network efficiency, potentially releasing the resources for compensatory functions in the case of high WM load [START_REF] Vermeij | Prefrontal activation may predict working-memory training gain in normal aging and mild cognitive impairment[END_REF].

One could hypothesize that WM overload might be present in patients with BD. Therefore, WM remediation programs in BD might promote adaptive responses and improve processing efficiency.

In line with previous studies on cognitive and functional remediation [START_REF] Deckersbach | RESEARCH: Cognitive rehabilitation for bipolar disorder: An open trial for employed patients with residual depressive symptoms[END_REF][START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF][START_REF] Preiss | Personalized cognitive training in unipolar and bipolar disorder: a study of cognitive functioning[END_REF][START_REF] Veeh | Cognitive remediation for bipolar patients with objective cognitive impairment: a naturalistic study[END_REF], residual depressive symptomatology was significantly decreased at the post-treatment visit. A possible explanation might be that the structure and support offered by the program favoured selfefficacy and empowerment [START_REF] Veeh | Cognitive remediation for bipolar patients with objective cognitive impairment: a naturalistic study[END_REF]. Moreover, BD is associated with impaired emotional regulation, which might be driven by poorer executive function, including WM [START_REF] Lima | Cognitive deficits in bipolar disorders: Implications for emotion[END_REF]. WM remediation could result in emotional and mood improvements via reduction of maladaptive forms of emotional regulation, such as rumination. As residual depressive symptoms are considered to be the strongest predictor of psychosocial functioning [START_REF] Bonnín | Clinical and neurocognitive predictors of functional outcome in bipolar euthymic patients: a long-term, follow-up study[END_REF], the reduction of depressive symptomatology might represent an additional benefit of this type of intervention.

Some authors suggested that mood and cognitive symptoms are independent sources of variation in the functioning of patients with BD [START_REF] Roux | Associations between residual depressive symptoms, cognition, and functioning in patients with euthymic bipolar disorder: results from the FACE-BD cohort[END_REF][START_REF] Samalin | Residual depressive symptoms, sleep disturbance and perceived cognitive impairment as determinants of functioning in patients with bipolar disorder[END_REF].

Other researchers proposed a reciprocal relationship [START_REF] Bonnín | Clinical and neurocognitive predictors of functional outcome in bipolar euthymic patients: a long-term, follow-up study[END_REF], and hypothesized that the mediation might occur for higher levels of depressive symptoms [START_REF] Roux | Associations between residual depressive symptoms, cognition, and functioning in patients with euthymic bipolar disorder: results from the FACE-BD cohort[END_REF].

Interestingly, despite the relatively high baseline MADRS score in our group (7.7 points on average, compared with less pronounced depressive symptomatology in previous studies [START_REF] Bonnín | Clinical and neurocognitive predictors of functional outcome in bipolar euthymic patients: a long-term, follow-up study[END_REF][START_REF] Roux | Associations between residual depressive symptoms, cognition, and functioning in patients with euthymic bipolar disorder: results from the FACE-BD cohort[END_REF][START_REF] Samalin | Residual depressive symptoms, sleep disturbance and perceived cognitive impairment as determinants of functioning in patients with bipolar disorder[END_REF]), all three domains (cognitive functions, depressive symptomatology and general functioning) were improved at the end of the intervention, but only depressive symptomatology and general functioning correlated with each other. The residual thymic symptom improvement could correlate with short-term changes in general functioning due to the high subjectivity of its measurement. The patients just feel better about their life and frame it more positively. Conversely, changes in the cognitive domains take longer to apply in real life because learning new skills, engaging in professional and personal activities require cognitive capacity and also self-confidence, especially at the beginning.

Though working memory improvement did not correlate significantly with the general functioning and depressive symptomatology improvement, the general functioning change became not significant after the adjustment for the working memory change.

Meanwhile, the global functioning change remained significant after controlling for the depressive symptomatology change. This emphasizes the possibility that, though the effect was small in the short term and was not sufficient to reach the statistically significant correlation, working memory change nevertheless, at least partly, drove the improvement in general functioning following the COGMED intervention.

History of head trauma and the number of previous manic, hypomanic or mixed episodes also were associated with the functioning improvement. This result should be interpreted with caution because only half of the patients indicated the number of previous episodes, and only six had a history of head trauma. However, this finding is intriguing.

Indeed, it was previously demonstrated that patients with a history of head trauma benefit from WM remediation [START_REF] Kim | Current Status of Cognitive Remediation for Psychiatric Disorders: A Review[END_REF], and comorbidity with BD seems to be common.

The exploratory study design allowed using less restrictive inclusion criteria and investigating this patients group. Our data indicate that they benefited from the intervention more than other patients in terms of functioning. Moreover, it has been proposed that the effect of interventions for cognitive dysfunction differs in function of the disease stage [START_REF] Vieta | Staging and psychosocial early intervention in bipolar disorder[END_REF]. Some studies found a negative correlation between the number of manic, hypomanic and mixed episodes and the efficacy of treatment [START_REF] Berk | Staging in bipolar disorder: from theoretical framework to clinical utility[END_REF], while our study found a positive correlation. Similar to our results, the chance of treatment efficacy on cognition was increased by 16% for every year of illness in patients with mood disorders treated with erythropoietin for cognitive impairment [START_REF] Miskowiak | Targeting Treatments to Improve Cognitive Function in Mood Disorder: Suggestions From Trials Using Erythropoietin[END_REF]. While previous longitudinal studies found no association between cognition and the number of thymic episodes [START_REF] Schouws | Five-year follow-up of cognitive impairment in older adults with bipolar disorder[END_REF], a recent study reported that patients with more manic or hypomanic episodes were more cognitively impaired and had poorer functioning (Solé et al., 2018). Moreover, neuroimaging studies in BD have shown progressive structural abnormalities (Hibar et al., 2018), related to the number of manic episodes. It could be hypothesized that patients with a higher number of manic, hypomanic and mixed episodes are characterized by progressive neurocognitive impairments, lower cognitive reserve and better treatment response to WM remediation.

Though the drop-out rate was as expected, and similar to what was previously reported in functional remediation studies [START_REF] Torrent | Efficacy of Functional Remediation in Bipolar Disorder: A Multicenter Randomized Controlled Study[END_REF], it raises concern that some patients lose their motivation to undergo such interventions despite their relative convenience, continuous active motivating, and relatively short duration. An aspect of selfmotivation, inherent to such interventions (one has to actively find time during the day to sit in front of a computer), could be one reason for the drop-out. Bipolar disorder patients are proven to have marked impairment in their ability to delay gratification, resulting in their choice of faster, but smaller rewards compared to healthy controls [START_REF] Ahn | Temporal Discounting of Rewards in Patients with Bipolar Disorder and Schizophrenia[END_REF]. In addition, though we did not find significant differences in cognitive measures between study completers and non-completers, younger individuals and those with a shorter disease duration were more likely to drop-out. This is following findings from a study with anorexia nervosa patients, that found that younger patients had a lower motivation to change [START_REF] Hillen | Motivation to change and perceptions of the admission process with respect to outcome in adolescent anorexia nervosa[END_REF].

Limitations and strengths

Some limitations should be noted. First, the naturalistic open-label, non-controlled design of this study precludes the assessment of causality with any degree of confidence.

Second, the small sample size and short study duration resulted in limited power to look at interaction effects among the domains and long-term effects. Even among this relatively small sample, eight patients did not complete the study. Moreover, the pharmacological treatment regimen was flexible, and this could have influenced the treatment response. Some patients were treated with benzodiazepines and had a history of head trauma, two conditions that should preclude their inclusion in cognition trials [START_REF] Miskowiak | Assessing and addressing cognitive impairment in bipolar disorder: the International Society for Bipolar Disorders Targeting Cognition Task Force recommendations for clinicians[END_REF]. However, drugs did not have any effect on the study outcomes. While the history of head trauma was correlated with a better response to intervention, the response remained significant even after the exclusion of these patients, and their inclusion increased the outreach and generalizability to real-life patients' populations because this comorbidity is common. Another limitation was the absence of screening for physical comorbidities, sleep disorders and ADHD that have been associated with cognitive decline in BD [START_REF] Berk | Staging in bipolar disorder: from theoretical framework to clinical utility[END_REF]. Another limitation is that this was a single-centre study, and therefore, a site effect on the outcome cannot be excluded.

Finally, the population was recruited in a psychiatric department where patients with more complex/severe disease and longer duration are followed, and this may limit the result generalizability to all patients with BD.

The main strength of our study is that it is the first of its kind and includes a sample of patients with a careful diagnosis of BD. Moreover, it is the first study to address the clinical meaningfulness of WM remediation using functional impairment grading. Although our sample was relatively small, it was congruent with previous studies. The mean level of functioning was in the range found in previous studies on the relationship between cognition and functioning in euthymic patients with BD [START_REF] Samalin | Residual depressive symptoms, sleep disturbance and perceived cognitive impairment as determinants of functioning in patients with bipolar disorder[END_REF][START_REF] Tse | Meta-analysis of predictors of favorable employment outcomes among individuals with bipolar disorder[END_REF].

Moreover, subjective and objective measures were included, as recommended by the ISBD taskforce [START_REF] Miskowiak | Assessing and addressing cognitive impairment in bipolar disorder: the International Society for Bipolar Disorders Targeting Cognition Task Force recommendations for clinicians[END_REF], to investigate the clinical significance and of the intervention and for research purposes.

Conclusions and future directions

The present study demonstrated a significant improvement of global functioning in patients with BD after computerized WM training. This intervention may represent an additional strategy to improve the general functioning in euthymic patients with BD and memory complaints. These preliminary results might direct future research in this area. A pertinent study approach could be a randomized controlled trial with a longer follow-up time, including multimodal approaches to combine pharmacological agents and WM remediation.

Although it was not the purpose of the present study, any adverse effects need to be assessed in future trials. Cost-effectiveness studies should be the next step to evaluate the applicability of this short treatment format.

  WM in everyday life. They were trained to use a computerized WM training program, called COGMED®[START_REF] Klingberg | Computerized Training of Working Memory in Children With ADHD-A Randomized, Controlled Trial[END_REF], and were given individual codes to connect to COGMED® through the web. This web-based program includes a battery of interactive game-like tasks to increase the visuospatial and auditory WM. The program includes 12 different WM tasks (eight per day). Each daily session lasts between 30 and 45 minutes. The typical WM training program included five sessions per week for five weeks (in total, 25 sessions). The difficulty level was adjusted automatically to the patient's maximum capacities in order to stimulate the cognitive effort. A progress index allowed evaluating the progress made by each patient.

Table 1 .

 1 Baseline demographic, clinical and neuropsychological characteristics of patients The other clinical characteristics and all neuropsychological performances were not different between leavers and completers. Finally, 50% of leavers (4/8) and 12.1% of completers (4/33) had a lifetime history of substance use disorder (excluding past six months

	with BD (N=40)
	The baseline FAST total score was significantly correlated with the residual
	depressive symptomatology (r=0.41, p=0.01 for MADRS, and r=0.38, p=0.03 for QIDS-

SR16), and positive history of head trauma (r= 0.38, p=0.02). The mean FAST and WM scores did not differ in function of the pharmacological treatment status (current treatment vs absence of current treatment, and one drug vs more than one drug).

3.2. Drop-out

Among the 40 recruited patients, 32 completed the COGMED® program and the post-treatment assessment. One patient abandoned the study before the first COGMED® session due to low motivation. Seven participants withdrew during the intervention after having completed a mean of 13.6 sessions (SD=5.09) in 2.4 weeks (SD=0.98). Following reasons for leaving the study were indicated by these patients: two participants complained of fatigue, one participant indicated that he had difficulties in concentrating in the previous session before leaving the study, and did not want to proceed, one participant expressed low general motivation. Three other patients did not indicate the drop-out reason. duration (mean 13.5 years, SD 10.30 vs 25.38 years, SD 9.57, p<0.01) and fewer of them used anticonvulsants (25% vs 69%, p=0.04) compared with patients who completed the study.

Table 2 .

 2 Comparison of the functioning before and after cognitive remediation Patients completed a mean of 22.97 (SD=5.23) COGMED® sessions. Compared with baseline, patients reported significantly fewer memory and attention complaints after the intervention (mean Mac Nair score at the post-treatment visit: 53.50, SD 18.08). The residual depressive symptomatology also significantly decreased, according to the clinician's and

Table 1 .

 1 Baseline demographic, clinical and neuropsychological characteristics of patients with bipolar disorder (N=40) History of drug/alcohol abuse/dependence, anxiety and eating disorders was measured with the Structured Clinical Interview for the DSM-IV (SCID-1) scale; MADRS, Montgomery-Asberg Depression Rating Scale; YMRS, Young Mania Rating Scale; FAST, Functional Assessment Staging Test; QIDS-SR16, Quick Inventory of Depressive Symptomatology; ASRM, Altman Self-Rating Mania Scale; MARS, Medical Adherence Rating Scale; STAI Form Y, State-Trait Anxiety Inventory, form Y; SF-36, Medical Outcome Study Short Form 36 health survey; Mac Nair, questionnaire to evaluate memory and attention complaints; TMT, Trail Making Test; Average of verbal fluency "p" and "animals" subtests is presented; WAIS-IV, Wechsler Adult Intelligence Scale, Fourth Edition; Information processing speed was measured with the WAIS-IV Coding and Symbol search subscale ; the Corsi block-tapping test is a part of the Wechsler Memory Scale, Third Edition, and was used to evaluate visuospatial memory; Stroop color item score -part of the Stroop Color-Word test with colors.

	Variable

Table 2 .

 2 Comparison of the functioning before and after cognitive remediation Student's t test if the hypothesis of normality is respected, or Wilcoxon signed-rank test if the hypothesis of normality not respected; FAST, Functional Assessment Staging Test.

	Variable	Initial visit (N=40)	Final visit (N=32)	Change,	p-value ‡
		Mean (SD)	Mean (SD)	Mean (SD)
	FAST total	26.80 (12.07)	18.53 (9.63)	-6.78 (4.65)	<0.0001*
	FAST total, without head trauma	23.73 (10.95)	18.00 (10.24)	-5.73 (3.87)	<0.0001*
	FAST autonomy †	2.33 (2.79)	1.72 (3.12)	-0.31 (2.60)	0.03*
	FAST occupational functioning †	9.45 (5.67)	7.63 (6.21)	-1.53 (3.35)	0.006*
	FAST cognitive functioning †	8.23 (3.03)	4.19 (2.66)	-3.97 (2.04)	<0.0001*
	FAST financial issues †	0.73 (1.63)	0.41 (1.29)	-0.09 (0.69)	0.63
	FAST interpersonal relationships †	4.75 (3.85)	3.94 (3.39)	-0.47 (1.95)	0.14
	FAST leisure time †	1.25 (1.81)	0.66 (1.18)	-0.31 (1.47)	0.31
	†Normality hypothesis not respected;
	‡

Table 3 .

 3 Comparison of the cognitive functioning tests before and after cognitive remediation Student's t test if the hypothesis of normality is respected, or Wilcoxon signed-rank test if the hypothesis of normality not respected; WAIS-IV, Wechsler Adult Intelligence Scale, Fourth Edition; TMT, Trail Making Test; Information processing speed was measured using the WAIS-IV Coding and Symbol search subscales; Average of verbal fluency results for the "p" and "animals" subtests is presented; Corsi block-tapping test is part of Wechsler Memory Scale, Third Edition, used to evaluate visuospatial memory; Stroop color item score is part of the Stroop test.

	Variable	Initial visit (N=40)	Final visit (N=32)	Change,	p-value ‡
		Mean (SD)	Mean (SD)	Mean (SD)
	WAIS-IV coding †	62.95 (14.61)	70.52 (16.96)	7.55 (9.94)	<0.0001*
	WAIS-IV symbol searching †	33.78 (10.65)	34.87 (8.93)	0.32 (11.19)	0.09
	TMT A	36.40 (11.97)	32.97 (17.48)	-3.06 (12.42)	0.18
	TMT B †	91.70 (58.61)	87.23 (76.91)	-4.42 (64.92)	0.25
	Information processing speed †	55.30 (52.70)	54.26 (62.11)	-1.35 (61.48)	0.96
	Verbal fluency total	28.13 (7.00)	31.47 (5.80)	2.27 (4.12)	0.006*
	WAIS-IV forward digit span	8.25 (2.10)	9.61 (2.16)	1.35 (1.72)	0.0001*
	WAIS-IV backward digit span †	6.93 (1.59)	8.84 (2.41)	1.81 (1.94)	<0.0001*
	WAIS-IV sequential digit span †	6.98 (1.90)	8.19 (1.97)	1.29 (1.79)	0.0002*
	Corsi block-tapping test forward	7.13 (1.65)	8.81 (2.06)	1.77 (1.65)	<0.0001*
	Corsi block-tapping backward	6.65 (1.46)	7.68 (1.64)	1.10 (1.35)	<0.0001*
	Stroop color item	37.03 (9.28)	40.83 (8.35)	4.00 (5.40)	0.0003*
	†Normality hypothesis not respected;
	‡


F E Jurin

C C Buron

S Clément

A Mehdi

B Lakard

N Martin

C Filiâtre

Conductive multilayer film based on composite materials made of conjugated polyelectrolytes and inorganic particles

Keywords: PEDOT:PSS, Multilayer, Conjugated polyelectrolyte, Polythiophene, Inorganic colloidal particle, Conductivity, Composite film

Poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) conjugated polymer blend and a cationic poly(3-hexylthiophene) (P3HT)-based conjugated polyelectrolyte incorporating imidazolium ionic side groups were first adsorbed as monolayer on inorganic colloidal particles. At the plateau of adsorption isotherm, adsorbed amount of PEDOT:PSS on alumina particles appears lower than the adsorbed amount of P3HT on silica particles. Both polyelectrolytes strongly modify the surface charge of particles as reveals by zeta potential variation. Functionalized particles were then assembled using Layer-by-Layer method in order to prepare (PDDA/Al2O3-PEDOT:PSS)n and (SiO2-P3HT/PEDOT:PSS)n conductive multilayer film. Thicker PDDA/Al2O3-PEDOT:PSS films were elaborated when a drying step was added during the build-up. The same effect was observed when composite particle concentration was increased from 1 to 10 g.L -1 . Electrical behavior was completely modified in comparison with PDDA/PEDOT:PSS assembly. After heating at 150°C, no conductivity was detected due to visible cracks on SEM images. For SiO2-P3HT/PEDOT:PSS assembly, thickness is higher than for PDDA/Al2O3-PEDOT:PSS assembly using the same experimental

INTRODUCTION

Conjugated polymers are nowadays an attractive class of materials to prepare thin conductive films for a wide range of applications such as organic light emitting diodes, electrochromic devices, sensors, capacitors or solar cells [START_REF] Reynolds | Conjugated Polymers: Properties, Processing, and Applications[END_REF]. Aqueous dispersion of conjugated polymers have notably attracted considerable attention leading to the printing of flexible, transparent, and biocompatible conducting films, which can be useful for a wide range of applications including energy [START_REF] Colberts | Aqueous Nanoparticle Polymer Solar Cells: Effects of Surfactant Concentration and Processing on Device Performance[END_REF][START_REF] Xie | Overcoming efficiency and stability limits in water-processing nanoparticular organic photovoltaics by minimizing microstructure defects[END_REF][START_REF] Prunet | Aqueous PCDTBT:PC71BM Photovoltaic Inks Made by Nanoprecipitation[END_REF] or biomedical devices [START_REF] Inal | Organic electrochemical transistors based on PEDOT with different anionic polyelectrolyte dopants[END_REF][START_REF] Inal | Conjugated Polymers in Bioelectronics[END_REF][START_REF] Khodagholy | In vivo recordings of brain activity using organic transistors[END_REF][START_REF] Zeglio | Conjugated Polymers for Assessing and Controlling Biological Functions[END_REF]. Because of its high conductivity, environmental and electrochemical stability, water solubility, biocompatibility and transparency in the visible range [START_REF] Heywang | Poly(alkylenedioxythiophene)s-new, very stable conducting polymers[END_REF], the current material of choice for many applications is poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). [START_REF] Wen | Scientific Importance of Water-Processable PEDOT-PSS and Preparation, Challenge and New Application in Sensors of Its Film Electrode: A Review[END_REF][START_REF] Greco | Ultra-thin conductive free-standing PEDOT/PSS nanofilms[END_REF] More recently, π-conjugated polyelectrolytes (CPEs), have emerged as a promising alternative for elaborating aqueous dispersion of conjugated polymers since due to the presence of ionic side chains, they can be dissolved in aqueous media. Their intrinsic structure allow them to combine the physicochemical behavior of the polyelectrolytes with the optical and electronic properties of conjugated polymers [START_REF] Lee | Conjugated polyelectrolytes: A new class of semiconducting material for organic electronic devices[END_REF][START_REF] Jiang | Conjugated Polyelectrolytes: Synthesis, Photophysics, and Applications[END_REF]. Poly(3-hexylthiophene) (P3HT)-based conjugated polyelectrolytes have been in particular developed to elaborate organic conductive films finding applications in organic transistors [START_REF] Schmidt | Smaller Counter Cation for Higher Transconductance in Anionic Conjugated Polyelectrolytes[END_REF][START_REF] Shao | A water-soluble polythiophene for organic field-effect transistors[END_REF] or organic solar cells [START_REF] Chevrier | Self-assembled conjugated polyelectrolyte-surfactant complexes as efficient cathode interlayer materials for bulk heterojunction organic solar cells[END_REF][START_REF] Kesters | High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells[END_REF][START_REF] Seo | Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer[END_REF].

To prepare thin conductive films based on PEDOT:PSS conjugated polymer blend or CPEs, several conventional techniques are available: dip coating, spin coating or drop casting. To elaborate complex film architectures, the layer-by-layer (LbL) technique is particularly appropriate due to its simplicity, robustness, low cost and versatility [START_REF] Schönhoff | Self-assembled polyelectrolyte multilayers[END_REF][START_REF] Xiao | Layer-by-layer assembly of versatile nanoarchitectures with diverse dimensionality: a new perspective for rational construction of multilayer assemblies[END_REF][START_REF] Decher | Multilayers thin films[END_REF]. Using this technique, PEDOT:PSS can be associated with traditional polyelectrolytes such as poly(allylamine hydrochloride) (PAH), polydiallyldimethylammonium chloride (PDDA) or Polyethylenimine (PEI). The electrical conductivity of the resulting films can be tailored to the targeted applications but generally remains lower than pure PEDOT:PSS film due to the use of insulating polyelectrolyte. Considering multilayer films, conductivity mainly depends on the chemical parameters used during the build-up process if no post treatment is applied.

To improve conductivity, which is an essential parameter for the performance of the organic devices, insulating polyelectrolytes can be replaced by CPEs or metal nanoparticles.

McCullough et al. have notably described the LbL assembly of anionic and cationic regioregular polythiophene-based CPEs leading to conductivity around 0.04 S.cm -1 [START_REF] Zhai | Layer-by-Layer Assembly of Polythiophene[END_REF].

More

Recently showing significant long-term stability and good photovoltaic performance [START_REF] Rider | Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface[END_REF][START_REF] Worfolk | Work Function Control of Interfacial Buffer Layers for Efficient and Air-Stable Inverted Low-Bandgap Organic Photovoltaics[END_REF]. Metallic silver nanoparticles were also combined with PEDOT:PSS leading to a conductive hybrid polyelectrolyte multilayer with an ordered architecture [START_REF] Knowles | Layer-by-Layer Assembled Multilayers of Polyethylenimine-Stabilized Platinum Nanoparticles and PEDOT:PSS as Anodes for the Methanol Oxidation Reaction[END_REF].

Exploiting metallic nanoparticles for LbL assembly represents a promising approach since in the colloidal dimensions, the total surface developed by one gram of particles, named specific area, is generally more than 100 m 2 . Consequently, particles can be used to adsorb a large amount of CPEs prior to their self-assembly by LbL technique with another polyelectrolyte leading to produce conductive hybrid multilayer film.

Based on this approach, we decided to investigate the assemblies of functionalized silica or alumina colloidal particles with PEDOT:PSS and an imidazolium-based polythiophene CPE (named here P3HT). Adsorption isotherms of PEDOT:PSS on alumina particles and P3HT on silica particles were first determined in order to maximize the adsorbed amount of conjugated polymers on particles. Composite Al2O3-PEDOT:PSS was then assembled with P3HT and SiO2-P3HT with PEDOT:PSS to prepare multilayer film. Effect of several parameters (composite particle and polymer concentrations, number of deposited bilayers, rinsing and drying steps) on film thickness and conductivity was investigated. Finally, scanning electron microscopy (SEM) was carried out to characterize the film morphology and to understand their electrical conductivity behavior.

EXPERIMENTAL Chemicals, solutions and substrates

Powder of alumina particles with an average particle size of 50 nm (TEM analysis provided by the supplier) were purchased from Sigma-Aldrich. Silica nanoparticles Bindzil 30/220 (30 wt.% SiO2 suspension in water, 20 nm average particle size) were purchased from Eka Chemicals AB. Particles were functionalized by adsorption of conjugated polyelectrolytes.

The anionic conducting polyelectrolyte poly(3,4-ethylenedioxythiophene) -poly(styrene sulfonate), PEDOT:PSS (Clevios PH) was purchased from Heraeus. Poly(3-[N-(1methylimidazolium-3-yl)hexyl]-thiophene-2,5-diyl bromide)), P3HT (Mn ~ 13 500 g.mol -1 , Ð = 1.27) was used as cationic polyelectrolyte and was synthesized as previously described [START_REF] Clement | Synthesis and characterisation of [small pi]-conjugated polymer/silica hybrids containing regioregular ionic polythiophenes[END_REF].

Poly(diallyldimethylammonium chloride), PDDA (Sigma-Aldrich, Mw 100 000-200 000 g.mol -1 ) was also used to build-up multilayer films. Other chemicals were obtained from Sigma-Aldrich and were used without further purification.

Polyelectrolyte and particles solutions were prepared in ultrapure water (Milli-Q Plus, Millipore, resistivity > 18 MΩ.cm). Polyelectrolyte concentrations were ranged from 0.25 g.L -1 to 1 g.L -1 and ionic strength was fixed at 3.10 -3 M using NaCl. Particles were dispersed in 3.10 -3 M NaCl solutions up to desired concentrations. pH of solutions was adjusted by addition of HCl or NaOH solution.

Substrates were oxidized silicon wafers (105 ± 0.5 nm of SiO2) purchased from ACM (France) and cleaned, before experiments, by using Piranha solution (3:1 volume ratio of concentrated H2SO4 (98 %) and H2O2 (30 %)) for 15 min, followed by rinsing with ultrapure water and dried under a nitrogen flow.

Polyelectrolyte adsorption on particles

To determine adsorption isotherm, series of PEDOT:PSS solutions in 3.10 -3 M NaCl at pH 3 and pH 5.5 with an initial concentration range between 50 and 1150 mg.L -1 were prepared.

Alumina particles were then added in each solution to reach a particle concentration equal to 10 g.L -1 . Adsorption of PEDOT:PSS was carried out during 4h or 22h.

For adsorption of P3HT on silica particles, polyelectrolyte solutions were prepared with an initial concentration range between 25 and 500 mg.L -1 in 3.10 -3 M NaCl at pH 9. Silica particle concentration was then adjusted at 1 g.L -1 and solutions were stirred during 4h.

Composite particles

Alumina particles coated by PEDOT:PSS, denoted Al2O3-PEDOT:PSS, were obtained from an alumina suspension at 10 g.L -1 (20 min in the ultrasonic bath) and a PEDOT:PSS concentration equal to 750 mg.L -1 in NaCl 3.10 -3 M. The pH of the solution was adjusted to 3 and the suspension was stirred during 4h followed by a centrifugation step (20000 rpm -10 min) to remove the excess of unadsorbed polyelectrolyte. The composite particles were dispersed in NaCl 3.10 -3 M using sonication and the pH of the suspension was adjusted to 7.

Silica particles coated by P3HT, denoted SiO2-P3HT, were prepared using a silica suspension at 1 g.L -1 and a P3HT concentration equal to 100 mg.L -1 in NaCl 3.10 -3 M. The pH of the solution was adjusted to 9 and the suspension was stirred during 4h. Afterward, the pH of SiO2-P3HT particles solution was adjusted to 3.

Fabrication of LbL films

Multilayer films were built-up by dip-coating and started by dipping substrates for 5 min into the solution containing cationic polyelectrolyte (PDDA) (or positive composite particles, SiO2-P3HT). Adsorption of the first polyelectrolyte layer was followed by a rinsing step of 30 s. Substrates were then dipped in a Al2O3-PEDOT:PSS (or PEDOT:PSS) solution for 5 min and rinsed during 30 s. This procedure was repeated n times to obtain a film of n bilayers referred to as (PDDA/Al2O3-PEDOT:PSS)n or (SiO2-P3HT/PEDOT:PSS)n LbL assembly.

The influence of a drying step before each new adsorption step was also analyzed by using a flow of warm air (around 60°C) during 10 s.

Characterization of particles and LbL films

Film thickness was measured with a Dektak 150 surface profiler (VEECO) equipped with a diamond stylus having a 12.5 µm tip radius. Zeta potential and average particle size were measured using a Zetasizer NanoZS from Malvern Instruments. A Shimadzu TOC-L analyzer was used to determine adsorption isotherm of polymer onto particles. Conductivity of composite multilayer films was measured using the van der Pauw four point probe technique [START_REF] Van Der Pauw | A method of measuring the resistivity and Hall effect of discs of arbitrary shape[END_REF][START_REF] Schroder | Semiconductor Material and Device Characterization[END_REF][START_REF] Blood | The Electrical Characterization of Semiconductors: Majority Carriers and Electron States[END_REF]. The conductivity values of LbL films were obtained using an upward temperature ramp from 30 to 150°C followed by a downward temperature ramp from 150 to 30°C (2°C.min -1 ). The temperature at 30, 50, 100 and 150°C was maintained during 10 min. The uncertainty about conductivity measurement is around 10%. Finally, surface morphology was investigated using scanning electron microscope, Quanta 450 W from FEI equipped with EDS EDAX APEX 2i for elemental analysis.

RESULTS AND DISCUSSION

Formation of Al2O3-PEDOT:PSS and SiO2-P3HT composite particles

The hydrodynamic diameter of alumina particles was first measured as a function of pH in Amphoteric Al-OH groups are present on the surface of Al2O3 particles dispersed in aqueous solution. As such, the surface of Al2O3 particles is positively charged in aqueous solutions at a pH below the point of zero charge (pzc), which was found to be around pH = 9 (Figure 2a).

At pH = 5, zeta potential is very high and the electrostatic repulsion forces between particles is close to its maximum. When the pH increases, the size of the particles increases until reaching a maximum size (around 2 µm) at pH 8, which is close to pzc. By moving away from the pzc to more basic pH's, the amphoteric groups become negatively charged leading to an increase of electrostatic repulsion forces, and consequently, a decrease of particles size.

Finally, it can be noticed that, in suspension, the size is higher than in the powder (50 nm) meaning that Al2O3 particles are aggregated. Indeed, during the dispersion process, van der Waals forces are too strong in the powder to overcome repulsive electrostatic forces if even these repulsive forces reach their maximum values for pH far from pzc.

For silica particles, silanol (Si-OH) surface groups in aqueous media have a pzc around pH = 2 (Figure 2b). Therefore, above this pH, surface of SiO2 nanoparticles is negatively charged.

A negative zeta potential was obtained for all investigated pH values with the magnitude increasing from a value of -7 mV at pH = 2.3 to a value of about -30 mV above pH = 7. In the range of investigated pH, silica particle size distribution is almost the same and the hydrodynamic diameter is around 24 ± 5 nm. Adsorption isotherms of PEDOT:PSS on alumina and P3HT on silica were carried out in order to determine the maximum adsorbed amount of each polyelectrolyte. Adsorption isotherm of PEDOT:PSS was performed with adsorption times equal to 4h or 22h. By comparing isotherms, the amount of adsorbed polymer was found to be similar which means that the equilibrium state is reached after 4h (See supplementary material). Equilibrium isotherms were then carried out at pH = 3 and pH = 5.5 where alumina particles are positively charged and PEDOT:PSS is negatively charged. The results (Inset of Figure 3a) show that the amount of adsorbed polyelectrolyte on the surface of particles depends on pH of suspension as it was shown in other studies [START_REF] Shin | Dispersion of aqueous nano-sized alumina suspensions using cationic polyelectrolyte[END_REF]. At pH = 3, the maximum amount of adsorbed polymer at the plateau is equal to 75 mg.g -1 while this amount is reduced to 40 mg.g -1 at pH 5.5.

Similar decrease in the amount of adsorbed polymer as a function of pH was observed during the adsorption of poly(acrylic acid sodium salt) (PAA) on alumina particles [START_REF] Bertrand | Influence of slurry characteristics on the morphology of spray-dried alumina powders[END_REF]. As shown previously, there are differences in terms of size and zeta potential of alumina particles at pH = 3 and pH = 5.5. The differences in diameter and zeta potential (250 nm and +34 mV at pH = 3 vs. 400 nm and +24 mV at pH 5.5) indicates that alumina particles in suspension at pH = 3 possess a higher specific surface area and that stronger attractive electrostatic interactions take place between polyelectrolyte and particles, which increase the amount of adsorbed polymer.

The maximum amount of PEDOT:PSS (75 mg.g -1 ) is reached for an initial polyelectrolyte concentration of about 750 mg.g -1 , which corresponds to an equilibrium PEDOT:PSS concentration equal to 40 mg.g -1 (Figure 3a). The adsorption isotherm of PEDOT:PSS doesn't correspond to the criteria to be fitted by the Langmuir model. However, the adsorption isotherm shape seems to be a high affinity adsorption isotherm [START_REF] Butt | Physics and Chemistry of Interfaces[END_REF]. The amount of adsorbed PEDOT:PSS vs. PEDOT:PSS concentration initially introduced is depicted in Figure 4a and the red curve represents 100 % of adsorbed polymer. At pH = 3, experimental points follow this straight line up to an initial PEDOT:PSS concentration of 750 mg.L -1 , which corresponds to the beginning of the adsorption plateau. The affinity between the polyelectrolyte and the particle is so strong that the concentration of free polyelectrolyte is close to 0 until the adsorption plateau is reached. At pH = 5.5, the total amount of polyelectrolyte initially introduced is adsorbed for polyelectrolyte concentration below 400 mg.L -1 . Adsorption isotherms of anionic polyelectrolytes such as PAA, poly(styrene sulfonate) (PSS) or poly(methacrylic acid) (PMAA) on alumina particles were proved to be also isotherms of high affinity [START_REF] Bertrand | Influence of slurry characteristics on the morphology of spray-dried alumina powders[END_REF][START_REF] Blokhus | Adsorption of poly(styrene sulfonate) of different molecular weights on α-alumina: Effect of added sodium dodecyl sulfate[END_REF][START_REF] Pettersson | Electrosteric Stabilization of Al2O3, ZrO2, and 3Y-ZrO2 Suspensions: Effect of Dissociation and Type of Polyelectrolyte[END_REF].

Adsorption isotherm of P3HT on silica particles at pH = 9 is shown in Figure 3b. The maximum amount of adsorbed P3HT is about 200 mg.L -1 . This amount is reached at an initial polyelectrolyte concentration of about 500 mg.L -1 which corresponds to an equilibrium P3HT concentration equal to 200 mg.L -1 . Adsorption data were fitted to the Langmuir monolayer adsorption isotherm (Figure 4b). From the linear fit of the Langmuir model, Γmax, K and b were calculated and were equal to 212 mg.g -1 , 7.36 mg.L -1 and 0.136 L.mg -1 , respectively.

According to Hall et al. [START_REF] Hall | Pore-and Solid-Diffusion Kinetics in Fixed-Bed Adsorption under Constant-Pattern Conditions[END_REF], the main parameters of the Langmuir isotherm can be expressed by a dimensionless equilibrium parameter RL, which is defined by the following relation:

= 1 1 + ( . )
with RL the equilibrium parameter, b the Langmuir constant (L.mg -1 ) and C0 the initial concentration (mg.L -1 ). The RL parameter indicates whether adsorption is rather unfavorable (RL> 1), linear (RL = 1), favorable (0> RL> 1) or irreversible (RL = 0). For P3HT adsorption, the initial concentrations are between 25 and 500 mg.L -1 and Langmuir constant b is 0.136 μg.mL -1 . Under these conditions, RL ranges between 0.015 and 0.227, which means that the adsorption of P3HT on silica particles is favorable. In order to investigate the effect of polyelectrolyte adsorption on the electrophoretic properties of Al2O3 or SiO2, functionalized particles were prepared. Initial polymer concentration was fixed at a value corresponding to the beginning of the isotherm plateau (750 mg.L -1 for PEDOT:PSS and 100 mg.L -1 for P3HT). However, sedimentation of composite Al2O3-PEDOT:PSS particles was clearly visible at pH = 3 while Al2O3 suspension was stable at the same pH. This was also observed for SiO2-P3HT suspension at pH = 9.

These basic observations indicate a modification of the surface charge of particles. Therefore, zeta potentials of composite polyelectrolyte-inorganic particles were measured as a function of pH. For Al2O3-PEDOT:PSS (Figure 2a), variation of zeta potential is completely different.

An isoelectric point (iep) appears at pH = 3 which also attests the adsorption of PEDOT:PSS on alumina particles. Starting from this pH, composite particles are negatively charged. In case of SiO2-P3HT composite particles, an iep is noticed as well. Due to adsorption of a cationic polyelectrolyte, iep is shifted towards a basic pH equal to 9 (Figure 2b). For Al2O3-PEDOT:PSS suspension, in order to avoid a partial desorption of PEDOT: PSS from alumina surface and to keep composite particles with enough negative charges for layer-by-layer adsorption, pH was fixed at pH 7. For SiO2-P3HT suspension, pH of suspensions was adjusted to maximize zeta potential, i.e. to increase electrostatic repulsive interactions between particles to avoid particle aggregation. Consequently, LbL assemblies were carried out with SiO2-P3HT suspension at pH = 3.

Polyelectrolyte/composite particle film: influence of build-up parameters.

(PDDA/Al2O3-PEDOT:PSS)n and (SiO2-P3HT/PEDOT:PSS)n multilayer films were built-up at polyelectrolyte and composite particle concentrations equal to 1 g.L -1 with or without a rinsing/drying step between each polymer adsorption. Thickness variations of (PDDA/Al2O3-PEDOT:PSS)10 film measured by profilometry are summarized in Table 1. Table 1 : Thickness of (PDDA/Al2O3-PEDOT:PSS)n and (SiO2-P3HT/PEDOT:PSS)n multilayer film versus build-up parameters. CPDDA 1 g.L -1 , CParticles 1 g.L -1 , CPEDOT:PSS 1 g.L -1 CNaCl 3.10 -3 M.

Assemblies n Build-up parameters

When a rinsing step is performed after polyelectrolyte adsorption, the average thickness of the film decreases for both types of LbL assemblies. This result is entirely consistent since the main objective of the rinsing step is to remove weakly adsorbed species (polymer chains or particles) adsorbed on the surface. The drying step seems to be essential for obtaining thicker films. Without this step, the thickness of the film does not exceed one hundred nanometers, which suggests that the particles are not incorporated in the films. Drying improves the adhesion of particles on the surface. Since composite particles remain on the surface, the chains of PDDA can be adsorbed thanks to the excess of negative charges brought by PEDOT:PSS. Construction of (PDDA/Al2O3-PEDOT:PSS)10 multilayer films requires then a drying step after each adsorption to exceed more than one hundred nanometers.

(SiO2-P3HT/PEDOT:PSS)n multilayer films are much thicker than (PDDA/Al2O3-PEDOT:PSS)n multilayer films obtained in the same conditions (Table 1). Indeed, for 10 adsorbed bilayers, thickness of multilayer films composed of composite alumina particles is approximately 600 nm whereas it exceeds 1.6 μm for multilayer films composed of composite silica particles. Use of silica nanoparticles (60 nm) seems to favor the growth of the multilayer films of (SiO2-P3HT/PEDOT:PSS)n. However, the particle size is not the only parameter that affects the thickness. Indeed, for multilayer films build up without particles, optical reflectometry analyses have demonstrated that the adsorbed amount of (P3HT/PEDOT:PSS)n is higher than the adsorbed amount of (PDDA/PEDOT:PSS)n. [START_REF] Jurin | Flexible and conductive multilayer films based on the assembly of PEDOT:PSS and water soluble polythiophenes[END_REF].

Polyelectrolyte/composite particle film: influence of polymer and particle concentrations

Concentration of composite particles is a major parameter that can affect the growth of a multilayer film. Indeed, when concentration of composite alumina particles increases, film thickness also increases even if a rinsing step is added during build-up (Figure 5). With an assembly of only 10 bilayers, the thickness of (PDDA/Al2O3-PEDOT:PSS)n films can reach 7 μm for a particle concentration of 10 g.L -1 while it is of about 500 nm at a concentration of 1 g.L -1 . Film prepared with the same experimental conditions and made of 10 bilayers, but without particles, has a thickness of about 500 nm. These results clearly indicate that using particles as conductive polymer carriers promotes the growth of multilayer films.

By introducing particles or not, multilayer film thickness can be tuned from a few tens of nanometers (polymers assembly) to a few tens of micrometers (polymer/particle assembly).

Finally, influence of the PDDA concentration was studied. Indeed, in a previous study [START_REF] Jurin | Electrical conductivity enhancement and wettability modification of (PDDA/PEDOT:PSS)n multilayer film[END_REF],

we showed that thickness of polyelectrolyte multilayer film decreases when concentration of PDDA is reduced. Therefore, the construction of multilayer films (PDDA/Al2O3-PEDOT:PSS)n was made with solutions containing different concentrations of PDDA: 0.25, 0.5 and 1 g.L -1 . Film thicknesses are summarized in Table 2. As for (PDDA/PEDOT:PSS)n multilayer films, the smaller the concentration of PDDA, the smaller the film thickness.

Indeed, thickness is equal to about 7 μm at a PDDA concentration of 1 g.L -1 whereas it is reduced to about 5 μm at a PDDA concentration of 0.25 g.L -1 . This decrease in thickness can be explained by a decrease in the amount of adsorbed cationic PDDA which in turn limits the adsorption of the anionic composite particles.

Film electrical conductivity and structural characterization

Electrical conductivity of (PDDA/Al2O3-PEDOT:PSS)n composite multilayer films was then determined by the van der Pauw method. Electrical conductivity was measured only for films prepared from particle suspensions concentration at 10 g.L -1 . It can then be assumed that below this value, the distribution of the composite particles is not compact and enough homogeneous to ensure an electrical percolation between PEDOT oligomers. Conductivity of the films was characterized at different temperatures according to the protocol described in the experimental section. Results reveal a slight increase of conductivity as a function of temperature up to values between 100 and 150°C (Figure 6). After this step, conductivity of (PDDA/Al2O3-PEDOT:PSS)10 films decreases and when the temperature is returned to 30°C, no conductivity is detected. Concentration of PDDA appears to modify conductivity: the smaller the PDDA concentration, the greater the conductivity. For example, conductivity is 0.094 S.m -1 for a film built up without a rinsing step at a PDDA concentration of 1 g.L -1 while it is doubled when PDDA concentration is 0.25 g.L -1 (0.192 S.m -1 ). Similar observations have already been done for multilayer films without colloidal particles [START_REF] Jurin | Electrical conductivity enhancement and wettability modification of (PDDA/PEDOT:PSS)n multilayer film[END_REF]. Indeed, when PDDA concentration is decreased from 1 g.L -1 to 0.25 g.L -1 during the preparation of a multilayer film made of 50 bilayers of PDDA/PEDOT:PSS, conductivity increases from 0.072 S.m -1 to 0.163 S.m -1 at 30°C. Decreasing the amount of insulating polymers embedded into the film leads to an increase in electrical conductivity of both the multilayer (polymer/polymer) and

(polymer/composite particles) films.

Concerning the effect of the number of bilayers, only 5 adsorbed bilayers are necessary to obtain a conductive (PDDA/Al2O3-PEDOT:PSS) film while no conductivity was detected for (PDDA/PEDOT:PSS) assemblies below 30 bilayers in the same experimental conditions.

Moreover, when a few numbers of bilayers are deposited, electrical conductivity of the multilayer films is improved. Indeed, conductivity of (PDDA/Al2O3-PEDOT:PSS)5 is approximately 0.400 S.m -1 whereas conductivity is about 0.200 S.m -1 when the number of bilayers is doubled. In the same time, thickness is multiplied by two between a film of 5 and 10 bilayers (Table 2). For multilayer films without particles, when 30 bilayers of PDDA/PEDOT:PSS are adsorbed, conductivity at 30°C is equal to 0.119 S.m -1 and decreases to 0.072 S.m -1 for 50 bilayers for a PDDA concentration of 1g.L -1 [START_REF] Jurin | Electrical conductivity enhancement and wettability modification of (PDDA/PEDOT:PSS)n multilayer film[END_REF]. However multilayer film conductivity usually increases for thin film until it reaches a plateau for thicker film depending on PEDOT:PSS weight ratio [START_REF] Smith | Layer-by-Layer Assembly of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)[END_REF]. In the present work, two explanations can be proposed. First, using PDDA polyelectrolyte, a progressive increase of insulating polymer into the polymeric matrix leads to a decrease of conductivity. At the opposite, this may also suggest that when the conductivity is measured by the van der Pauw method, only the upper part of the film participates in the conductivity phenomenon. Indeed, in the equation proposed by van der Pauw, resistivity depends on film thickness and measured electrical resistances.

Investigations are in progress to clearly identify the major factor involved in conductivity decrease.

Elemental analysis and morphology observations of the multilayer films were then carried out. First, EDX analysis confirms the presence of composite alumina particles within the multilayer films since it reveals the presence of oxygen, aluminum and sulfur elements which refers to the alumina particles and PEDOT:PSS chains (See supplementary material).

Observations of (PDDA/Al2O3-PEDOT:PSS)10 films by scanning electron microscopy (Figure 7) show slight cracks on the multilayer films prepared with a PDDA concentration of 0.5 and 1 g.L -1 . Moreover, after a thermal cycle with a maximum temperature of 150°C (Image d), the cracks appear more numerous and wider. No cracks were observed for a PDDA concentration of 0.25 g.L -1 before and after heating. When concentration of PDDA is high, multilayer films are thicker. During drying step, internal stresses in the multilayer film are intensified, which cause a widening of the cracks already present and/or create new ones. Heating process accentuates crack formation that explains why films are no longer conductive.

Multilayer films made of SiO2-P3HT composite particles were also characterized at 30 °C using the van der Pauw method. Although these films seem to be conductive (measurements with a multimeter), measurements by van der Pauw method have shown that the ohmic character of the sample contacts is not respected (no significant linear interpolation of I-V measurements). So, these measurements do not present the validity criteria. SEM investigations have revealed that composite particles modify the structure of the multilayer films. (SiO2-P3HT/PEDOT:PSS)10 films made with rinsing steps does not show visible cracks at this magnification but the surface is not homogeneous (Figure 8a). Therefore, films made of 20 bilayers of (SiO2-P3HT/PEDOT:PSS) were built-up. Unfortunately, cracks were visible on these films due to the thickness increase. In addition, multilayer films made without a rinsing step (Figure 8b) exhibit large cracks. As for multilayer films composed of composite alumina particles, the presence of cracks causes a poor circulation of electrons which does not allow multilayer films (SiO2-P3HT/PEDOT:PSS) to be conductive.

CONCLUSIONS

LbL films were built-up by assembly of conjugated polymers and composite polymerinorganic particles. Particles were used as carriers of conducting polymers in the aim to modulate the thickness and to increase the amount of conducting polymers into the films.

Alumina and silica particles were functionalized with PEDOT:PSS conjugated polymer blend and an ionic-based 3HT, respectively. The best conditions for the adsorption of polyelectrolytes on particles were first determined to ensure a stable suspension. Thickness of (PDDA/Al2O3-PEDOT:PSS)n films varies from about 70 nm to 7 µm depending on parameters used during the LbL assembly: composite particle and insulating polymer concentrations, number of deposited bilayers, rinsing and drying steps. Electrical conductivity was only measured for the highest concentration of composite alumina particles (10 g.L -1 ).

Hybrid films made of 5 bilayers reach a conductivity of 0.4 S.m -1 while 30 bilayers of (PDDA/PEDOT:PSS) are necessary to obtain the same value. So conducting polymer supported by particles allows to modulate the film thickness and to reduce the number of dipping steps and so the time to built-up LbL conductive films. However, cracks are observed after heating (PDDA/Al2O3-PEDOT:PSS)n films at 150°C and no conductivity is then detected. Films made of (SiO2-P3HT/PEDOT:PSS)n are not conductive even though two conductive polyelectrolytes compose the film.

Consequently, the use of particle-supported polyelectrolyte during the growth of polyelectrolyte multilayer film is an interesting strategy to decrease the steps in film production process which is usually time and products consuming. Moreover the conductivity and the thickness of PEDOT:PSS-based thin films can be optimized. Nevertheless, mechanical properties of these composite films are strongly impacted that could be an interesting way to elaborate temperature sensor that provide an irreversible evidence of a high-temperature deviation.

3. 10 - 3 M

 103 aqueous NaCl solution by dynamic light-scattering (DLS) measurements using the Zetasizer Nano ZS. As shown in Figure1, the alumina particles reach their smaller size below pH = 5 with an average particle size around 250 nm at pH = 3 or pH = 11.

Figure 1 :

 1 Figure 1: Size distribution of alumina particles vs. pH of suspension (NaCl 3.10 -3 M).

Figure 2 :

 2 Figure 2 : Zeta potential vs. pH of Al2O3 and Al2O3-PEDOT:PSS particles (a) and SiO2 and SiO2-P3HT particles (b).

Figure 3 :

 3 Figure 3 : Adsorption isotherms of PEDOT:PSS on Al2O3 at pH = 3 and pH = 5.5 (a) and P3HT on SiO2 at pH = 9 (b).Contact time: 4 hours at RT.

Figure 4 :

 4 Figure 4 : Adsorbed amount of PEDOT:PSS on Al2O3 as a function of initial concentration at pH = 3 and pH = 5.5 (a).Plots of Langmuir adsorption isotherm for P3HT-SiO2 system (b).

Figure 5 :

 5 Figure 5 : Effect of composite particle concentration on (PDDA/Al2O3-PEDOT:PSS)10 film thickness with (R) or without a rinsing step (No R).

Figure 6 :

 6 Figure 6 : Electrical conductivity of (PDDA/Al2O3-PEDOT:PSS)10 at different temperatures.The conductivity of multilayer (PDDA/Al2O3-PEDOT:PSS)n films appears to be temperature

Figure 7 :

 7 Figure 7 : SEM images of (PDDA/Al2O3-PEDOT:PSS)10 films built-up at two PDDA concentration: 0.25 g.L -1 and 1 g.L -1 before (a, b) and after heating at 150°C (c, d). Cparticles = 10 g.L -1 .

Figure 8 :

 8 Figure 8 : SEM images of (SiO2-P3HT/PEDOT:PSS)n films built-up with rinsing step (a, b) or without rinsing step (c, d). Cparticles 1 g.L -1

Table 2 )

 2 . Rinsing step after each adsorption induces a slight decrease of conductivity compared to those obtained without rinsing. Weakly adsorbed polyelectrolytes which are removed during rinsing seem to not participate in conductivity phenomenon.

	PDDA concentration	Number of	Rinsing	Thickness	Conductivity
	(g.L -1 )	bilayers N	step	(nm)	(S.m -1 )
	1	10	-R	7095 6826	0.094 0.069
	0.5	10	-R	6428 5278	0.106 0.105
	0.25	10	-R	5169 4329	0.192 0.123
	0.25	5	-R	3122 2868	0.481 0.441

Table 2 :

 2 Thickness and conductivity of (PDDA/Al2O3-PEDOT:PSS)n multilayer. Ccomposite particles 10 g.L -1 .

ACKNOWLEDGEMENTS

This research was funded by the Conseil Régional de Bourgogne Franche-Comté (PhD grant).

This work was partly supported by the French RENATECH network and its FEMTO-ST technological facility. The authors also thank the CNRS and the University of Montpellier for financial support.