Cedric Gil-Jardine
email: cedric.gil-jardine@chu-bordeaux.fr

MD Samantha Al Joboory

Saint Jammes Juliane Tortes

Saint Jammes

MD d Guillaume Durand

MD Romain Brunschwig

MD a , Éric Tellier MD PhD Pierre Catoire

Eric Tellier

Regis Ribereau-Gayon

MD PhD Michel Galinski

MD PhD Louis-Rachid Salmi

MD PhD Cédric Gil-Jardiné

MD Régis Ribéreau-Gayon

MD Philippe Revel

MD Guillaume Valdenaire

MD PhD Karim Tazarourte

Emmanuel Lagarde

Prevention of post-concussion-like symptoms in emergency room patients: Results from a two-center randomized controlled study comparing an early single-session Eye Movement Desensitization and Reprocessing intervention with usual care

Keywords: Stress, emergency room, Eye Movement Desensitization and Reprocessing, postconcussion-like symptoms, post-traumatic stress disorder, clinical trial

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

Background

In 2012, the most recent national survey in France revealed that 10.6 million people came or were taken to the emergency room (ER), sometimes on several occasions, as 18 million visits were recorded that year. Although more than 80% of individuals attending the ER leave within a few hours without hospitalization, 1,2 recent studies [START_REF] De Leon | Baseline predictors of fatigue 1 year after mild head injury[END_REF][START_REF] Friedland | Function after motor vehicle accidents: a prospective study of mild head injury and posttraumatic stress[END_REF][START_REF] Mclean | Health status, not head injury, predicts concussion symptoms after minor injury[END_REF][START_REF] Stovner | Headache after concussion[END_REF] have consistently documented that 10-20% of injured patients will suffer for several months from very diverse symptoms after the event and that this may lead to a potentially significant decline in their quality of life. This decline could delay or prevent the resumption of school or work activities and also change social and family relationships. Each year in France, approximately 2 million people are confronted by difficulties of varying degrees, but the causes are often unidentified and may be unrelated to the traumatic event. This relationship remains difficult to understand because these symptoms, including headaches, concentration disorders, memory problems, stress intolerance, personality change, and irritability, appear to be non-specific. Such symptoms have been described for more than 50 years in association with head trauma, and in this context, are referred to as post-concussion syndrome (PCS). However, it is now accepted that these symptoms are not specific to head injuries and can also occur in other patients who visit the ER, [START_REF] Mclean | Health status, not head injury, predicts concussion symptoms after minor injury[END_REF][START_REF] Smith-Seemiller | Presence of post-concussion syndrome symptoms in patients with chronic pain vs mild traumatic brain injury[END_REF][START_REF] Laborey | Specificity of postconcussion symptoms at 3 months after mild traumatic brain injury: results from a comparative cohort study[END_REF] which greatly expands the size of the affected population. In a cross-sectional observational study of 31,958 high school athletes, Iverson et al. [START_REF] Iverson | Factors Associated With Concussion-like Symptom Reporting in High School Athletes[END_REF] found that 19% of uninjured boys and 28% of uninjured girls reported having a symptom burden that resembled a diagnosis of PCS based on the International Classification of Diseases, 10th Revision (ICD-10); [START_REF] Iverson | Factors Associated With Concussion-like Symptom Reporting in High School Athletes[END_REF] subsequently, this diagnosis has frequently been described as post-concussion-like symptoms (PCLS).

The symptoms of PCLS are very similar to, and sometimes exactly the same as, two previously published dimensions of post-traumatic stress disorder (PTSD), i.e., hyperactivation of the nervous system and cognitive and emotional numbing. Thus, most researchers have hypothesized that PCLS and PTSD share, at least in part, the same causal pathway in which stress plays a key role. [START_REF] Laborey | Specificity of postconcussion symptoms at 3 months after mild traumatic brain injury: results from a comparative cohort study[END_REF][START_REF] Lagarde | Association of symptoms following mild traumatic brain injury with posttraumatic stress disorder vs. postconcussion syndrome[END_REF][START_REF] Gil-Jardiné | Stress and lasting symptoms following injury: Results from a 4-month cohort of trauma patients recruited at the emergency department[END_REF] This would be particularly relevant for prevention because only studies that have specifically investigated PTSD are sufficient in number and quality to identify credible modes of intervention. [START_REF] Bryant | Post-traumatic stress disorder vs traumatic brain injury[END_REF] This led our research group to consider using stress management interventions in the ER in the hope of improving outcomes for traumatized patients. To date, the psychotherapeutic intervention that has proven superior to all other methods for the prevention and treatment of PTSD is Eye Movement Desensitization and Reprocessing (EMDR). [START_REF] Shapiro | Eye Movement Desensitization and Reprocessing (EMDR): Basic Principles, Protocols, and Procedures[END_REF][START_REF] Bradley | Potentially inappropriate prescribing and cost outcomes for older people: a cross-sectional study using the Northern Ireland Enhanced Prescribing Database[END_REF][START_REF] Seidler | Comparing the efficacy of EMDR and trauma-focused cognitive-behavioral therapy in the treatment of PTSD: a meta-analytic study[END_REF][START_REF] Bisson | Psychological therapies for chronic post-traumatic stress disorder (PTSD) in adults[END_REF][START_REF] Tarquinio | Early Psychological Preventive Intervention For Workplace Violence: A Randomized Controlled Explorative and Comparative Study Between EMDR-Recent Event and Critical Incident Stress Debriefing[END_REF] In particular, a brief single trauma-focused EMDR protocol, the Recent Traumatic Episode Protocol (R-TEP) method, [START_REF] Shapiro | Early EMDR Intervention (EEI): A Summary, a Theoretical Model, and the Recent Traumatic Episode Protocol (R-TEP)[END_REF] was developed and can be used in the context of the ER. EMDR is based on alternated bilateral stimulation that could be conducted using visual (eyes movements) or sensitive (taping) stimulation.

Our research group tested this method in a randomized open-label single-center pilot study of 130 patients with a high risk of PCLS that was conducted in the ER of Bordeaux University Hospital. The patients were randomized into three groups: a 15-minute reassurance session, a 60-minute session of EMDR, and usual care. The proportions of patients with PCLS at 3 months were 18%, 37%, and 65% in the EMDR, reassurance, and control groups, respectively. [START_REF] Gil-Jardiné | Emergency room intervention to prevent post concussion-like symptoms and post-traumatic stress disorder. A pilot randomized controlled study of a brief eye movement desensitization and reprocessing intervention versus reassurance or usual care[END_REF] The present study was designed to replicate this trial with greater statistical power using patients from two sites.

Methods

Study Design

The study population and design of the SymptOms Following Trauma Emergency Response 3 (SOFTER 3) trial have been previously published. [START_REF] Gil-Jardiné | Prevention of post-concussionlike symptoms in patients presenting at the emergency room, early single eye movement desensitization, and reprocessing intervention versus usual care: study protocol for a two-center randomized controlled trial[END_REF] Briefly, this was a two-center open-label randomized controlled trial designed to assess the effects of an early EMDR R-TEP session on PCLS at 3 months compared with those of usual care in patients who presented to the ER.

The secondary objectives included comparisons between the EMDR R-TEP and control groups regarding PTSD at 3 months, self-reported stress at ER discharge, self-assessed recovery expectations at discharge and 3 months, and self-reported pain levels at discharge and 3 months.

Sites and Patients

All patients who came or were brought to the adult ER at one of the study sites following an event that led to an injury or with a new acute medical condition were included in the present study. The inclusion criteria were as follows: ≥18 years of age, conscious, able to provide informed consent, affiliated with social security, and able to understand the study procedures and to comply with them for the entire length of the study; only French speakers were enrolled in the study. Whatever the cause of injury, the event must have occurred in the past 24 hours. Patients who attended the ER for medical reasons were eligible if their condition was acute and if they were presenting to the ER for this reason for the first time. To select the patient most at risk for PCLS at 3 months we used a risk score derived from the results of our previous studies that assessed the determinant of PCLS. This score was computed as follows: female gender, +1; current use of anxiolytics/antidepressant, +1; and perceived health status prior to admission: excellent or very good (0), good (+1), poor (+2), and bad (+3). To be eligible for enrollment in the study, a patient needed to score above the pre-defined threshold of 1 on the 3-item assessment procedure; this was designed to select patients at risk for PCLS. This score was developed using data from previous studies conducted in the ER setting of the Bordeaux Teaching Hospital. [START_REF] Laborey | Specificity of postconcussion symptoms at 3 months after mild traumatic brain injury: results from a comparative cohort study[END_REF][START_REF] Lagarde | Association of symptoms following mild traumatic brain injury with posttraumatic stress disorder vs. postconcussion syndrome[END_REF][START_REF] Gil-Jardiné | Emergency room intervention to prevent post concussion-like symptoms and post-traumatic stress disorder. A pilot randomized controlled study of a brief eye movement desensitization and reprocessing intervention versus reassurance or usual care[END_REF] Patients who were unable to provide written informed consent, unwilling to be contacted at 3 months, and/or under the influence of acute drug or alcohol use or dependence that, in the opinion of the site investigator, could interfere with adherence to study requirements were excluded from the study.

Participants were recruited from among patients who presented to the ERs of the University Hospitals of Bordeaux (Groupe Hospitalier Pellegrin) and Lyon (Groupement Hospitalier Edouard Herriot) and who were determined to have a high risk of PCLS. The identification and recruitment of potential study participants was carried out by emergency personnel under supervision of the project manager. Priority was given to the clinical evaluation and care of each patient, and the recruitment procedure was only initiated when the patient's condition allowed it. First, oral consent for participation was sought during the risk assessment stage.

Then, patients who fulfilled the inclusion criteria and were assessed as having a high risk for PCLS were presented with the objectives and procedures of the study and invited to sign an informed consent form.

Data collected

For all patients, we recorded demographic information and the reason for their visit to the ED.

Patients were also asked to report their level of stress, recovery expectation and acute pain using a numeric ten-level Likert scale.

Intervention

At both sites, patients were allocated to one of the two arms of the study using block randomization. Patients in the EMDR group received a 1-hour psychotherapeutic intervention based on the R-TEP protocol, [START_REF] Shapiro | EMDR and early psychological intervention following trauma[END_REF] which incorporates and extends Shapiro's early EMDR intervention protocols [START_REF] Shapiro | Eye Movement Desensitization and Reprocessing (EMDR): Basic Principles, Protocols, and Procedures[END_REF] into an integrative and comprehensive intervention that accounts for the fragmented and unconsolidated nature of recent traumatic memories as well as the need for safety and containment; these sessions were carried out by trained psychologists. Following the eight phases of the standard EMDR protocol it introduces four new procedural concepts (Traumatic Episode, Episode Narrative, "Google Search/ Scan" for identifying disturbing fragments and Current Trauma Focused processing strategies).

All psychologists involved in the study received a one-day specific training for the R-TEP protocol together with a continuous remote mentoring. A standardized questionnaire was completed by the psychologists at the beginning and end of the EMDR session to record the level of disturbance using a Likert scale (0-10) on the Subjective Unit of Disturbance (SUD) scale, [START_REF] Wolpe | The Practice of Behavior Therapy[END_REF][START_REF] Wolpe | Post-traumatic stress disorder overcome by eye-movement desensitization: a case report[END_REF] and free text commentary was provided to record the details of the session.

In a post-hoc analysis, the skill level of each psychologist was evaluated by an EMDR supervisor blind to the intervention content as well as the 3-month outcomes. Skill level was defined based on professional background, level of formation in EMDR practice (1 or 2), EMDR certification, and experience in the R-TEP protocol prior to the training delivered for the purpose of the study. Fidelity to the protocol was not assessed.

Patients in the usual care group were medically and psychologically managed by the ER staff without the intervention of a study psychologist. Inclusion in the study was only possible on days when psychologists were deployed in the ER.

Follow-up Assessments

Patients were contacted by phone 3 months after their ER visit using the phone number provided at the time of ER recruitment. Although several attempts were made to contact patients when necessary, the attempts were stopped when the delay exceeded 4 months after ER discharge. Symptoms were assessed using a standardized questionnaire administered by a research assistant blind to the randomization group.

Outcomes

The primary outcome was the proportion of patients with PCLS at 3 months as measured using the Rivermead Post-Concussion Symptoms Questionnaire. [START_REF] King | The Rivermead Post Concussion Symptoms Questionnaire: a measure of symptoms commonly experienced after head injury and its reliability[END_REF] The definition of PCS in the Rivermead Questionnaire includes the following symptoms: headache, feelings of dizziness, nausea/vomiting, sleep disturbances, fatigue, irritability, noise sensitivity, depression, frustration, poor memory, poor concentration, taking longer to think, blurred vision, light sensitivity, double vision, and restlessness. All variables were measured using a Likert scale that ranged from 0 (not experienced at all) to 4 (a severe problem). Consistent with the PCS definition in the context of mild head injury, patients were defined as having PCLS if they reported at least three symptoms of moderate to high severity.

The secondary outcomes included the presence of PTSD (defined using the PTSD Checklist, 5 th version), [START_REF] Blanchard | Psychometric properties of the PTSD Checklist (PCL)[END_REF] self-assessed recovery expectation at discharge, self-reported chronic pain at 3 months, and self-reported acute pain at discharge. All variables were assessed during the 3month follow-up phone interview.

Sample Size and Statistical Analysis

Based on previous pilot studies, [START_REF] Gil-Jardiné | Emergency room intervention to prevent post concussion-like symptoms and post-traumatic stress disorder. A pilot randomized controlled study of a brief eye movement desensitization and reprocessing intervention versus reassurance or usual care[END_REF] this protocol shows a PCLS incidence of 47% in patients with a score ≥2. The goal of the present study was to document a decrease of 15% in PCLS prevalence in the EMDR group. Thus, based on a 5% type I error rate and a power level of 80%, the required sample size was 169 patients in each group. Further considerations for 20% of participants lost to follow-up and 5% lost due to missing data for the main variables resulted in an expected number of 223 patients in each group.

The analyses for the primary and secondary outcomes were conducted on all patients who completed follow-up at 3 months. The prespecified stratified analysis was carried out with considerations for study center, stress level, and individual PCLS risk score. An additional post hoc analysis was conducted in the intervention group to assess the potential impact of psychologist skill level. Differences between patients who completed the study and those who were lost to follow-up were assessed for all variables. All statistical analyses were performed blind to arm allocation.

Ethics, Confidentiality of Data, and Data and Safety Monitoring Board guidelines

This research project received a positive endorsement from the French Comité de Protection de Personnes (CPP), Ouest II-Angers-N° RCB = 2017-A01462-51-N° CPP = 2017/36. The study was registered on ClinicalTrial.gov (NCT03400813).

Results

Figure 1. Study Flow Chart

Table 1. Patient characteristics

Between January and July of 2018, 1,855 patients were admitted to the ER at times when psychologists were available; of these patients, 313 (200 at Bordeaux and 113 at Lyon) were eligible for the study and were randomized into one of the two groups (156 in the intervention group and 157 in the control group). Of these 313 patients, 94 were lost to follow-up; thus, 219 patients were ultimately included in the final analysis (Fig. 1). Independent of follow-up, the patient characteristics at inclusion were similar between the intervention and control groups (Table 1). The proportion of patients lost to follow-up in the two groups did not differ.

Delivery of the Intervention

A total of 31 psychologists participated in the study, representing a total of 984 hours of time present in the ER. All of the psychologists had been previously trained in EMDR 26 The median duration of the EMDR sessions was 50 minutes (interquartile range: 30-90); we did not observe any difference according to whether or not a PCLS was present at 3 months. SUD scores decreased between the beginning and end of the EMDR sessions (difference: -3.9, 95% confidence interval [IC95%]: -4.5 to -3.3).

Effectiveness

There was no difference between the groups in terms of the primary outcome, i.e., the rate of PCLS (EMDR: 53.8% vs. Control: 49.6%). However, among the secondary outcomes, more cases of PTSD were observed in the intervention group than the control group (9.4% vs.

2.7%, p = 0.04). The occurrence of chronic pain was similar between the two groups (41% vs. 39%, p = 0.78), and the levels of acute pain at discharge did not differ (median [inter-quartile range]: 9 [7-10] vs. 9 [6-10], p = 0.89).

Table 2. Primary and secondary outcomes.

Table 3. Presentation of the impact of psychologist skill level on PCLS occurrence at 3 months.

Post hoc Analyses

The analysis of PCLS according to psychologist skill level indicated that the qualifications of the practitioner may have influenced the outcome because the incidence of PCLS at 3 months was lower among patients who were seen by the most qualified and skilled psychologists (Table 2). There was no association between an incomplete session and an increased risk of PCLS. However, a high self-assessed stress level at admission (>6) was strongly associated with an increased risk of PCLS in the EMDR group (76.9% vs. 40.9%; Table 3 and Fig. 2).

The overall incidence of PCLS did not differ between the two study centers (Bordeaux: 50.7%, IC95%: 41.4-57.4; Lyon: 54.2%, IC95%: 32.9-59.2). However, the incidence of PCLS in the EMDR group was 48.8% (IC95%: 37.5-60.1) at Bordeaux and 69.2% (IC95%: 48.1-84.9) at Lyon. Figure 2 presented Relatives Risks of post-concussion like symptoms occurrence after stratification on different factors. The difference in PCLS incidence between the intervention and control groups according was not related to patients' reasons for attending the ER.

Discussion

The results of the present trial revealed that an early EMDR R-TEP session performed during the ER stay did not reduce the incidence of PCLS at 3 months compared to usual ER care. Moreover, there was a higher incidence of PTSD in the intervention group, and the intervention resulted in an increased incidence of PCLS at 3 months among patients in the highest quartile of self-assessed stress at admission. Finally, there was an association between psychologist qualification level and success of the intervention.

The present study failed to confirm the results obtained during the SOFTER 2 trial. [START_REF] Gil-Jardiné | Emergency room intervention to prevent post concussion-like symptoms and post-traumatic stress disorder. A pilot randomized controlled study of a brief eye movement desensitization and reprocessing intervention versus reassurance or usual care[END_REF] In that study, there was a substantially lower rate of PCLS among patients treated by a psychologist in an EMDR session compared to those treated with usual care in the ER. More specifically, 6 of 34 patients in the EMDR group had PCLS at 3 months compared with 24 of 37 patients in the control group. There are several possible reasons for this discrepancy between the studies.

Only two experienced psychologists were involved in the previous pilot study, whereas 31 psychologists with heterogeneous levels of experience were recruited for the present trial. Of these 31 psychologists, only 8 had previous experience with the R-TEP protocol. The present study found clear positive associations between the outcome of the intervention and the various indicators used to assess the psychologists' experience and skill. Although it is possible that this can explain the present results, the assessment of the psychologists' competencies was not planned in the initial protocol and was only conducted after the effectiveness results were known. Therefore, this should remain a hypothesis, but it is also indicative of the need to carefully control the level of training provided to EMDR therapists because the short training period may have been insufficient. [START_REF] Chen | Efficacy of Eye-Movement Desensitization and Reprocessing for Patients with Posttraumatic-Stress Disorder: A Meta-Analysis of Randomized Controlled Trials[END_REF] Having less experienced and/or trained psychologists might also have reduced patient adherence to the protocol and increased the number of refusals. Thus, future studies should evaluate fidelity to the intervention protocol.

Approximately 30% of patients included in the present trial were lost to follow-up, but the characteristics of the patients who answered the 3-month questionnaire did not differ from those who did not. The proportion of refusals in the SOFTER 3 trial (~40%) was significantly higher than that in the SOFTER 2 trial (~20%). There is no clear explanation for this difference, and it may have influenced the results. In fact, it is possible that the patients who agreed to participate in the study differed from those who did not, which might explain why the expected number of patients was not achieved.

Further analysis of the discrepancies between the present trial and the previous pilot study, which produced more encouraging results, revealed differences in the psychologists' reports about the nature of the points of disturbance in the EMDR sessions. In the pilot study, the psychologists primarily addressed issues that were not directly related to the event that led a patient to the ER, whereas in the present study, a majority of the intervention reports mentioned disturbance points that were directly related to the event. It was also noted that patients with 3-month PCLS exhibited a significant decrease in SUD scores between the beginning and end of the EMDR session.

The present findings also differed from those of some studies in the literature. [START_REF] Tarquinio | Early Psychological Preventive Intervention For Workplace Violence: A Randomized Controlled Explorative and Comparative Study Between EMDR-Recent Event and Critical Incident Stress Debriefing[END_REF][START_REF] Gil-Jardiné | Emergency room intervention to prevent post concussion-like symptoms and post-traumatic stress disorder. A pilot randomized controlled study of a brief eye movement desensitization and reprocessing intervention versus reassurance or usual care[END_REF][START_REF] Kutz | The Effect of Single-Session Modified EMDR on Acute Stress Syndromes[END_REF] A study conducted in Israel reported very promising results following a single session of early modified EMDR provided in a general hospital setting by psychologists who were experienced in EMDR practice. [START_REF] Kutz | The Effect of Single-Session Modified EMDR on Acute Stress Syndromes[END_REF] In that study, patients reported the presence of acute stress syndrome and suffered from intrusion distress following accidents and terrorist bombing attacks. However, at the 4-week and 6-month follow-up assessments, the immediate responders in the terror victims group remained symptom free.

The second key finding of the present study concerned the high level of adverse effects associated with the intervention in patients who described themselves as experiencing high levels of stress. When EMDR is performed by an unqualified practitioner, insufficient attention may be paid to the importance of initially establishing sufficient stabilization and calming, which should be part of the protocol when applied correctly. Importantly, the issue of managing patients with high levels of stress or dissociation remains. In response to this challenge, modified and adapted EMDR-type early intervention protocols have been developed to assist victims. [START_REF] Shapiro | EMDR and early psychological intervention following trauma[END_REF][START_REF] Shapiro | The recent traumatic episode protocol (R-TEP): An integrative protocol for early EMDR intervention (EEI)[END_REF] Additionally, in the present study, perceived stress was evaluated using a 10-point Likert scale that had never been validated for stress assessment in the ER. Nonetheless, this scale provided a method with which to measure variations in subjective stress between admission and discharge, and a similar 5-point Likert scale for acute stress ("not at all" to "strongly") had previously been validated. [START_REF] Hampel | PCLS : Post-concussion like symptoms ; Reported pain : "Do you have pain ?" Yes/No ; Self-assessed stress : 10-level lickert scale from 0 (no pain) to 10 ([END_REF] The use of a 10-point scale likely did not influence the validity of the acute stress measure, especially because this variable was a secondary outcome and was assessed in a post hoc analysis. However, patients in the EMDR group who were experiencing high stress, defined as a numeric score >6, reported many more symptoms than did those in the control group.

Conclusions

Among patients admitted to the ER in the present study, a single EMDR R-TEP session did not reduce the incidence of PCLS at 3 months, particularly among patients who reported high levels of stress at admission. The present results suggest that it will be necessary to collect more data to determine the available treatment options that can be offered to patients attending the ER. Furthermore, the present results must be applied with caution, particularly due to the large degree of heterogeneity in the skill level of the psychologists employed in this study. Regardless of this issue, clinicians should continue attempts to identify the best care options for traumatized patients who present to the ER.

(Level 1 : 9 ;

 19 Level 2: 22), 8 had practiced the R-TEP protocol prior to the training delivered for the present study, and 4 were certified in EMDR practice. The median number of interventions performed by each psychologist was three (inter-quartile range: 1.75-4.5). Of the 106 EMDR 12 sessions performed for patients who completed the follow-up assessment, 66 were completed.

Figure 2 .

 2 Figure 2. Subgroup analysis: Relative Risk of post-concussion like symptoms occurrence after stratification on different factors.

Table 2 :

 2 Presentation of the impact of psychologist skill level on PCLS occurrence at 3 monthsSkill level of the psychologist was evaluated by an EMDR supervisor blinded from both interventions delivery and 3-month outcomes

					108	51.4
	reported	79	53.7	69	46.3	55	54. 5	53	48.6
					72	64.3
	Chronic pain followed	54	66.7	47	65.3	37	66. 1	35	62.5
					88	49.4
	Current daily pain	52	41.6	64	54.7	41	47. 1	47	51.6
					117	53.4
	Thinks having been evaluated by	154	98.7	13	8.3	105	99. 0	12	10.6
	psychologist in the ED
	At ED discharge
	Reported pain	72	61.5	72	63.1

39.7


I Elaboudi
email: i.elaboudi@ism.u-bordeaux1.fr

S Lazare

C Belin

D Talaga

C Labrugère

From polymer films to organic nanoparticles suspensions by means of excimer laser ablation in water

Keywords: 79.20.Ds, 78.67.Bf, 82.70.-y I. Elaboudi ( )

This study highlights the preparation of organic nanoparticles (NP) by laser ablation (LA) of polymeric materials in water. Experiments focused on poly(ethylene terephtalate) (PET) were carried out with the KrF laser pulse (248 nm). Size distribution and concentration of nanoparticles were deduced from suspensions turbidity measurements with the aid of Mie model, by Atomic Force Microscopy (AFM) on the basis of a statistical study and Scanning Electron Microscopy (SEM). The obtained results show that assemblies of spherical NP with a mean diameter 50 nm were synthesised. Composition and surface chemistry of NP were investigated using the Confocal Micro-Raman Spectroscopy (CMRS) and X-ray Photoelectron Spectroscopy (XPS). It indicates that NP are graphitic carbon rich and have a polymeric structure like polyacetylene. The possible mechanisms responsible of NP synthesis by under water LA of polymers was briefly discussed by investigating other polymers targets.

Introduction

Laser ablation (LA) in liquids presents a unique and fast method to synthesise nanosized objects [1][2][3] such as nanoparticles (NP), nanowires and nanotubes directly in suspensions that can be further studied, used or processed into a final material whose functionality requires the presence of the new nanostructure. Many synthetic strategies have been developed in recent years to produce and study numerous elemental or compound NP due to their high potential of application in industry (nanotechnologies), in medicine (diagnostics, imaging and treatment of cancer [4], for instance), in biology (immunolabeling, sensing [5][6][7]), and in research (material, electronics, atmosphere, spectroscopy, catalysis, toxicology [8], microscopies. . .). It is a promising technique for the controlled fabrication of nanomaterials via rapid quenching of ablated species at the interface between solid target and liquid. Numerous studies were focused on synthesis of diamond NP [1], elemental metals (Ag, Au) [9][10][11] alloys and oxides [12][13][14] NP. However, little work has been devoted to prepare organic NP by means of laser ablation in liquids. It is, therefore, the purpose of this work to systematically investigate ablation in water of polymeric targets studied for a long time in our laboratory [15] and for which a significant amount of data on ablation in air is available for comparison. In our previous papers, we brought the first evidence of the NP formation by underwater excimer laser ablation of PET and PC [16,17] by studying the suspension properties as a function of ablation dose (number of pulses). In this paper, we provide additional results more concerned with the chemical composition of NP. An ultimate goal of this work, not yet achieved, would be to provide enough understanding to pre-design a polymeric target for the synthesis of specific NP with predictable properties.

In this framework a number of careful experiment controls (see experimental part) has been setup in order to have a quantitative evaluation of LA efficiency in water: amount of ablated polymer, suspension concentration. These considerations are usually overlooked in recent reports of NP laser ablation of elemental target but necessary for complex multi-element polymeric targets as well as concerns of the chemical modifications of the ablated surface. Interpretations of UV-VIS spectroscopic results, with approximated MIE theory [START_REF] Bohren | Absorption and Scattering of Light by small Particles[END_REF], an appealing approach to probe the "as prepared suspension", are often puzzled by complications due to size-distribution and to aggregation of NP in water which do not allow direct quantification. Other analytical tools (see below) require the deposition of NP on a substrate by drop drying and may loose some of properties of native isolated NP produced in water by the laser pulse.

A set of polymers poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), polyimide (PI), poly(methyl metacrylate) (PMMA) and polystyrene (PS) (see below) is studied and the mechanisms involved in the laser ablation are briefly discussed in this paper.

Experimental

The used polymers are poly(ethylene terephthalate) (PET, Mylar D 75 μm), poly(ethylene naphthalate) (PEN, Teonex), polyimide (PI, Kapton), poly(methyl metacrylate) (PMMA), and polystyrene (PS).

NP suspensions preparation: The KrF excimer laser irradiation setup giving a square homogeneous spot is described in detail in the recent publications [16]. Samples were immersed inside a spectroscopy type quartz cell which is filled with ∼3 ml of ultrapure water (Milli Q).

NP investigation: Suspensions can be measured quickly and directly by UV-VIS attenuation spectrometry performed on a Hitachi Model-3200. MIE type interpretation was done with our home-made program (Mathcad) called Mathmie using approximated parameters like index of refraction, density, size, and shape of particles. Other techniques require the deposition of NP on a substrate. It is done by spin coating at 1000 rpm on mica for Atomic Force Microscopy (AFM) just before analysis with a Thermomicroscope CP Research system using intermittent contact mode. Scanning Electron Microscopy (SEM) analysis of NP was done using a JEOL JSM-840 scanning microscope. Samples were also prepared by the same method used for AFM (spin coating) analysis but supported on HOPG (high oriented pyrolytic graphite) substrate. To investigate chemical composition of NP by confocal micro-Raman spectroscopy (CMRS, Jobin Yvon Labram spectrometer equipped with Ar ion laser at 514.3 nm), a layer of particles was prepared on glass substrate by drying a drop of suspension. For X-ray Photoelectron Spectroscopy (XPS) analysis (VG spectrometer ESCALAB 220-XL, AlKα (1486 eV) some suspension was dropped onto the clean surface of indium. The photoelectron spectrum was done at constant pass energy of 20 eV. The peaks were fitted using a symmetric Lorentzian/Gaussian mixed function (L/G: 25-35%) with the aid of VG Scientific software. In a blank analysis, a drop of ultrapure water was also investigated after being dried in the same conditions to take into account water impurities and indium pollution.

Results

In our previous study [16], we demonstrated NP formation by LA of PET Mylar D and PC in water. The effect of number of pulses on particle size distribution (PSD) was studied and tendency of NPs to aggregation was concluded due to the increase of NP concentration with ablation dose. Figure 1 shows SEM image of NP prepared from PET Mylar D by LA at 500 mJ/cm 2 and 100 pulses. We believe it is the evidence of such aggregates of NP. The mean size of these NP is ∼50 nm as judged in the Fig. 1. This result can be compared to the AFM analysis [17] which was further used to measure the size distribution and indicated the presence of very small particles ∼1 nm in diameter [16]. These could not be looked for by SEM because of focussing adjustment which is much more time consuming. In addition, analysis of dielectric NP could reduce the resolution (from 1 to 30 nm) of SEM technique due to the accumulation of charges on NP surfaces. In order to check whether fluence is an important parameter influencing NP formation, LA with the KrF excimer laser (248 nm) was performed on PET Mylar D at various fluences. NP hydrosols were prepared with 60, 100, 200, 300, and 500 mJ/cm 2 and at fixed number of pulses (100). Figure 2 presents UV-Vis spectra of the obtained suspensions. It shows that eightfold increase of fluence induces fivefold increase of suspension extinction at 248 nm. Therefore, it is possible that NP attenuate the laser fluence (20% is absorbed and scattered after 500 mJ/cm 2 and 100 pulses irradiation) before arriving at the target surface and leads to LA efficiency decrease (by reducing the etching depth) at high fluences. Growth of both size and density of populations of particles explain the increase of suspension extinction with fluence. Approximated Mie theory can be used to support further conclusions by simulations as shown in Fig. 2. To calculate extinction coefficients of suspensions, optical properties of NP were needed; thus, they can be approximated to those of PET Mylar D on one hand (see [16]). On the other hand, Raman spectra show carbon and polymer like NP formation (see below); therefore, the optical characteristics of carbonaceous NP [START_REF] Hong | [END_REF] can be considered like in Fig. 2. The calculation procedure of extinction and scattering coefficients was already described in [17,[START_REF] Bohren | Absorption and Scattering of Light by small Particles[END_REF] where the shape of NP and some aggregates of NP are assumed to be spherical (as an approximation). A high extinction at shorter wavelengths can be interpreted according to Mie theory as a high density of small particles. Because neither PET nor NP are strong absorbers outside the UV region [START_REF] Hong | [END_REF]20], a high extinction at longer wavelengths is attributed to scattering of a high density of larger particles essentially. Table 1 shows results of simulation indicating the possible formation of new populations of NP with larger size (80 nm, 1200 nm) and disappearance of population of NP with small size (40 nm). This behaviour could be explained by the aggregation phenomenon which is strongly favoured at high concentration of particles produced by large dose of LA. Numerous experiments on LA of PET Mylar D were done in air and vacuum in past studies. Gaseous and solid products of ablation were already investigated and interpreted with photochemical and photo-thermal effects which are the main ablation mechanisms at the KrF laser wavelength (248 nm) [15]. In these conditions product collection mode is limited to deposition on surface an approach which only allows aggregates, whereas "in water ablation" allows preparation of more dispersed single NP that could be addressed individually for analysis or used further. NP were characterised by CMRS and XPS, two spectroscopies that are sensitive to nm thick particles of matter. XPS (Fig. 3) allows identification of elemental composition whereas CMRS mainly described previously [16] identifies bond vibrations. With XPS, the depth of analysis is given by the elastic photoelectron mean free path and is about 50 Å. Figure 3 presents two sets of ablation condition (B) low fluence at 23 mJ/cm 2 , 900 pulses and high fluence (C) at 640 mJ/cm 2 , 40 pulses. The first fluence is lower than the ablation threshold of PET Mylar D in air (Ft = 30 mJ/cm 2 ); however, formation of NP was indeed observed and independent recent study of laser kinetic ablation made underwater confirmed the reduction of the ablation threshold of PET (≈14 mJ/cm 2 ) [21] by doing it in water. This effect will be presented in an upcoming paper.

NP and other particles investigated by CMRS are completely different from that of starting PET film. In the Raman spectra of some products obtained at high fluence (640 mJ/cm 2 , 100 pulses), display bands attributed to C=C, C-C, and C≡C bonds characteristic of polyacetylene with The XPS spectrum of pristine PET Mylar D is displayed in Fig. 3(A) and is in good agreement with data previously reported [22]. It is compared to those of NP (B) and (C), the main differences are presented in Table 2. Figure 3 displays also XPS spectra of NP prepared at 23 mJ/cm 2 (B) and 640 mJ/cm 2 (C). For comparison, the main differences between XPS spectra are presented in Table 3. It appears that NP are richer in carbon than pristine PET, as indicated by the ratio C-C/C1s of samples (here C-C means carbon singly bonded to carbon and C1s is total carbon). For the high fluence case (C), the O/C ratio is reduced from 1.13 to 0.77, whereas below threshold (B), the drop of O/C is significantly less (0.92). During laser irradiation at any fluence (23, 640 mJ/cm 2 ), the ratio COOR/C1s is reduced upon ablation which indicates that the ester group is preferentially dissociated during LA. The most striking result is the twofold increase of the ratio COOH/C1s near the ablation threshold (sample B). In order to check if there is a contribution of the substrate pollution, a drop of pure water was deposited above and was analysed by XPS under the same conditions as the previous samples B and C. The ratio of COOH/C1s was about 2.17, which is much lower than those obtained in case of B and C. This could indicate a possible hydrolysis reaction of PET in addition to photochemical degradation and therefore may explain the reduction of the ablation threshold of this polymer in water.

Similarly to PET Mylar D, underwater LA of PEN, PI, PMMA and PS allows the preparation of organic NP suspen-sions. We obtained NP having a 3 nm in height and 80 nm in width (on average) (see Table 3). Fluence effect on PSD produced from these polymers was examined by a statistical analysis of AFM images. For this purpose, PEN, PI and PS were subjected to KrF LA in water at fluences 100 and 500 mJ/cm 2 . For the case of PMMA, the fluences 625 mJ/cm 2 and 1 J/cm 2 were applied; the number of pulses was fixed to 100 for all preparations. The conclusion is that height and width of NP increases with fluence in case of all polymers. This influence is small only in the case of PEN, but significantly larger for PMMA, PS and PI. These NP were also identified by CMRS. We obtained only a graphitic carbon in case of LA of PI, similar products to PET Mylar D in case of PEN, and finally, some oligomers of polystyrene and graphitic carbon were identified in case of LA of PS. Upon LA of PEN, PI and PS, graphitic carbon production could be expected because of high absorption coefficients of these polymers at laser wavelength (248 nm) [START_REF] Lazare | Laser Processing of Engineering Materials[END_REF] (see the section below). In case of PMMA which is the less absorbing, Raman spectra show formation of little graphitic carbon particles, but most of NP analysed have a spectrum which was similar to that of PMMA film revealing no great damage of this polymer during NP formation. Lazare et al. [START_REF] Weisbuch | [END_REF] highlighted PMMA nanofibers synthesis upon LA in air due to liquid expulsion that follows PMMA melting and expansion. It is possible that NP of PMMA detected here, were produced by this mechanism.

Discussion

Briefly, it is interesting to speculate on the particular point of the ablation mechanisms leading to the formation of the nanoparticles. Two main pathways may be put forward one called "condensation" and the other "fragmentation". In the case of carbon NP, it is thought that they form by saturated carbon vapour condensation in the plume [START_REF] Kabashin | Chap[END_REF] during the expansion phase after the heating phase due to absorption of the laser pulse. Condensation is, for instance, due to low vapour tension of carbon atoms, which makes even at elevated temperature, carbon particles growth possible. Therefore, it is a purely thermal pathway that follows the atomisation of the polymeric matter by the ablation process. On the other hand fragmentation is a mechanical effect due to the hydrodynamical expansion of the heated volume. It results in the breaking the material into small particles due to the rapid thermoelastic expansion and tensile stress (Model of Grandy). Therefore, it may be a low temperature pathway preserving the chemical structure of the polymer. This would explain the formation of the polymer-like NP.

Conclusion

By KrF laser ablation of various polymers in water several types of nanoparticles were formed. Analysis by dedicated techniques indicates that they are mainly composed of carbonaceous NP and polymer-like NP. Suspensions are rich in particles aggregates which are formed concomitantly to isolate NP. The fluence effect on PSD was examined using Mie theory and the conclusion is that increasing fluence enhances aggregation of NP. Varying the polymer structures allowed the observation of how NP populations depend on them. Possible mechanisms are either condensation or fragmentation or both.

Fig. 1 Fig. 2

 12 Fig. 1 SEM image of organic NP prepared by laser ablation of PET Mylar D in water at fluence 500 mJ/cm 2 and 100 pulses delivered at 1 Hz

Table 1 2 Fluence

 12 Particle size distributions (PSD) and density of NP used in the approximated Mie model to fit UV-Vis spectra in Fig.

Fig. 3

 3 Fig. 3 XPS spectrum of (A) virgin PET Mylar D film. XPS spectra of ablation products prepared by LA of PET: (B) at low fluence 23 mJ/cm 2 , 900 pulses; and (C) at high fluence 640 mJ/cm 2 , 40 pulses

Table 2

 2 Data from curve fitting of spectra displayed in Fig.3(A),

	(B), (C)
		C-C: 284 eV, COOR: 288 eV, COOH: 289 eV
		A (PET)	B (NP low F)	C (NP high F)
	O/C	1.13	0.92	0.77
	C-C/C1s	48.9	5 5 .3	5 9 .7
	COOR/C1s	12	5.4	4 .8
	COOH/C1s	2.9	5 .8	3

Table 3

 3 Nominal height and width of NP issued from LA of different polymers and determined by statistical study of AFM images. All suspension were prepared at fluence 500 mJ/cm 2 , 100 except PMMA which was ablated at 1 J/cm 2 , 100 pulses

	Polymer	Nominal height (nm)	Nominal widths (nm)
	PET Mylar D	4	1-50
	PEN	3	60-80
	PI	2	80
	PS	0.5	60-90
	PMMA	4	80

Acknowledgements Regional Council of Aquitaine and CNRS (Centre National de Recherche Scientifique) of France.


L Abe

E A R I S T I D I

A Domiciano

B Lopez

IMAGING INTERFEROMETRY USING INTERFEROMETRIC REMAPPED ARRAY NULLING

We present a beam combining technique capable of direct instantaneous imaging. The particularity is that the image (or PSID, point source intensity distribution) is formed in a pupil plane. We present the concept on a theoretical basis, comment on the PSID for onand off-axis sources and finally compare it to Fizeau imaging. Future experiments and reflections about this concept are finally considered.

Introduction

The combination of long baseline interferometry and high dynamic range technique is foreseen for future spatial mission like DARWIN/TPF. In a direct imaging context Labeyrie [START_REF] Labeyrie | Interferometry in Space, proc. SPIE[END_REF] proposed the hyper-telescope (HT) or pupil densification concept which aims at forming a filled output pupil from a diluted array of telescopes, which is, in other words, a 2D generalization of Michelson interferometry (Fig. 1a). Although a tremendous signal to noise gain can be achieved using the HT concept, some problems can occur in the image interpretation of complex sources. The IRAN is conceptually very close to the HT, and preliminary studies tend to show that it is more suitable for interferometric imaging in the classical way. The first section of this paper is recalling the Michelson (HT) image formation. A second section is describing how images are obtained with IRAN. Then an intuitive comparison with the Fizeau mode is made. Finally, we briefly present the possibility to couple IRAN with a coronagraph.

Summary of the Hyper-Telescope (Michelson) Concept

This "densified pupil" is then used to form a focal plane image. The image can be divided into 2 components: a fringe pattern due to the telescope array arrangement, and a modulating envelope related to the diffraction pattern of single apertures (i.e. elementary array aperture scaled in the densification process). It is well known in two telescopes Michelson interferometry that when pointing off-axis from a source, the envelope is moving slower (proportionally to the densification factor) than the fringe pattern, so that the white fringe intensity decreases when it reaches side lobes of the modulating envelope. Thus, image interpretation become difficult if objects are present beyond the FOV defined by the first lobe of the envelope (also called the Zero Order Field).

3I R A N C o n c e p t

Beam Recombination

Contrarily to classical multi-aperture recombination in OLBI where optical path difference is modulated in order to extract the fringes signature, the IRAN principle is to directly form a fringed pupil. Each of the N ×(N -1) 2 pairs of apertures produces two tilted wavefronts in the pupil and form fringes by coherent addition. The beam combination therefore involves an image plane, where we have brought them closer to each other, and preserving their optical axis parallel. A common lens then reimages the pupil according to the described fringe formation process (Fig. 1b).

. 2 O nA x i sP o i n tS o u r c e

All the fringes relative to each baseline give rise to a pseudo-PSF which we call "Point Source Intensity Distribution" (or PSID) with a stronger coherent peak. The coherent addition of all the cophased fringe pattern is similar to the interference pattern (IP)o ft h eN apertures geometrical configuration, that is,

IP(u, v)=FT N i=1 δ x i ,y i .

Off-axis Point Source and Object/Image Relationship

If we consider an off-axis point source, then the corresponding wavefront will be tilted against the instrument optical axis. As depicted in Figure 3, each diffraction pattern at the focus of each telescope will be displaced with the same amplitude, in the same direction. As these patterns are transported to the recombiner, each of them carries a global piston phase shift (shown in Fig. 3), corresponding to the optical path difference relative to the zero-OPD case (on-axis source). These pistons affect each fringe pattern associated to each baseline, and translate them according to the differential piston contributions. We demonstrated formally (Vakili et al. 2003) that the PSID is preserved, making it almost invariant by translation.I ndeed, the pupil being fixed imposes that it does not translate at all with the fringe pattern and therefore cannot be qualified of "invariant" by translation. Nevertheless, interesting possibilities of image reconstruction can be applied (under study) since the modulation of this interference pattern is uniform over the entire FOV (see below).

Field of View

As shown in Figure 2c, the PSID is limited by the geometrical pupil image of single apertures (supposed to be identical). This means that the FOV is only limited by the optical FOV of individual telescopes. The linear dimension (FWHM) of the coherent peak (angular size λ/B) depends on the inter-PSF distance in plane b) of Figure 2, and the FOV depends on the size of the pupil image which can theoretically be arbitrary large. But regarding the energy conservation, the bigger the pupil image, the lower the energy in the coherent peak, so a compromise must be found depending on the science objectives.

Analogy with the Fizeau Recombination

In a Fizeau configuration, one states that the output pupil is homothetic to the entrance pupil, and that the focus image is the Fourier Transform of this output pupil, with the bonus of being invariant by translation. We can decompose the Fizeau image formation in 2 steps. Take an arbitrary geometrical arrangement of apertures, which can be mathematically written as a sum of kronecker δ in a 2D plane. Then, convolve this array with identical telescopes' pupil functions. By Fourier transforming this function, we obtain the Fizeau PSF, which is the Fourier transform of the δ array, modulated by the Fourier transform of the telescope pupil function, i.e. the Airy pattern (Fig. 4c). The IRAN works the same, with the difference that, instead of convolving the telescope pupil function to the δ array, we convolve the PSF (i.e. Airy pattern) of each telescope (supposed to be identical). By Fourier transforming this function, we obtain the IRAN PSF, which is the Fourier transform of the δ array, modulated by the fourier transform of the Airy pattern, i.e. the telescope pupil function (Fig. 4d). This strong analogy helps understand why the object-image relationship is preserved. The gain here is ∼16 (peak-to-peak intensity ratio).

In order to estimate the intensity gain for a given telescope configuration over a classical Fizeau image, we can translate the dilution of the flux over the modulating envelope: the pupil shape, which determines the FOV in the IRAN case, and the single aperture diffraction pattern for the Fizeau image (determined by the aperture diameter parameter D tel ). Under these considerations, we can derive the following approximation,

Gain = B 2 D 2 tel 1 FOV 2 λ/D
where FOV λ/D is the field of view expressed in units of λ/D, B is the maximum baseline of the array configuration, and D tel the apertures diameter. Figure 4 illustrates the gain of the IRAN versus Fizeau beam combination in a configuration where FOV λ/D =25λ/B, B = 100 m and D tel = 1 m. It should be noted that if you want to increase the FOV, you will reach a limit where the gain will become a loss because the dilution of the PSID energy will occur on a relatively large array compared to the fringe size.

Nulling Mode

By defining a PSF in the pupil plane, we had some reflections about using "equivalent" high dynamic range imaging techniques like coronagraphy for the IRAN. We checked wether an analog to focal plane coronagraphy (i.e. phase or occulting mask) could be used successfully, but we found that only pupil interference set-up (A.I.C, Gay & Rabbia 1996) could provide the desired nulling effect (see Vakili et al. 2003 for further details).

Fig. 1 .

 1 Fig. 1. The hyper-telescope a) versus IRAN b) beam recombination. The IRAN pupil-PSF is the result of the N×(N-1) 2

Fig. 2 .

 2 Fig. 2. Illustration of an IRAN image. a) The telescope configuration, b) the densified images and c) the resulting PSID in the pupil plane.

Fig. 3 .

 3 Fig. 3. Keys to understanding the off-axis source effects on the IRAN PSF.

Fig. 4 .

 4 Fig. 4. Analogy to the Fizeau recombination. a) Positions in an array of telescopes (the "δ array"). b) The Fourier transform of this δ array. c) The fizeau PSF which is b) modulated by the PSF of a single aperture. d) The IRAN pupil PSF which is b) modulated by the pupil function of a single aperture.

Fig. 5 .

 5 Fig. 5. Comparison of the IRAN gain against a classical Fizeau mode, where the maximum baseline is 100 m, the apertures diameter is 1 m, and the FOV is about 25 λ/D.The gain here is ∼16 (peak-to-peak intensity ratio).

UMR 6525, Laboratoire Universitaire d'Astrophysique de Nice, 06108 Nice Cedex

2, France 2 Observatoire de la Côte d'Azur, 06304, Nice Cedex 04, France

Our team is at the moment evaluating the performance and feasability of the IRAN concept in a laboratory experiment. Once validated, a detailed study of the optimal multiple-aperture beam recombination has to be carried out for a future operational test instrument.


Le Cam Vincent
email: vincent.le-cam@lcpc.fr

Bourquin Frédéric
email: frederic.bourquin@lcpc.fr

Cottineau Louis-Marie
email: louis-marie.cottineau@lcpc.fr

A WIRELESS SENSOR NETWORK FOR DAMAGE DETECTION AND HEALTH MONITORING OF CABLES

Acoustic emission emerges as one of the reliable methods for cable health monitoring. Practical implementation involves numerous acoustic sensors to be placed along cables, as well as a supervising system. The Public Works Laboratory has developed such a platform. One version has been produced, and actually equips several bridges (Tancarville bridge, Iseron bridge, Aquitaine bridge,...). The system provided reliable results and was able to warn on-time bridge operators in case of critical health state. This article aims at presenting recent progresses towards better reliability of break detection by means of DSP, and increased flexibility provided by a TCP/IP -WIFI wireless implementation.

Introduction

In view of safety, structure durability and cost efficiency, detection of wire breaks is a strategic component of cable monitoring. Knowing the number of breaks and their location contributes to understand the health state of the structure. Acoustic emission emerges as one of the favourite non-destructive evaluation techniques. A great challenge is to develop reliable and versatile systems. This article especially describes the wire breaks detection by an acoustic emission method and the use made of emergent technologies such as the WIFI and Digital Signal Processing.

Principle of the acoustic emission 2.1 Acoustic wave in cables

The configuration of pre-stressed internal and external cables as well as those in bracing cables or suspension bridges implies that when a wire breaks, its two ends re-anchor close to the crack location. At this moment, an acoustic wave is generated in two opposite directions in the corresponding strand and neighbouring ones. Multistrand configurations may lead to the problem of detecting ruptures in internal strands. Fortunately, since corrosion and fatigue coupled to corrosion are major causes of ruptures, internal strands should undergo much less ruptures than external ones.

2.2Rupture detection

Detection of a wave by means of acoustic sensors that transmit this information to a supervisor means that a rupture occured in the cable. Sensor detection consists of recording the amplitude of the wave and the time to arrival at sensor location.

2.3Rupture location

Each sensor that has detected the event sends its amplitude and time to a supervisor that "knows" all sensor relative locations (L12, L23,...). Then by a simple calculation the supervisor can determine the point of rupture. First, the supervisor identifies sensors 1, 2 and 3. Sensors 1 and 2 are the two sensors that received the wave first. The break thus occurred between these two sensors. Sensor 3 and sensor 1 enable one to compute the speed "V" of the wave. From these data it determines "X" as follows :

V = L31 / ∆t31 (1) X = ½* (L12 -v * ∆t12) (2)
Where ∆tij denotes the delay in event detection between sensor i and sensor j.

This principle not only enables one to detect wire breaks but it also yields their location along the cable. The wave amplitude sent by each sensor provides information on the energy released during rupture. -Sensors are placed 10 m apart from one another

Results

Clear-cut results have been obtained with this system. First, laboratory tests show an accuracy of the order of ±15cm for crack location when sensors are located 10m apart from one another. In situ, the system provided with strategic informations that justified the cable replacement of Aquitaine bridge. Once took down, cables have been analyzed; the actual number and location of ruptures proved to be in agreement with those detected by the acoustic system [START_REF] Brevet | Surveillance Acoustique des câbles de ponts. Application aux ponts en béton précontraint[END_REF]. Therefore this system in its wire version proves reliable in delivering critical internal informations on cable health in full scale real life applications 4 A wireless and smart sensor network

Motivations

Even if the first generation gives useful informations for crack detection, two main evolutions are being added to the acoustic system :

-In view of flexibility, cost efficiency, wireless sensors have been designed, -In view of increased reliability in wire breaks detection, sensors are made "intelligent".

Wireless TCP/IP (WIFI) solution

More than only wireless sensors, the goal is to obtain the capabilities offered by sensor networks.

Sensor network architectures are more and more appropriate in many cases. Sensor network topologies allow to organize the monitoring in a way that complies with the nature of what should be measured and the final goal of the monitoring. For example, variable sensor density as well as heterogeneous or combined measures are easily supported.

Moreover, "network" means that entities could exchange data between each other. Sensor data don't have inevitably to go through a supervisor. Sensors can be grouped in sub-networks that take care of only part of the monitoring. Computation and analysis can be decentralized through the sensor network. A hierarchy can be implemented : one sensor can supervise a group of sensors that measure temperature, whereas another manages strain measurements... Thus only relevant informations are furnished to the main supervisor.

For many years, as a consequence of the exponential computer science development, the TCP/IP protocol has emerged. It has now become a world wide standard protocol. It appears very interesting to implement this protocol in sensors not only because of its perennity but also because TCP/IP can transit now through a new wireless standard called WIFI (standards 802.11a, b, and g, 802.16 coming) [START_REF] Gast | 11 Wireless Networks -The definitive guide[END_REF]. Sensor TCP/IP protocol has often been cited as a "heavy protocol" [START_REF]Wireless Sensor Networks for Area Monitoring and Integrated Vehicle Heakth Management Applications[END_REF]. But thanks to new electronical components (micro-controller, DSP, processor, memory storage...) this protocol can easily be embedded in the memory space of a processor. The TCP/IP software stack code can be optimized and customized to minimize its size and its CPU needs.

Without a standard communication protocol, wireless sensors are often dedicated to only one kind of measurement and systems are usually in a "star topology" around a supervisor (each sensor doesn't know the other ones). Morevover, sensor evolution is then more difficult because the protocol has to be rewritten according to the new type of data whereas TCP/IP offers a standard pipe between entities.

That problem is well underlined by the example of the MEMs inexpensive wireless sensor accelerometers [START_REF] Valoff | Presettable Micromachined MEMS Accelerometers[END_REF] that the Intel's EcoSense research project [START_REF] Krishnamurthy | How to build a network of 500 or 5000 sensors ?[END_REF] tries to connect while maintaining a high level of network performance and using the wireless TCP/IP.

Nevertheless, in case of very local and dedicated measures such as seismic instrumentation, specific development could be more suitable [START_REF] Lynch | Advanced Wireless Structural Monitoring : Past, Present and Future[END_REF].

Smart sensors -Use of a DSP core

The waves produced by wire breaks have a velocity of about 5000 m/s and a frequency band under 30kHz. In order to have a good resolution, waves are sampled at 300kHz by sensors.

With such a sampling frequency, the first generation of sensors (using Motorola 68hc11 microcontroller) was only able to record a thereshold value exceed and its moment. Then those two datas are sent to the supervisor.

Thanks to electronical evolutions and especially in signal processing, DSPs (Digital Signal Processors) allow high speed sampling and simultaneous "high level" treatment : numerical filtering, FFT,... The DSP familly is large and subject to permanent evolution (integration, low power consumption...).

L.C.P.C. retained DSPs that embed a TCP/IP stack, a large memory storage, and an Analog to Digital Converter.

Platform development

The new health monitoring platform integrates the wireless TCP/IP protocol (802.11b) for communication between network nodes. Each entity (supervisor, sensors) is viewed as in a classical office data network. Sensor role is extented : waves are permanently compared to a threshold. If exceeded, the sensor records the complete signal on a 8ms period including a 1ms pre-trigger delay. Then it sends this temporal signal to the supervisor using its embedded TCP/IP wireless stack.

Wireless platform :

Preliminaries results

The laboratory wireless sensor network system has been developed and works fine : signals over the threshold are detected, wireless transmitted to the supervisor that analyses them for tracking wire breaks. Amplitude distorsion is less than 0.5% and the transmission delay of a signal from a sensor to supervisor is less than 10ms.

Autonomy

Wireless sensor networks must ensure long-term monitoring. Thus, one of the challenges to reach in sensor development is to guaranty a sensor autonomy that allows suitable periods (from weeks to half a year at least).

In wireless sensors, the most demanding is the communication module that embeds an antenna and all the RF communication's components. In our Wifi case power consumption range is : 10mW to 400mW.

Consequently, sensor algorithms are developed with the following philosophy: "sensor permanently measure but seldom talk". The sensor is always the entity that decides when to transfer informations. Since ruptures are very rare events, the Wifi module is permanently OFF and only turned ON by the DSP when such an event occurred.

In addition, sensor is composed of very few lowpower consumption electronic components : a piezzoelectric (0 mW), an ADC ( < 5mW), a DSP AD-BF533 ( < 43mW), the Wifi module (< 250mW when ON).

The above paragraph underlines a serious limitation of wireless systems : sensor autonomy. When sensor have to be permanently online (in real-time applications for example), this increases its power consumption and long monitoring periods are not possible. The ways that should be explored are low-power consumption and node exchange optimization.

Conclusion

Use of the TCP/IP protocol provides notable advantages : even if radio link is disturbed, typical rate remains equal to 1 Mb/s; a wifi area can easily be increased by adding other Access Points (in "roaming mode"); TCP/IP protocol is independent of the physical link, thus, sensors can be connected if a wire connection is more appropriate (in bridges for example). This world-wide protocol is "layer-level" designed, so any modification in data exchange (with supervisor, other sensors…) only consists of high-level software programming.

The initial platform and works in progress prove that cable health monitoring can be performed by a non destructive acoustic method. During the coming year, a first complete and industrial wireless platform will be designed and attached to real cables under real conditions in order to validate and improve the system.

Many ways have to be explored. First, DSP functionalities are under-used : a studied evolution is the frequential signature analysis to separate real ruptures detection from noise and to increase scientific knowledge in this domain. Another way is the development of the network capacities offered by the TCP/IP protocol : while paying attention to power consumption, sensors are able to communicate with each other and the sensor network could be hierarchized in agreement with instrumentation needs.

According to the number of industrialized sensors and selected technologies, the unit price should be included between a few tens and a few hundreds of dollars.

Due to its modularity, the wireless sensor network could equip smart structures and be used for other kinds of instrumentations such as vibration measurement using accelerometers, wind monitoring of bridges using pressure sensors, fatigue monitoring using deformation gauges, temperature measurement, or chemical measurement to quote a few .

Figure 1 :

 1 Figure 1: Rupture detection by acoustic sensors

Figure 2 :

 2 Figure 2: Rupture location by acoustic sensors

A

  first generation system has been designed and produced. It now equips several french civil engineering constructions such as Aquitaine Bridge in Bordeaux, Tancarville bridge, Normandie bridge.

Figure 3 :

 3 Figure 3: Iseron and Tancarville bridges Main features : -Sensors are linked by an RS485 serial wire, -The physcial measure is performed by an accelerometer, -Supervisor is a central PC to witch the serial link is connected, -Sensors use an accelerometer and detection is made by permanent comparison of the signals to a pre-determined threshold (around 0.8g),

Figure 5 :

 5 Figure 5: The wireless sensor network architecture

Figure 6 :

 6 Figure 6: The wireless platform in development The supervisor consists of a user-friendly interface software that computes messages from sensors to perform the rupture tracking. WIFI area is performed by a traditional Access Point. Sensors use an Analog Device BlackFin533 DSP (100MHz processor frequency) and an Intersil PCMCIA wifi board.

Figure 7 :

 7 Figure 7: Sensor (oscilloscope snapshot) and supervisor measure of a shock