EA Philippe H Robert
email: robert.p@chu-nice.fr

Alexandra Koening

Hélène Amieva

Sandrine Andrieu

Francois Bremond

Roger Bullock

Mathieu Ceccaldi

Bruno Dubois

Serge Gauthier

Paul-Ariel Kenigsberg

Alexandra König

Hélene Amieva

François Bremond

Stéphane Nave

Jean M Orgogozo

Julie Piano

Michel Benoit

Jacques Touchon

Bruno Vellas

Jerome Yesavage

Valeria Manera

Philip P Foster

Andrea Bozoki

Serge Colle

Liliane Colson

Salome Imbert

Pierre David Nicaise

Jessica Petit

Guillaume Roth

Elisa Sacco

Karim Saltarin

TECHNOLOGY domain ( Alexandre Tifratene

Baptiste Derreumaux

Consuelo Fosty

Rachid Granata

Michael Guerchouche

Carol Lixon

BUSINESS-ECONOMY domain ( Sullivan O'

Laure Chantepy

Philippe Caton

Isabelle Corradi-Paumier

Cécile Cridelich

Frederic Couriol

Pierre Foulon

Jean François Herbert

Romain Le Cozannet

Patrick Malléa

Céline Jacquet

Recommendations for the use of Serious Games in people with Alzheimer

Keywords: Alzheimer's disease, mild cognitive impairment, frailty, serious games, recommendations, rehabilitation, SWOT analysis, non pharmacological treatment

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

INTRODUCTION

Due to the increasing average lifespan, the occurrence of neurodegenerative disorders such as dementia has risen by unprecedented levels, thus engendering high socio-economic costs. Alzheimer's disease (AD) is the most common type of dementia, and affects one in eight aged 65 and older (Alzheimer's Disease International, 2011). Prevalence studies estimate that the number of people affected will reach 81.1 million worldwide by 2040 [START_REF] Ferri | Global prevalence of dementia: a Delphi consensus study[END_REF]. As a consequence, the early detection and the treatment of AD and related disorders (ADRD) are considered as research priorities [START_REF] Ballard | Alzheimer's disease[END_REF].

In the last decades there has been a growing interest in employing Information and Communication Technologies (ICT) to help assess and evaluate patients' functional impairments, as well as to help and support patients in everyday activities [START_REF] Wichers | Momentary assessment technology as a tool to help patients with depression help themselves[END_REF]. Concerning clinical assessment, ICT play an important role allowing the development of new methods to evaluate more objectively behavioral and functional deficits [START_REF] König | A review of current information and communication technologies: can they be used to assess apathy?[END_REF]. This is important for clinical activity, as well as for research purposes [START_REF] Robert | Recommendations for ICT use in Alzheimer's Disease assessment: Monaco CTAD expert meeting[END_REF]. But beyond being important for assessment, ICT can also play a key role in the patients' treatment, stimulation, and rehabilitation. This is the underlying idea for the development of Serious Games (SG), which are digital applications specialized for purposes other than entertaining, such as training and educating, informing, communicating, marketing, leading societal/ideological impact on specific subjects, or enhancing user's aptitudes or cognitive/physical functions.

The elderly population (above 50 years) represents now a considerable portion of digital gamers (e.g., 14% in Germany [START_REF] Biu | Gamer-Statistiken[END_REF]29% in USA ESA, 2011), which is predicted to increase. For this reason, SG may represent a low-barrier, motivating, sustainable and relatively cheap method to improve, or at least delay the onset of impairments in selected social, sensorymotor, and emotional functions [START_REF] Mccallum | Gamification and serious games for personalized health[END_REF]. Recently, some studies have started to investigate the effectiveness of SG in people with ADRD (McCallum and Boletsis, 2013), but the field is still completely uncharted, and the clinical, ethical, economic, and research impact of the employment of SG in these target populations has never been rigorously addressed and discussed.

The purpose of the present methods paper is to put together recommendations for the development and use of SG in patients with ADRD and frailty gathered from stakeholders in the field, and to analyze systematically the Strengths, Weaknesses, Opportunities and Threats (SWOT) of employing SG with these patients. We will now briefly describe the target population and review the results of the few studies that addressed the employment of SG in that area, before describing in more detail the aims of the present work.

ADRD AND FRAILTY

AD is a neuro-degenerative disorder where memory functions are primarily affected at the early stages of the disease [START_REF] Agüero-Torres | Natural history of Alzheimer's disease and other dementias: review of the literature in the light of the findings from the Kungsholmen project[END_REF]. Persons with AD generally deteriorate progressively through multiple stages over several years. Despite some discrepancies, there is a growing consensus in subdividing the course of AD into three stages: (1) a preclinical/asymptomatic stage, only revealed by biomarker evidence; (2) a predementia phase, characterized by the impairment in memory or other cognitive domains not negatively affecting social and/or occupational functioning (also known as Mild Cognitive Impairment, MCI) (3) a dementia phase, in which cognitive disturbances significantly interfere with the capacity of independent living [START_REF] Dubois | Revising the definition of Alzheimer's disease: a new lexicon[END_REF]. At this stage, cognitive symptoms are often associated to behavioral and psychological symptoms such as apathy or agitation [START_REF] Aalten | Behavioral problems in dementia: a factor analysis of the neuropsychiatric inventory[END_REF].

MCI is defined as a cognitive decline greater than expected for an individual's age, but which does not interfere notably with activities of daily life [START_REF] Petersen | Mild cognitive impairment: clinical characterization and outcome[END_REF][START_REF] Gauthier | Mild cognitive impairment[END_REF]. Some people with MCI remain stable or return to a normal state over time, but more than half progress to dementia within 5 years. Therefore, MCI can be regarded as a risk state for dementia and its early identification could offer opportunities for preventative interventions [START_REF] Albert | The diagnosis of mild cognitive impairment due to Alzheimer's disease: recommendations from the National Institute on Aging-Alzheimer's Association workgroups on diagnostic guidelines for Alzheimer's disease[END_REF].

Another concept that has recently attracted the attention of researchers and clinicians is that of frailty, defined as a multidimensional geriatric syndrome characterized by increased vulnerability to stressors as a result of reduced capacity of different physiological systems [START_REF] Kelaiditi | Cognitive frailty: rational and definition from an (I.A.N.A./I.A.G.G.) international consensus group[END_REF]. Traditionally, the concept of frailty has focused principally on the physical domain [START_REF] Fried | Frailty in older adults: evidence for a phenotype[END_REF]. Recent work has started to study more deeply the cognitive impairment due to physical frailty, and lead to the definition of cognitive frailty, defined by the simultaneous presence of both physical frailty and cognitive impairment without the presence of a concomitant neurological disease (see [START_REF] Kelaiditi | Cognitive frailty: rational and definition from an (I.A.N.A./I.A.G.G.) international consensus group[END_REF] for a review). Cognitive frailty is viewed as a potential precursor of neurodegenerative processes with good potential for reversibility, and thus is the ideal target of early interventions.

STATE OF THE ART: THE USE OF SERIOUS GAMES WITH PEOPLE WITH ADRD

There is evidence that SG can successfully be employed to train physical and cognitive abilities in elderly people (e.g., [START_REF] Anguera | Video game training enhances cognitive control in older adults[END_REF]see Wiemeyer and Kliem, 2012 for a review of the studies employing SG in prevention and rehabilitation of elderly people). Recently, some studies have started to investigate the effectiveness of SG in people with AD, MCI, and related disorders. [START_REF] Mccallum | Dementia Games: a literature review of dementia-related Serious Games[END_REF] performed a literature review of the experimental studies conducted to date on the use of SG in neurodegenerative disorders. In summary, the results of the 15 reported studies suggest that: (1) physical games (or exergames, i.e., games that promote physical fitness) can positively affect several health areas of the players with mild AD and MCI, such as balance and gait [START_REF] Padala | Wii-fit for improving gait and balance in an assisted living facility: a pilot study[END_REF], and voluntary motor control [START_REF] Legouverneur | Wii sports, a usability study with MCI and Alzheimer's patients[END_REF]; (2) cognitive games (i.e., games which target cognitive improvement) can improve a number of cognitive functions, such as attention and memory [START_REF] Stavros | Computer based cognitive training for patients with mild cognitive impairment (mci)[END_REF][START_REF] Weybright | Effects of an interactive video game (Nintendo Wii) on older women with Mild Cognitive Impairment[END_REF][START_REF] Rosen | Cognitive training changes hippocampal function in mild cognitive impairment: a pilot study[END_REF] and visuo-spatial abilities [START_REF] Yamaguchi | Rehabilitation for dementia using enjoyable video-sports games[END_REF]; (3) both physical and cognitive games can have a positive impact on social and emotional functions, for instance they can improve the mood and increase positive affect and sociability [START_REF] Weybright | Effects of an interactive video game (Nintendo Wii) on older women with Mild Cognitive Impairment[END_REF][START_REF] Boulay | A pilot usability study of MINWii, a music therapy game for demented patients[END_REF][START_REF] Yamaguchi | Rehabilitation for dementia using enjoyable video-sports games[END_REF] and reduce depression [START_REF] Férnandez-Calvo | Efficacy of cognitive training programs based on new software technologies in patients with Alzheimer-Type dementia[END_REF]. Very few studies investigated the effects of the use of games for social/emotional health (which encourage the players to link with their friends and/or improve their social and emotional life) in dementia, but the results are encouraging [START_REF] Boulay | A pilot usability study of MINWii, a music therapy game for demented patients[END_REF].

Despite these promising results, a number of studies showed that elderly people and people with ADRD have problems in using many of the SG currently available on the market. Their difficulties include problems in getting familiar with the game technology and embarrassment about using the tools designed for the game (e.g., [START_REF] Wollersheim | Physical and psychosocial effects of Wii video game use among older women[END_REF][START_REF] Legouverneur | Wii sports, a usability study with MCI and Alzheimer's patients[END_REF]. Furthermore, certain games were considered too demanding or even risky for elderly people (e.g., [START_REF] Sohnsmeyer | Effect of a six-week intervention with an activity-promoting video game on isometric muscle strength in elderly subjects[END_REF]. These difficulties derive from the fact that most of the SG currently employed have been developed for entertainment purposes (e.g., the Nintendo Wii Fit, Wii Sports, and Big Brain Academy) and with a "typical healthy user" in mind.

To overcome this problem, SG targeting specifically ADRD are starting to emerge (e.g., [START_REF] Benveniste | The MINWii Project: Renarcissization of patients suffering from Alzheimer's Disease through video game-based music therapy[END_REF][START_REF] Nor Wan Shamsuddin | Virtual environment design guidelines for elderly people in early detection of dementia[END_REF][START_REF] Tarnanas | Ecological validity of virtual reality daily living activities screening for early dementia: longitudinal study[END_REF], along with guidelines ensuring their usability among the targeted populations (e.g., [START_REF] Bouchard | Developing SG specifically adapted to people suffering from Alzheimer[END_REF][START_REF] M146 Fua | Designing serious games for elders[END_REF]. However, these recommendations are still very sparse.

AIMS OF THE PRESENT WORK

As outlined in the previous paragraphs, there is an increasing interest in employing SG with patients with frailty and ADRD, and a general feeling that these cutting-edge applications may open new avenues for clinical treatment and experimental investigation of important issues. However, the field is completely uncharted. Strong evidence concerning the effectiveness of SG as clinical tools is still missing, together with a consensus on how, when and for what purposes SG should be employed. Furthermore, the ethical, social, and economic consequences of the employment of SG in these clinical populations have never been systematically investigated. As the topic is very new and relatively few scientific papers have been published so far, we believe that it would benefit from a structured dialog and discussion between different stakeholders: including people working in the health domain (e.g., clinicians, neuropsychologists, geriatricians, etc.), people working on ICT (e.g., engineers and SG designers), people working in the business domain (e.g., product marketing, business development), as well as patients and their caregivers (e.g., people working on nursing homes, family caregivers, etc.).

Starting from these considerations, we organized a two-round workshop (IA workshop) with stakeholders in the field with the aims of (a) analyzing systematically the employment of SG in frailty and ADRD (SWOT analysis), and (b) gathering recommendations for the development and use of SG targeting these populations.

METHODS

WORKSHOP STRATEGY

The analyses and recommendations reported in the present paper were collected and discussed during the workshop "Innovation Alzheimer 2013" (IA workshop 2013), organized by the CoBTek (Cognition-Behavior-Technology) Research Unit of the University of Nice-Sophia Antipolis (UNS) in Nice, France. CoBTek's main mission is to use ICT, particularly imaging and video analytic techniques to: (1) Improve diagnostics and treatment of behavioral and cognitive symptoms in ADRD (2) Develop new strategies in order to prevent, help and assist elderly people (3) Improve autonomy in the elderly.

The IA workshop 2013 had a two-step design (two rounds plus a web survey).

FIRST ROUND

The first round took place in Nice on November 7th, 2013, and involved 50 participants including health care professionals and family association representatives (n = 25), ICT engineers (n = 10), representatives of companies involved in ICT, and economical experts (n = 15). It started with a 3-h plenary session, where recent works employing SG in elderly people and people with ADRD were presented. The aim was to ensure that participants coming from different backgrounds were familiar with terms such ADRD, frailty, and SG. After the plenary session, participants were divided in three 2-h parallel sessions:

SWOT analysis session

Participants (2 groups of 4 participants) were presented with items concerning SWOT of SG in ADRD deriving from a former literature review, and asked to prioritize them, as well as to generate new ideas. The list of items proposed to the participants, as well as the new items that were proposed during the session, are reported in the Results section.

Serious Game design session

Two groups of 4 participants worked separately to design a SG targeting patients suffering from ADRD.

Recommendation session

One group of 34 participants worked to generate practical recommendations for the development and use of SG in ADRD. Why, Where, When, What, and How questions were used as a guideline to structure the session. The list of the 6 questions asked to the participants is reported in Table 1, and included two general questions and four questions focused on the use of SG in patients with ADRD. First, participants were presented with the questions, and asked to respond to them through a brainstorming carried out in small groups. After the brainstorming, participants were presented with a list of responses to each question, and were asked to rate the importance of each item on a 0-3 scale (0 = not important at all/not adapted at all; 1 = not very important/not very adapted; 2 = important/adapted; 3 = very important/very adapted).

After voting, participants were presented with a list of practical recommendations for the development of SG adapted to people with ADRD that emerged from a previous literature review, and were asked to comment them and to generate new ideas. The complete list of items proposed to the participants is reported in the Results section. At the end of the three parallel sessions, all participants were involved in a 1-h plenary discussion, where all the groups' works were presented and commented.

WEB SURVEY

Based on the results collected from the recommendation session during the first round, a web survey was proposed to all CNR members, representatives of the French Memory centers, and members of the European FP7 project VERVE. 30 experts completed the survey, including health care professionals (N = 17), representatives of companies involved in ICT and economical experts (n = 6), ICT engineers (n = 1), and researchers (N = 6). Participants were presented with the same list of questions and items used during the first round, and asked to rate the importance of each item on a 0-3 scale (0 = not important at all/not adapted at all; 1 = not very important/not very adapted; 2 = important/adapted; 3 = very important/very adapted).

SECOND ROUND

The second round took place on November 15th during the 6th edition of Clinical Trials on Alzheimer's Disease (CTAD) conference held in San Diego (November 14-16th, 2013). The Consensus Group included 10 clinicians (geriatricians, epidemiologists, neurologists, psychiatrists, psychologists), 1 ICT engineer, and 1 representative from the pharmacological industry.

The week of the second round participants received a first draft of the recommendations. During the second round, the CoBTeK team presented the preliminary results collected during the first round and the web survey, and asked for comment on these results within a group discussion, which was audio recorded. The objective was to validate the manuscript draft, and to collect new and/or different ideas coming from international experts.

RESULTS

SWOT ANALYSIS

Participants of the SWOT analysis session (round 1) were presented with items concerning SWOT of the use of SG in people with ADRD based a literature review (e.g., [START_REF] Rizzo | A SWOT Analysis of the field of VR rehabilitation and therapy[END_REF], and asked to comment and prioritize the items, as well as to generate new ideas. The reported results (summarized in Table 2) include the findings that emerged from the SWOT analysis session of round 1, commented on and integrated by all the participants of the first round during the general discussion, and by the participants of the consensus group during the second round.

Strengths

Interface adapted to the user. In the context of SG it is possible to create game interfaces adapted to the users' capacities and interests. Motivation is a crucial aspect for the success of rehabilitation programs, but differs from one person to another [START_REF] Leone | What are you interested in?"-A survey on 601 nursing homes residents activities interests[END_REF]. One of the advantages of SG is that it is possible to generate games with different story-plots (e.g., cooking, travelling), but targeting the same ability (e.g., physical activity and executive functions). Furthermore, each SG can be potentially individualized. For example users may be able to generate personalized avatars, and the game can incorporate pictures or videos of familiar objects and environments. Adaptation is important also when it comes to the users' impairments. For instance, people with visual problems may receive auditory cues instead of visual cues, and people with memory problems may be prompted with repeated instructional cues. Taking into account the users' impairments is particularly important when targeting people with dementia-related disorders.

Gaming factors to enhance motivation, positive mood and improve assessment. SG have a playful character, which is supposed to enhance motivation and to improve the users' mood; an aspect which is particularly important when targeting clinical populations. Gaming factors can also improve assessment abilities. Plato said, "You can discover more about a person in an hour of play than in a year of conversation." Observing and quantifying a person's behavior when participating in a SG may provide information even more reliable than those acquired with traditional performance assessments, because the person engaged in a gaming task is less focused on the fact of being "tested," which is usually reported as very stressful [START_REF] Cassady | Cognitive test anxiety and academic performance[END_REF].

Independent practice and self-assessment. SG, due to their "light" interfaces, their playful aspect and interactivity, and their high level of immersion (for the VR-based SG) can be employed for self-assessment and home-based skill practice. These are common components of most rehabilitation programs, considered crucial to promote activity automation, and generalization of the learned skills to every-day behavior. The possibility to carry out an autonomous activity can also lead to mood improvements and self-esteem enhancement. Furthermore, the home-based and independent practice would contribute to reduce the costs sustained by insurances and by the public healthcare system.

Safe testing and training environment.

When developing ecological assessment and rehabilitation instruments, it is necessary to consider safety risks (for instance, the risks due to driving errors, but also the risks of burning oneself with a hot pan). SG represent an optimal solution to this problem, minimizing these kinds of risks.

Promoting social bonding. Social isolation and lack of social interactions are often reported as crucial problems by elderly people and people with dementia-related disorders [START_REF] Cattan | Preventing social isolation and loneliness among older people: a systematic review of health promotion interventions[END_REF]. SG may play a role in promoting social interactions. For instance, there are SG that can be played by multiple players physically co-present, or in groups. Some multi-player SG can be played online by people connected from remote locations. In this case the social interaction is more limited; even if some preliminary studies suggest that participants (undergraduate students) perceive the interactions developed during online SG as good quality social interactions [START_REF] Mansour | The impact of multi-players serious games on the social interaction among online students versus face-to-face students[END_REF], the prevalence and extent of social activities in online group games are intrinsically different from real world social interactions (e.g., [START_REF] Ducheneaut | Alone together?: exploring the social dynamics of massively multiplayer online games[END_REF]. for their limited ecological validity [START_REF] Chaytor | The ecological validity of neuropsychological tests: a review of the literature on everyday cognitive skills[END_REF]. The same concern applies to diagnostic laboratory testing. An important strength of SG, especially of those based on VR, is that they can offer rehabilitation environments which simulate real life environments and activities.

Enhanced ecological validity. Traditional clinical (nonpharmacological) rehabilitation methods have been criticized

Control of stimulus delivery. Another strength of SG (and of all the tests and rehabilitation programs involving ICTs) compared to traditional rehabilitation methods is the possibility to systematically control and deliver stimuli, which represents a basic foundation of all human research.

Cuing stimuli for error-free learning. This dynamic stimulus delivery and control also allows the presentation of cuing stimuli that could be used for "error-free" learning and similar approaches.

Performance analysis in real time. The evaluation of patient performance typically involves a post-hoc examination of numeric data and subsequent translation of that information into graphic representations. Videotapes of the activities have been used to provide a more naturalistic performance evaluation, but it is often difficult to correlate these observations to the other numerical data. SG offer the therapists (and caregivers) the possibility to record and visualize the activity immediately after it has been recorded, and to measure the patient performance in real-time.

Real-time feedback delivery. In the context of SG, it is possible to provide feedback on performance and on accomplished activities, which is considered a key element to improving learning and rehabilitation of functional activities.

Promoting learning processes. When thinking about elderly people and people with dementia, there is a tendency to focus on rehabilitation and recovery of lost functions. However, neuroplasticity can also be improved by learning new things and activities, and by promoting positive affect and stress reduction [START_REF] Davidson | Social influences on neuroplasticity: stress and interventions to promote well-being[END_REF]. SG can be easily employed for this purpose. The employment of ICT when playing a SG represents a learning opportunity and challenge for elderly people and people with dementia-related disorders. Furthermore, SG has been shown to be able to improve mood and to decrease stress [START_REF] Russoniello | The effectiveness of casual video games in improving mood and decreasing stress[END_REF]. be accumulated for performance comparisons needed for diagnostics, research and for training purposes.

Low-cost duplicable environments

Weaknesses

Interface challenge I: non-naturalistic interactions. Current SG platforms are still limited in the degree to which they allow users to naturalistically interact with them. Progress has been made, but it is necessary to invest more in usability testing, especially when designing SG for elderly and fragile people. Many elderly people are not used to interacting with high-tech interfaces, and the initial approach with SG may cause them a high cognitive and affective load, which may significantly slow down the learning curve. It is necessary to avoid any extra, non-automatic cognitive effort that could represent a distraction for the patient, and thus limit the assessment and rehabilitation processes.

The interface challenge 2: wires and displays. In order to use SG on ordinary PC/laptops or televisions, it is often necessary for the user to connect a number of cables and modify the setups. This may be too difficult for non-expert users, especially for elderly, and fragile people.

Immature engineering process. Building, testing, and maintaining a rehabilitation system based on SG is a very complex process, and there is a lack of a standard methodology. Developers must integrate disparate knowledge in both engineering and rehabilitation that include sub-areas such as tracking, displays, interaction, computer graphics, simulation, human factors, biokinesiology, cognitive psychology, and so forth.

Expensive equipments. To be effective, some VR-based SG need fully immersive projection displays (i.e., CAVE, Powerwalls, Immersadesks), which are very expensive, and thus not easy to install where the assessment/rehabilitation programs usually take place (hospitals, day clinics, etc.).

Poor platform compatibility. Most of SG are not inter-operable, and the applications are not written in a simple and reconfigurable manner. This represents an important problem, especially because the target places where the SG should be used (e.g., hospitals, nursing homes) are equipped with disparate (often not regularly updated) systems. Furthermore, many SG applications require a fast and reliable internet connection, which is sometimes not available, or not optimized.

Software difficult to use. Healthcare professionals are not programmers. Consequently, in order to maximize the usability and usefulness of rehabilitation program based on SG, great care needs to be placed on building an intuitive front-end interface.

Similarly, the applications used to visualize performance results should be intuitive and easy to use. These remarks also apply to home-based SG managed directly by the patients and their caregivers.

Lack of generalization.

Although SG try to promote ecological interactions and to create naturalistic environments, it is difficult to generalize the skills learned during SG to the real life context. For instance, learning to recognize other peoples' emotions from facial expressions in a SG does not guarantee that the user will actually demonstrate an improved ability to recognize emotions via facial expressions in a real social interaction context.

Addiction. Like many digital games, SG could cause addiction [START_REF] Griffiths | Excessive online computer gaming: implications for education[END_REF]. Even if SG have mainly training purposes (e.g., enhancing memory and attention), people may spend too much time playing, and reduce the amount of time they dedicate to other activities (e.g., physical activity, hygiene, etc.).

Side effects. Side effects can be observed especially in the VRbased SG [START_REF] Cobb | Virtual reality-induced symptoms and effects[END_REF]. Cybersickness is a form of motion sickness with symptoms reported to include nausea, vomiting, eyestrain, disorientation, ataxia, and vertigo. After effects may also include symptoms such as disturbed locomotion, changes in postural control, perceptual-motor disturbances, flashbacks, drowsiness, fatigue, and generally lowered arousal. When using simpler SG (not VR-based), after effects may include headaches due to prolonged fixation of the display. In order to obtain usable and safe SG for fragile people, it is necessary to adapt them to the specific patients' deficits and problems.

Opportunities

Emerging advances in technology. Progress in the Graphics/Video Integration brings more visual realism to SG, representing an important component for developing effective rehabilitation programs. Another positive development is the trend toward wireless technology, which reduces the need for complex wire configurations. Similarly, due to the increase of demands in the consumer market, display devices are becoming cheaper. For VR-based SG, autostereoscopy makes stereo viewing possible without the need to wear any special equipment (i.e., polarized or active shutter-type glasses), a feature that can enhance the overall usability of VR systems.

Real-time data analysis. One of the strengths of rehabilitation programs based on SG is the possibility to collect and analyze performance data in real-time, and to provide the user with rapid performance feedback. This is possible in the context of cognitive tasks (e.g., providing results of memory or attention based games), but as well in the domain of motor tasks, thanks to the emergence of real-time motion analysis coupled with direct acquisition of motion data.

Gaming industry drivers.

The recent growth in the interactive gaming-industry area, and its interest in the domain of health and ageing, will continue to drive the development of products targeting specific populations, such as people with dementia-related disorders.

Intuitive appeal to the public. The general public is very attracted to the idea of using SG with therapeutic or prevention targets. This interest is increased by the attention devoted to this domain by the media.

New professions.

In order to create, develop, test and sell SG targeting specific populations, such as people with dementia-related disorders, it is necessary to rely on multi-disciplinary teams including people with different backgrounds and abilities, such as engineers, healthcare professionals, and businessmen. This process will lead to the creation of new professions (e.g., engineers with a deeper knowledge of specific health-related issues), and will represent an economical opportunity to create new job positions.

Closeness between scientific, technical, and clinical communities.

The interdisciplinary work between the communities of researchers, engineers and clinicians that focus on SG for rehabilitation purposes are becoming more frequent.

SG as research instruments. SG can improve our knowledge of brain functioning. For instance, they can be easily used in association with brain imaging techniques, as they allow to interact in complex environments almost without moving (e.g., using a joystick). This feature of SG explains the increase of funding dedicated to projects that try to integrate the use of SG in domains such as neuropsychology and behavioral neuroscience. SG may also be employed as motivating factors to facilitate participants' recruitment, and to reduce the incidence of drop-outs.

Telerehabilitation. Due to the flexibility of the Internet, the idea of delivering rehabilitation and therapy to patients in remote locations has been a popular topic. The application of SG within a tele-rehabilitation format is the next logical step, allowing a therapist to check the utilization and progress of patients, or to manually modify the game parameters. Similarly, this would allow technicians to perform maintenance operations without the need to go to the users' home.

Big market. The number of people with dementia-related disorders is very high and destined to increase, and thus the tools developed to assess and rehabilitate dementia-related impairments are potentially addressing a large number of users. This makes the market interesting for high-tech companies, as well as for insurance companies.

Threats

Ethical challenges. Professionals must carefully consider and address incumbent ethical threats concerning the use of SG with fragile people, and the fact that the immersion in an environment too realistic may sometimes cause more problems than benefits. Also, questions concerning the disclosure of private personal information in the domain of internet-based SG should be addressed carefully.

Poor integration with the clinical practice. Although SG can be employed as clinical tools, they are poorly integrated in standard clinical and neuropsychological practice. In order to be effective, SG should be used only after a standard clinical and neuropsychological assessment of patient's deficits and abilities has been performed. This standardized evaluation (independent of the evaluation provided by SG) should be used to decide which SG should be employed with a specific patient, and with which level of difficulty.

Lack of assessment methodology. The field lacks standard methodologies to verify the effectiveness and efficacy of the use of SG. In order to understand if SG achieve the participants objectives, it is necessary to establish rigorous methods to assess the users' performance improvements, based on the methods used for clinical assessment. Clinicians and therapists should be involved directly in the assessment stage, especially during feasibility studies.

Lack of feasibility and efficacy studies.

The interest in employing SG with patients with ADRD emerged only recently (see [START_REF] Mccallum | Dementia Games: a literature review of dementia-related Serious Games[END_REF], and as a consequence the field is completely uncharted. Rigorous feasibility and efficacy studies will be necessary to develop a better understanding of the physical, psychological, and social effects and consequences of employing SG in these populations, and to predict and anticipate possible adverse consequences.

Lack of regulation.

The field lacks regulations concerning the development, marketing and use of SG with sensitive populations, such as people with dementia-related disorders. It is extremely important to create standard rules for companies and institutions that create, test, sell and use SG, in order to guarantee that SG are safe, useful, and respect ethical principles. This lack of regulations is responsible for some false promises made in the past through media advertisement.

Lack of business model. Most SG are designed with the aim to create a prototype, and not a product for the market. As a consequence, the business model behind SG is not clear (e.g., should SG be free for the final user? who should pay for them, and for their cost of development? how should copyright issues be managed?). This lack can delay investments that game industries and companies working on ICT would devote to SG applications.

Too few cost/benefit proofs. The field lacks definitive cost/benefit analyses. Such analyses must spell out both the clinical and economic benefits of SG, weighed against the costs for using them over already-existing traditional methods.

Technological vs. clinical tools. SG are often considered by the general public as interesting and engaging technological tools. However, SG targeting assessment and rehabilitation of dementiarelated disorders should be considered primarily as clinical tools, and thus employed with the same care and cautions that apply to all clinical instruments.

Side effects.

As discussed in the "Weaknesses" section, side effects (such as cybersickness, perceptual-motor disturbances, drowsiness, fatigue, and headaches) may occur in users after using SG applications. If these aftereffects are not understood and managed properly, there is the possibility that a developer, clinician, researcher, or supporting institution could be held responsible for them.

The perception that the technological tools will eliminate the need for the clinician. This issue belongs to the more general problem of using technology-based instruments in the health sector. Although supporters of new technologies point out that SG are simply tools that extend the therapist's expertise, there still exists a view in some clinical quarters that any technology serves to subvert the clinical relationship.

Unrealistic expectations.

First-time SG users often experience unrealistic expectations, mostly based on overhyped media representations. This can lead to some disappointment when starting to use SG.

Poor academic and professional acceptance. SG are viewed by many researchers and clinicians as expensive toys gaining no scientific and clinical credibility. The perception of SG in therapy and rehabilitation contexts is slowly starting to change, and SG are gradually being accepted as tools providing new treatment options. This perception change is partly due to the increased number of articles published in mainstream journals, along with conferences dedicated to this topic. However, there is still a consistent part of the scientific community that derides the idea of using SG for scientific and clinical purposes. Controlled clinical trials will be necessary to evaluate the effectiveness of SG targeting dementiarelated disorders, and to increase the academic and professional acceptance.

Technophobia. The fear or dislike of ICT is still common in many elderly people, as well as in some of their family caregivers and healthcare professionals. This can make it difficult to employ SG as assessment and rehabilitation tools for some of the present generation of elderly people.

Serious Game design.

Participants working on the SG design session of the first round were asked to provide a definition of SG, and proposed the following: "A SG is a solution that combines entertainment and motivation to facilitate learning and social bonding in the context of an activity." Furthermore, they discussed about two SG currently under development at the CoBtek research unit: "Panic at the Nursing Home" devoted to the training of nursing home professional staff, and "battle ship" (http:// www.azagame.fr/) devoted to the physical and cognitive training of patients with ADRD. Finally, participants were asked to generate ideas for a SG adapted to people with ADRD. They proposed the following games: (1) a cultural game taking place in a museum, where players can navigate different collections of artifacts, and in each location they are asked questions and play games to train their memory and executive functions; (2) a social quiz game (similar to the board game "Trivial Pursuit") to be played by people with ADRD together with young people, with the aim of training memory, and promote inter-generational bonding.

Recommendations. Participants in the recommendation session of round 1 were asked to:

• Respond to 6 questions (see Table 1); after a brainstorming with a free response format, participants were presented with a list of items for each question, and asked to rate the importance of each item on a 0-3 scale (see Methods). • Comment and discuss practical recommendations for the development and use of SG in people with ADRD.

QUESTIONS

The results of the questions (mean scores of the ratings assigned to each item, collapsed across participants from round 1 and from the online survey) are reported in Figure 1. The additional elements emerged during the brainstorming of round 1, and the additional comments provided by participants of round 2 are reported below.

Generals questions

-SG for whom? i.e., what should be the target population for SG?

Based on a literature review (e.g., [START_REF] Mccallum | Dementia Games: a literature review of dementia-related Serious Games[END_REF], and on the authors' clinical expertise, participants were asked to rate if SG are adapted to the following categories of target people: AD (dementia stage), MCI, people with frailty, family caregivers and health professionals.

The results of the ratings showed that SG are considered between "adapted" and "very adapted" to: people with MCI, people with frailty syndrome, and family and professional caregivers. The highest score was assigned to the use of SG for people with MCI. SG were rated between "not very adapted" and "adapted" to people with AD and other dementia-related disorders. The experts in round 2 agreed that most of the SG currently available on the market are not very adapted to AD and dementia-related disorders, but suggested that designing SG for these target populations, though challenging, should be considered a priority. Furthermore, they highlighted that professional caregivers should be sensitized to the opportunity to use SG for dementia-related disorders. SG may also be employed as an engagement tool to train caregivers and provide them with more "active" dementia-related information, as opposed to the traditional standard training methods. The additional categories of target populations proposed during the brainstorming of round 1 included: all people; people with handicap, people suffering with phobias, autism, eating disorders, sleep disorders, depression, apathy, behavioral disturbances, chronic illnesses, people with reduced autonomy, people alone, and isolated, and old/new friends of people with impairments.

-SG for what? i.e., Why is it interesting to use the SG?

Participants were asked to rate if SG are adapted for the following purposes: to stimulate, assess and rehabilitate the patients, and to train caregivers and healthcare professionals. The results of the ratings showed that SG are considered between "adapted" and "very adapted" for patient stimulation, assessment, and rehabilitation and for the training of caregivers and healthcare professionals. The highest score was assigned to the use of SG for patients' stimulation. The additional categories proposed during the brainstorming of round 1 included: for everything, to have fun, to share and communicate, to help to address fears and phobias, to train in using new technologies, to help to accomplish everyday tasks, to educate, to improve the sense of self-efficacy, FIGURE 1 | Results of the recommendation questions. Mean ratings provided by participants of round 1 of the recommendation session and participants of the online survey to the two general questions (light gray) and the four questions focused on ADRD (dark gray). Rating scale: 0-3 scale (0, not adapted at all; 1, not very adapted; 2, adapted; 3, very adapted).

to work, for prevention purposes, and to reduce the cost of healthcare for the society, social security, and associations.

Questions focused on AD, related disorders, and frailty

-Why should SG be employed with patients with ADRD? What is the clinical target?

Participants were asked to rate whether SG are important to target AD patients: agitation, apathy, physical activity, cognitive impairment, impaired/reduced social interaction, and reduced functionality in activities of daily living. These categories were selected as they represent some of the most common symptoms of dementia, occurring in up to 90% of people with ADRD [START_REF] Cerejeira | Behavioral and psychological symptoms of dementia[END_REF]. As the terms "agitation," "apathy," and "functionality in activities of daily living" are terms that can be employed with different meanings in different clinical populations, we will briefly clarify their meaning in the context of ADRD.

Agitation is a neuropsychiatric symptom defined as "inappropriate verbal, vocal, or motor activity that is not judged by an outside observer to result directly from the needs or confusion of the agitated individual" [START_REF] Cohen-Mansfield | Can agitated behavior of nursing home residents with dementia be prevented with the use of standardized stimuli?[END_REF]. Confusion, discomfort, and unmet needs may underscore agitation, but the outward behavioral expression is usually the result of a need not being addressed [START_REF] Cohen-Mansfield | Measurement of inappropriate behavior associated with dementia[END_REF] as well as an attempt to overcome passivity [START_REF] Yu | The association of physical function with agitation and passivity in nursing home residents with dementia[END_REF]. Those unmeet needs, often present in AD patients, particularly institutionalized, are addressed quite successfully by non-pharmacological interventions such as music therapy [START_REF] Gerdner | Effects of individualized versus classical "relaxation" music on the frequency of agitation in elderly persons with Alzheimer's disease and related disorders[END_REF][START_REF] Remington | Calming music and hand massage[END_REF] and physical activity [START_REF] Alessi | A randomised trial of a combined physical activity and environment intervention in nursing home residents: do sleep and agitation improve?[END_REF][START_REF] Buettner | AD-venture program: therapeutic biking for the treatment of depression in long-term care residents with dementia[END_REF], and thus agitation should be susceptible to improvement through the use of SG.

Apathy is one of the most common neuropsychiatric symptoms of AD, occurring in almost 65% of dementia patients [START_REF] Ferri | Global prevalence of dementia: a Delphi consensus study[END_REF][START_REF] König | A review of current information and communication technologies: can they be used to assess apathy?[END_REF] being associated with a higher degree of global functional impairment [START_REF] Doron | Estimation of physical activity monitored during the day-to-day life by an autonomous wearable device (SVELTE project)[END_REF] and therefore loss of autonomy in activities of daily living [START_REF] Boyle | Executive dysfunction and apathy predict functional impairment in Alzheimer disease[END_REF][START_REF] Scarpini | Treatment of Alzheimer's disease: current status and new perspectives[END_REF][START_REF] Lechowski | Persistent apathy in Alzheimer's disease as an independent factor of rapid functional decline: the REAL longitudinal cohort study[END_REF]. [START_REF] Marin | Apathy: a neuropsychiatric syndrome[END_REF] defined it as a lack of motivation, interest, emotion, or feeling. Recently, [START_REF] Robert | Proposed Diagnostic Criteria for Apathy in Alzheimer's Disease and other Neuropsychiatric Disorders[END_REF] proposed a consensus definition in terms of a set of diagnostic criteria for apathy in Alzheimer's disease (AD). According to those criteria, an apathy diagnosis should meet the following requirements: first, the core feature of apathy, diminished motivation, must be present for at least 4 weeks; second two of the three dimensions of apathy (reduced goal-directed behavior, goal-directed cognitive activity, and emotions) must be present; third there should be identifiable functional impairments attributable to apathy. Most recent models consider apathy particularly in terms of an "absence of responsiveness to stimuli as demonstrated by lack of self-initiated action" [START_REF] Andersen | Ability to perform activities of daily living is the main factor affecting quality of life in patients with dementia[END_REF][START_REF] Levy | Apathy: a pathology of goal-directed behaviour. A new concept of the clinic and pathophysiology of apathy[END_REF]. Due to their playful nature, SG may be particularly adapted to target apathy deficits.

Finally, impairment in functionality in activities of daily living represents a major diagnostic requirement for AD (American Psychiatric Association, 1994). Several studies have investigated the link between cognitive functioning and daily functioning, both necessary to live autonomously. Cognitive domains such as memory, executive functioning, visuospatial functions, and object perception have all been found to correlate with activities of daily living impairment [START_REF] Perry | Relationship between functional and neuropsychological performance in early Alzheimer disease[END_REF][START_REF] Glosser | Visual perceptual functions predict instrumental activities of daily living in patients with dementia[END_REF][START_REF] Plehn | The relationship between neuropsychological test performance, social functioning, and instrumental activities of daily living in a sample of rural older adults[END_REF][START_REF] Jefferson | Object perception impairments predict instrumental activities of daily living dependence in Alzheimer's disease[END_REF][START_REF] Farias | Longitudinal changes in memory and executive functioning are associated with longitudinal change in instrumental activities of daily living in older adults[END_REF]. Those activities can be divided into basic activities of daily living such as self-maintenance skills, and Instrumental activities of daily living (IADL) such as preparing a meal and handling finances [START_REF] Lawton | Assessment of older people: selfmaintaining and instrumental activities of daily living[END_REF]. Complex IADL activities are sensitive to cognitive decline in early stages of AD, whereas basic activities remains preserved until later and more severe stages of the disease [START_REF] Stern | Measurement and prediction of functional capacity in Alzheimer's disease[END_REF][START_REF] Barberger-Gateau | A hierarchical model of domains of disablement in the elderly: a longitudinal approach[END_REF][START_REF] Wilms | Limitations in activities of daily living: towards a better understanding of subthreshold mental disorders in old age[END_REF][START_REF] Peres | Restriction in complex activities of daily living in MCI: impact on outcome[END_REF]. SG could be employed to train those activities, in order to promote a longer independent life at home.

The results of the ratings showed that SG are considered between "important" and "very important" to target apathy, motor activity, cognitive impairments, and diminished social interaction, with the highest score assigned to cognitive impairment. SG were rated between "not very important" and "important" to target agitation and reduced functionality in activities of daily living. The experts during round 2 suggested that SG could be further employed to target agitation. As seen above, very often agitation in people with dementia-related disorders is caused by the desire to overcome passitivity [START_REF] Yu | The association of physical function with agitation and passivity in nursing home residents with dementia[END_REF]. SG may represent an interesting and fun activity that could be possibly linked to an intervention: for instance, showing old pictures, or listening to well-known songs may have a calming effect. The experts also suggested that SG may be employed in order to improve patients' functionality in activities of daily living, especially for patients with MCI and patients in the AD pre-clinical stages. Importantly, SG should target depression and anxiety in AD and dementia-related disorders. Finally, the experts advised that SG should start to target a single ability/problem (e.g., the fear of falling down) and then extend progressively to other abilities (e.g., memory). The additional categories proposed during the brainstorming of round 1 includes: to create an intergenerational link, to improve mood, to combine different targets (e.g., physical + cognitive activity), to teach new abilities, to give personalized information on therapeutic goals, to help the patient to orient (knowing the time and day, etc.), to improve sleep and eating disorders, to promote cultural aspects.

-When (how frequently) should SG be used in patients with ADRD?

Participants were asked to rate if SG are adapted to be used everyday, once a week, and on request. The results of the ratings showed that SG are considered between "adapted" and "very adapted" to be used every day, once a week, and on request, with the highest score assigned to the use of SG every day. The additional categories proposed during the brainstorming of round 1 included: frequently and regularly.

-Where should SG be used for patients with ADRD

Participants were asked to rate if SG are adapted to be used at home, at the hospital, in the day centers and at the nursing homes.

The results of the ratings showed that SG are considered between "adapted" and "very adapted" to be used at home, in the nursing homes, and in the day centers, with the highest score assigned to the use of SG in day centers. SG were rated between "not very adapted" and "adapted" to be used at the hospital. An additional category proposed during the brainstorming of round 1 was: in the waiting rooms -Whom should patients with ADRD play SG with?

Participants were asked to rate whether SG are adapted to be used by the patient alone, and by the patient together with a therapist, a family caregiver, a professional caregiver, and a robot. The results of the ratings showed that SG are considered between "adapted" and "very adapted" to be used with a therapist, a professional caregiver and a family caregiver, with the highest score assigned to the use of SG with the therapist. SG were rated between "not very adapted" and "adapted" to be used alone and with a robot. The experts in round 2 added that SG could be played by the patient alone in the preclinical stages and the early clinical stages, to decrease the time, burden and effort the caregiver dedicates to the patient. The additional categories proposed during the brainstorming of round 1 included: in a group, with nurses.

PRACTICAL RECOMMENDATIONS FOR THE DEVELOPMENT OF SG TARGETING ADRD

Participants of the recommendation session of the first round were also presented with a list of recommendations for the development of SG targeting ADRD gathered from a literature review of the field (e.g., [START_REF] Bouchard | Developing SG specifically adapted to people suffering from Alzheimer[END_REF][START_REF] M146 Fua | Designing serious games for elders[END_REF]. These recommendations are reported below, together with the comments and integrations provided by the participants of the recommendation session (first round), and the additional comments provided during the second round.

Based on literature review, in order to be used for assessment and rehabilitation purposes of dementia-related disorders, SG should:

1. Keep track of the patient's cognitive abilities. The game should produce an in-game estimation of the patient's cognitive abilities. This would allow the clinician, caregiver and patient to assess the impact of the game on cognitive performance, and could be used to adapt the level of game difficulty to the actual performance achieved. 2. Determine an appropriate number of steps for the challenge.

Each challenge should be completed in a correct number of steps. A high enough number of steps would correctly train the cognitive abilities of the patients. However, too many steps could overload them and lower the benefits of the game. 3. Keep the player in his "flow zone." Flow represents the feeling of complete and energized focus in an activity, with a high level of enjoyment and fulfillment. Maintaining flow will make the game more enjoyable, improving the learning experience. 4. Promote naturalistic interactions. Employing naturalistic interactions should allow a significant reduction of the learning time, and thus optimize the effects of the game experience. 5. Use user-friendly interface for home-based exercises. Design choices should be made in a way to facilitate home-based rehabilitation training. 6. Take advantage of the multimodal aspect. It is important to train sensory and motor modalities at the same time. When possible, multi-sensory interactions should be introduced both as input and output. 7. Take into account the impairments of the users. It is crucial to consider that ageing and dementia have important effects on the sensory, motor, and cognitive system. Patients with dementia can suffer from visual and hearing problems, and have troubles with many aspects of memory (e.g., working memory, semantic memory, episodic memory, prospective memory). Furthermore, they can show a decline in the processing speed, have impairments in some executive functions (such as reasoning) and visuo-spatial abilities, and show some motor impairments. SG should take these impairments into account, for instance, keep the visual scenes as simple as possible, provide instructions, and cues repeatedly and at the right time (possibly using different modalities, e.g., auditory and visual cues), minimize actions requiring a fast response and requiring complicated movements, and so on (see [START_REF] M146 Fua | Designing serious games for elders[END_REF] for a review).

The participants of the first round of the IA workshop confirmed that all the recommendations reported above are relevant for the application of SG to people with ADRD. In addition, the following recommendation emerged: 8. Take into account the social and cultural background of the user. To promote ecological interactions, it is necessary to situate the game in the correct physical place (e.g., a city well known to the user) and cultural environment.

DISCUSSION AND FUTURE RESEARCH DIRECTIONS

The recommendations and analyses reported in the present paper have been carried out by and for healthcare professionals, patient, and family associations' representatives, engineers, and companies involved in the development of SG. Even if the overestimated expectations, frequently presented by the media, are far from being achieved, our SWOT analysis showed that SG can be considered as useful tools for professionals involved in the care of patients suffering from ADRD. However, more interdisciplinary work should be done in order to create SG specifically targeting these populations. The employment of SG in the domain of rehabilitation of ADRD is still in an early phase of development, characterized by some early encouraging research results. The many listed strengths provide a justification for continuing in this direction. Weaknesses do exist (in particular limitations concerning the difficulty that patients experience while using SG interfaces), as well as some threats, but none are terminal, especially when addressed in collaboration with the different partners involved in the development of SG for rehabilitation. In order to acquire more academic and professional credibility and acceptance, the field of SG would need to invest more in research. Specifically, researchers need to start collecting incremental data over numerous small-scale studies to test and evolve usability, usefulness, and access of SG targeting people with dementia-related disorders. At the same time, ethical, professional, and cost/benefit issues need to be considered and addressed. The field will need to face a number of challenges, which will benefit from the multidisciplinary collaboration between engineers, researchers, clinicians, healthcare professionals, patients, and family caregivers. For instance, it would be important for the companies and manufactures involved in the design and commercialization of SG to work closely not only with people in the ICT domain, but also with people coming from the health domain and with researchers. National and international agencies (such as the CNR in France-Centre national de reference en Santé) aiming to facilitate the communication and exchange between scientific community and private companies, may play a major role in driving this interdisciplinary exchange.

The recommendations gathered during the IA workshop addressed several abilities and deficits that SG may target in people with ADRD, as well as different ways to employ SG based on the patients' impairments, and may be used as guidelines to design SG specifically adapted to people suffering from these disorders. A limitation of the present work is that the participants of the IA workshop were asked to provide recommendations concerning the development and use of SG for people with frailty and ADRD, but we did not specify at which stage of the disease progression. As neuro-degenerative diseases are characterized by different stages of impairment, future investigations should better clarify which recommendations are adapted to the different disease stages.

Table 1 | Questions for recommendation session. General questions

 1

	SG for whom? i.e., what should be the target population for SG?
	SG for what? i.e., Why is it interesting to use the SG?

Questions focused on dementia-related disorders

	Why should SG be employed with patients with ADRD? What is the
	clinical target?
	When (how frequently) should SG be used in patients with ADRD?
	Where should SG be used for patients with ADRD
	Whom should patients with ADRD play SG with?

Frontiers in Aging Neuroscience www.frontiersin.org March

2014 | Volume 6 | Article 54 | 3

Table 2 | Summary of a SWOT analysis of the use of SG in ADRD.

 2

	Strengths

Frontiers in Aging Neuroscience www.frontiersin.org March 2014 | Volume 6 | Article 54 | 6

ACKNOWLEDGMENTS

The IA Workshop was supported by the Innovation Alzheimer association, The Centre National de Référence Santé à domicile et autonomie (CNR Santé) and by grants from the FP7 European Commission Seventh Framework Programme VERVE project, the Fondation Jean Louis Noisiez, the foundation Dream It, and COVEA. A special acknowledgment to all the participants of the IA Workshop: HEALTH domain (Gregory Bens-Sadoun, Pierre Bonhomme, Renaud David, Nathalie Fernandez, Sarah Gomez, Françoise Gugonis, Cathrine

Conflict of Interest Statement: Philippe H. Robert received research grants from: EEC, ANR, CoVEA, Eisai, and Lundbeck. He also received honorarium from Lundbeck, Lilly, Novartis, Jansen, Roche, Ipsen. François Bremond has declared associations with the following companies: Genious, Interactive4D, SolarGames. Stéphane Nave is full time employee at Hoffmann La Roche, Basel. Jean M. Orgogozo is a consultant for Novartis-Bases and is a corporate partner of Pharnext Biotech, Paris. The other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


E Markiewicz
email: eric.markiewicz@univ-valenciennes.fr

B Langrand

T Dupuy

Strain rate sensitivity analysis of spot-weld heat affected materials

Keywords: Spot Weld, Heat affected zone, Dynamic testing, Material behaviour

L'histoire thermo-mechanique dans chaque ZAT est analysée à l'aide d'une simulation EF du procédé de soudage. La micro-structure de chaque ZAT est alors reproduite sur des échantillons de traction à l'aide d'un simulateur thermo-mécanique Gleeble. La micro-structure des ZAT ainsi simulées est comparée à celle des ZAT d'un point soudé afin de valider la procédure. Les échantillons sont testés en traction uniaxale à différentes vitesses de déplacement. Le comportement du métal de base et des matériaux affectés thermiquement s'avèrent dépendents de la vitesse de déformation alors que la déformation à rupture semble quant à elle insensible à la vitesse de déformation dans le cas de matériaux affectés thermiquement.

Strain rate sensitivity analysis of spot-weld heat affected materials.

Analyse de l'influence de la vitesse de déformation sur le comportement des matériaux affectés thermiquement d'un point soudé par E. Markiewicz *, B. Langrand, T. Dupuy **

Introduction

In structural crashworthiness computations, explicit FE codes for fasteners commonly use a single element (e.g., a generalised spring, a beam, a spring type beam) connected to the shell elements by tied interfaces. The assembly's behaviour is a function of the welded point and the metal plate surrounding the joint. Local strains in the plate around the welded point contribute locally to the joint's observed non-linear macroscopic behaviour. It is thus necessary to integrate the local strains within a certain volume of the metal plate into the behaviour of the single macro element. In the model proposed by Combescure [START_REF] Combescure | A finite element to simulate the failure of weld points on impact[END_REF], the whole welded region is replaced by a single non-linear beam, which represents the region's homogenised behaviour. The main interest of this beam model is that it couples the loads in different directions, which is an improvement compared to previous solutions in which the joint models barely combined shear and tensile forces. The mechanical behaviour of the beam model is considered to be elastic-plastic, and a damage model is implemented in order to simulate the failure of the welded region. In its present version, the model does not consider strain-rate effects. Strain-rate dependence can be added to this model by introducing a material stress vs. strain curve depending on the strain rate (e.g., Johnson-Cook model) in the expression of the flow rule.

The authors have developed Arcan device for characterising the mechanical properties of joints in pure and combined modes I/II under quasi-static loading conditions [START_REF] Langrand | Non-linear and failure behaviour of spotwelds -A "global" finite element and experiments in pure and mixed modes I/II[END_REF]. The parameters of the elasticplastic-damage joint model [START_REF] Combescure | A finite element to simulate the failure of weld points on impact[END_REF] were characterised using the pure opening and shear experimental results and validated using combined loading tests or pull-out, coach-peel and single-lap shear tests results [START_REF] Langrand | Non-linear and failure behaviour of spotwelds -A "global" finite element and experiments in pure and mixed modes I/II[END_REF]. This experimental procedure has been extended by the authors to dynamic loading conditions using hydraulic jack and Split Hopkinson Pressure Bars (SHPB) apparatus [START_REF] Langrand | Strain rate dependence in spot welds: non-linear behaviour and failure in pure and combined modes I/II[END_REF]. Experimental results have shown that the yielding, hardening and failure of spot welds were strain-rate dependent. However strains and strain rates are non-uniformed in the plate surrounding the nugget because strain and stress states are tri-axial and materials are heterogeneous (Base Material, Heat Affected Zone and Welded Nugget), making very challenging the improvement of the flow rule of the beam model for dynamic loadings with a strain rate dependent model. The welded region is a very complex zone for which the actual material properties are complex and highly variable from point to point. For mild-steel (XES) considered in this work, the HAZ may be separated into four materials with different micro-structures (i.e., Grain Coarsened HAZ, Grain Refined HAZ, Inter-Critically HAZ and Sub-Critically HAZ). Consequently their material properties are affected. To obtain a material specimen from each of these regions, large enough for tensile testing, is challenging knowing the size of the heat-affected zone. It is proposed to analyse the thermomechanical history in each HAZ by FE simulation of the welding process and then to replicate their micro-structure on tensile sample using a Gleeble thermo-mechanical simulator. Uni-axial tensile specimens are then tested at different displacement rates in order to study the strain-rate dependency of these materials.

Simulated HAZ materials

A finite element model is developed with the Sysweld FE code to study the thermo-mechanical history in the spot weld. The FE analysis of the welding process was performed by the industrial partner of this project using his own methods and input data. The force and the intensity were defined by the automotive manufacturers and the metallurgy industry. The welding FE simulation was validated by comparing the temperature of the electrodes and the size of the welded nugget observed after one or several welding cycles. Nugget formation kinetics were validated for the case of two-sheets, as well as three-sheets joining. The predicted and the measured thermal histories were in a good accordance for the magnitude of the temperature even if a slight discrepancy in the heating and the cooling rates could have been observed [START_REF] Srikunwong | Influence of electrical-thermal physical properties in resistance spot welding modelling[END_REF]. Figure 1 presents the temperature distribution given by Sysweld at the end of the welding process. From the welding process FE simulation, the thermal transformation has been analysed in the different nodal positions from the centre of the nugget to the HAZ. The thermal transformation in the different HAZ of interest has been interpolated based on the numerical results of the FEA of welding process. Results in Table 1a are used as input data of a thermo-mechanical simulator in order to reconstruct the micro-structure in these HAZ materials. A thermo-mechanical simulator Gleeble 3500 has been used to affect the mild-steel material. Copper alloy electrodes deliver an electric discharge in the specimen. The temperature was measured by three thermocouples in order to verify the temperature homogeneity during the heat. The heating and cooling rates depend on the size of the specimen. The specimen is a tip of sheet metal plate (1.16mm thick) with the overall length and width equal to 120mm and 12mm respectively. Once heat affected, these tips will be machined with electro-erosion-machining to produce the tensile specimens to be tested at high rate of strain. First tests were conducted with the inter-electrode distance of 60mm. Heating rate up to 3000°C/s was measured, but the cooling rate was 25°C/s only. Reducing the interelectrode distance to 30mm has made it possible to increase the heating rate to values close to those given in Table 1a but the cooling rate was 100°C/s only. It was not possible to decrease again the distance between the electrodes (it won't be possible to machine tensile specimen with an homogeneous material between the griping areas) and a water hardening system was unavailable; so a air quenching system has been developed to increase the cooling rate to 250°C/s.

Simulated HAZ materials samples were produced at least with the thermal properties summarised in Table 1b. As already explain, the cooling rate was about 250°C/s. The maximum temperatures reached in the simulated HAZ materials were close to those observed in the welding simulation (Table 1a).

There is a more or less important deviation between heating rate depending on the HAZ considered due to the size of the specimen that remains too large to reach upper values. It has been verified first that the micro-structure of the simulated HAZ materials was homogeneous along the effective length (30mm) of the heated samples. Then, the micro-structure of the simulated HAZ materials was compared to the micro-structure of the HAZ materials in a spot weld. No difference can be observed between both micro-structures of SC-HAZ material. The shape and the size of the grains in this HAZ are close to those of the base material. The micro-structure of the simulated IC-HAZ material is in a good agreement with the IC-HAZ of the spot weld. The geometry and the dimensions of the grains are very close. They are typically representative of a fine-grained micro-structure. Little difference can be observed on the micro-structure of the GR-HAZ materials. The grain size in the spot weld is a little finer than the simulated GR-HAZ material. The morphology and the size of the grains in the GC-HAZ differ slightly in both the simulated and the spot weld materials. The simulated GC-HAZ material has a coarse-grained micro-structure, such as the material in the spot weld. The number of grains is however more important and batten-shaped grains can be observed. Moreover, needle-shaped microstructure (Widmanstäetten) is missing in the simulated material.

Strain-rate dependence of HAZ materials behaviour

The strain rate sensitivity of the HAZ materials and Base Metal behaviour is studied with the uni-axial tensile specimens shown in Figure 2. The specimen was subjected to uni-axial stress loading at different rates of strain, using a hydraulic jack and the experimental device, shown in Figure 2. Tests were performed at 5mm/mn (0.008/s), 0.01m/s (0.8/s) and 1m/s (90/s). Engineering stress vs. nominal strain curves obtained from tensile tests on HAZ materials in Figure 3 have exhibited inhomogeneous yielding between the elastic region to plastic deformation, known as Piobert-Lüders bands that form and propagate across the length of the sample. These P-L bands are common for mild steels when subjected to aging mechanisms, here thermal heating and cooling.

The Engng. stress vs. Engng. strain responses in Figure 3 has a notably dependence to the strain rate whatever the materials. The yield and ultimate stresses have increased with the strain rate. The yield stress is however more influenced by the strain rate than the ultimate stress. The material behaviour evolves consequently to rigid plastic behaviour at high rate of strain.

The fracture stress and fracture strain are used to compare the materials under uni-axial loading (triaxiality is equal to 0.33). These properties are affected by the strain rate in the case of the base metal. The fracture strains are gradually reduced with the strain rate: 0.61 at 5mm/mn and 0.50 at 1m/s. Similar fracture stresses were observed at 5mm/mn and 0.01m/s (150MPa), but the fracture stresses have increased more significantly to 175MPa at 1m/s displacement rate. The fracture stress and fracture strain are on the contrary not influenced by the strain rate for the heat affected materials.

Conclusions

In this paper, the thermo-mechanical history in each HAZ is analysed by FE simulation of the welding process. The micro-structure of each HAZ material is then replicated on tensile sample using a Gleeble thermo-mechanical simulator. The micro-structure of the simulated HAZ materials is compared to those of the HAZ materials in a spot weld. Apart for the GR-HAZ material which differs slightly the procedure to simulate HAZ materials is validated. Uni-axial tensile specimens are tested at different displacement rates. The behaviour of the base metal and the HAZ materials are strain-rate dependents but the fracture strain is observed to be not strain rate sensitive for the heat affected materials.

The parameters of viscoplastic plastic model can now be identified using the true stress vs. true strain diagrams from the experimental data. Finally, a refined FE model of the Arcan specimens can be setup to make it possible the prediction of the non-uniformed local stress, strain and strain rate fields developing under quasi-static and dynamic loading conditions. Based on these FEA results, the flow rule of the beam model with a strain rate dependent model will be developed.

Figure 1 :

 1 Figure 1: Temperature distribution and time histories in the spot weld. Contours are given in °C.

Figure 2 :

 2 Figure 2: Uni-axial specimen and experimental device for tensile tests at high rate of strain.

Figure 3 :

 3 Figure 3: Engng. Stress vs. Engng. Strain diagrams at different strain rates (0.008, 0.8 and 90s -1 ). (a) SC-HAZ, (b) IC-HAZ, (c) GR-HAZ, (d) GC-HAZ (right), Base Metal.

Table 1 : Thermal transformation of the welding process. Temperature is given in °C and rates in °C/s.

 1

	HAZ		(a) from the FEA			(b) by the Gleeble
		Max. Temp.	Heating rate	Cooling rate	Max. Temp.	Heating rate	Cooling rate
	GC	1250	4833	2754	1260	3608	258
	GR	1100	4421	2283	1096	3545	267
	IC	900	3743	1654	902	3155	233
	SC	750	3005	1183	745	2801	237

Acknowledgements

The authors are grateful to the European Union, the Regional Council Nord-Pas-de-Calais, the CISIT research program (http://www.cisit.org), and the French Ministry of Defence and Ministry of Higher Education and Research for their financial support of the DRSC Laboratory (Collisions and Rapid Structural Dynamics) with the participation of ONERA/DADS/CRD, UVHC/LAMIH and CNRS.