Chloé Morel
email: chloe.morel@univ-lorraine.fr

Henri Schroeder
email: henri.schroeder@univ-lorraine.fr

Claude Emond
email: claude.emond@pksh.ca

Jonathan D Turner
email: jonathan.turner@lih.lu

Eric Lichtfouse
email: eric.lichtfouse@gmail.com

Nathalie Grova
email: nathalie.grova@lih.lu

Brominated flame retardants, a cornelian dilemma

Introduction

Dying immediately in flames or from poisoning in the long run is the cornelian dilemma induced by the use of brominated flame-retardants. Indeed, these compounds have been intensively used to slow down fires in houses and buildings, thus increasing escape time from about 2 min to 20 min. They were initially thought to be safe, notably because their adhesive properties make them unlikely to reach human tissues. However, recent research has disclosed that brominated flame-retardants are developmental neurotoxicants that trigger neurodevelopmental deficits in young children [START_REF] Gray | Developmental neurotoxicity and autism: a potential link between indoor neuroactive pollutants and the curious birth order risk factor[END_REF] (Fig. 1). Since infants and young children have been exposed to brominated flame-retardants since the early 1970s, we are now observing the long-term developmental impact of this 50-year-old ticking time bomb. Here we discuss the occurrence of brominated flame-retardants, exposure at home, neurodevelopmental diseases and alternative flame retardants.

Industrial products

Brominated flame-retardants are industrial chemicals used to reduce the risk of fatal fire injuries [START_REF] Ryan | The brominated flame retardants, PBDEs and HBCD, in Canadian human milk samples collected from 1992 to 2005; concentrations and trends[END_REF]. They are extensively incorporated into consumer products such as domestic insulation, carpets, electronic devices, clothes, toys, and plastic products. They inhibit fire ignition or slow down the speed of fire combustion, and, consequently, they reduce the risk of fire-injury and property damage. Brominated flame-retardants belong to a wide family of more than 175 different flame retardants [START_REF] Birnbaum | Brominated flame retardants: cause for concern?[END_REF]. They occur in building materials worldwide and are still incorporated in many new materials [START_REF] Frederiksen | Human internal and external exposure to PBDEs -a review of levels and sources[END_REF][START_REF] Malliari | Children's exposure to brominated flame retardants in indoor environments -a review[END_REF].

Brominated flame-retardants are particularly abundant indoors. For instance, electronics and furniture contain polybrominated diphenyl ethers, circuit boards contain tetrabromobisphenol A, and polystyrene building materials contain hexabromocyclododecane. These three compounds are predominant both in indoor dust and in human tissues [START_REF] Frederiksen | Human internal and external exposure to PBDEs -a review of levels and sources[END_REF]). Hexabromocyclododecane has not been imported in the USA since 2018 (USEPA 2020). In Europe, hexabromocyclododecane and polybrominated diphenyl ethers are strictly controlled by legislation and import restriction, both in marketable goods and in waste (Sharkey et al. 2020a). Tetrabromobisphenol A is still used in manufactured products in Europe and only regulated at concentration above 0.1%. Implementating a new regulation in Europe could be an effective method to reduce the release of tetrabromobisphenol A in the environment [START_REF] Lenroot | A review of semi-volatile organic compounds (SVOCs) in the indoor environment: occurrence in consumer products, indoor air and dust[END_REF][START_REF] Malkoske | A review of the environmental distribution, fate, and control of tetrabromobisphenol a released from sources[END_REF].

Electronics and textile are major contributors of brominated flame-retardants in the environment. Here, the three largest producers of e-waste worldwide are the European Union with 10.07 million tons (Mt), China with 7.21 Mt and the USA with 6.3 Mt, which are also global leaders as manufacturers and exporters of textile (Sharkey et al. 2020b). In the Middle East, Australia, and Africa, levels of brominated flame-retardants are usually low in indoor spaces [START_REF] Malliari | Children's exposure to brominated flame retardants in indoor environments -a review[END_REF].

Exposure at home

Indoor air quality is a major source of concern in health risk assessment [START_REF] Azuma | Risk assessment concepts and approaches for indoor air chemicals in Japan[END_REF]. Indeed, indoor air is much more polluted than the outdoor atmosphere. As a consequence, opening windows regularly is recommended to evacuate airborne contaminants. In sharp contrast, during the previous decades, many residences were almost sealed to save energy, e.g. to avoid heating loss. This has highly reduced air exchange with the exterior, thus increasing the concentration of pollutants. Breathing indoor gases and airborne particulate matter are therefore a major source of inhaled or ingested exposure [START_REF] Koivisto | Source specific exposure and risk assessment for indoor aerosols[END_REF]. Recently, pollutant exposure further escalated during the coronavirus disease 2019 (COVID-19) pandemic due to people lockdown and rising teleworking [START_REF] Moretti | Characterization of home working population during COVID-19 emergency: a cross-sectional analysis[END_REF]. Today, we spend approximately 80-90% of our time indoors, thus getting exposed to both indoor contaminants, and outdoor contaminants that enter homes and buildings via ventilation [START_REF] Hussein | Modeling regional deposited dose of submicron aerosol particles[END_REF]. Remarkably, this exposure to mixtures of several chemicals induces adverse health effects in an additive or even supra-additive way [START_REF] Lenroot | A review of semi-volatile organic compounds (SVOCs) in the indoor environment: occurrence in consumer products, indoor air and dust[END_REF].

The concentration of brominated flame-retardants, to which we are exposed indoors from dust particles, exceeds in general that of outdoor air [START_REF] Fromme | Polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD) and "novel" brominated flame retardants in house dust in Germany[END_REF][START_REF] Harrad | Indoor contamination with hexabromocyclododecanes, polybrominated diphenyl ethers, and perfluoroalkyl compounds: an important exposure pathway for people?[END_REF][START_REF] Mcgrath | Concentrations of legacy and novel brominated flame retardants in indoor dust in Melbourne, Australia: an assessment of human exposure[END_REF][START_REF] Ni | HBCD and TBBPA in particulate phase of indoor air in Shenzhen, China[END_REF]. Exposure mainly occurs through ingestion of foodstuff and inhalation of dust and airborne particles, increasing the body's burden of internal brominated flame-retardants. Health concerns have arisen following the detection of some brominated flame-retardants released into the indoor environment and subsequently taken up into human tissues [START_REF] Birnbaum | Brominated flame retardants: cause for concern?[END_REF][START_REF] Usepa | Risk evaluation for cyclic aliphatic bromide cluster (HBCD)[END_REF]. Due to their persistence in the environment and their lipophilic properties, polybrominated diphenyl ethers and hexabromocyclododecane accumulate in lipidic tissues [START_REF] Rivière | Dietary exposure to perfluoroalkyl acids, brominated flame retardants and health risk assessment in the French infant total diet study[END_REF]Sharkey et al. 2020a). Children are particularly exposed to brominated flame-retardants because of their hand-to-mouth behaviour and their playing on dusty grounds [START_REF] Landrigan | Children's health and the environment: public health issues and challenges for risk assessment[END_REF]). Indeed, dust ingestion is the main route of exposure in children followed by indoor air inhalation and dermal contact.

Fig. 1 Brominated flame-retardants are synthetic chemicals incorporated in a wide variety of products such as plastics, textile and electric equipment, to make them less flammable. Once thought to be safe, there is now growing concern on their toxic effects because they occur widely in natural media

High risk for children's brains

There is growing evidence that human sensitivity to brominated flame-retardants changes at different life stages [START_REF] Street | Current knowledge on endocrine disrupting chemicals (EDCs) from animal biology to humans, from pregnancy to adulthood: highlights from a national Italian meeting[END_REF]. In particular, the toxicity of brominated flame-retardants may induce diseases a very long time after initial body contamination. In other words, an adult disease may result from foetal exposure [START_REF] Grova | Epigenetic and neurological impairments associated with early life exposure to persistent organic pollutants[END_REF]). This phenomenon is rather similar to the poorlyexplored phenomenon of 'pollutant resurrection' whereby some pollutants stay trapped as bound residues in complex media, e.g. soils, then are liberated years later to induce a second wave of toxicity [START_REF] Charles | Pesticide resurrection[END_REF]). The earlylife window of susceptibility of 1000 days between the child's conception and the age of two years old is crucial in the development of health and diseases because major changes occur during this time period in gene expression by epigenetic modifications as the infant develops [START_REF] Barker | Infant mortality, childhood nutrition, and ischaemic heart disease in England and Wales[END_REF]. Therefore, pollutant exposure may modify epigenetic mechanisms and, in turn, implement changes that will cause adult diseases. When development is disrupted during organogenesis, there is no 'going back' and the damage becomes irreversible.

The brain is probably the most sensitive of all organs during development. For instance, exposure to polybrominated diphenyl ethers is estimated to be responsible for a cumulative loss of 162 million intelligent quotient points in American children from 2001 to 2016 [START_REF] Gaylord | Trends in neurodevelopmental disability burden due to early life chemical exposure in the USA from 2001 to 2016: a population-based disease burden and cost analysis[END_REF]). This loss is higher than that of well-known neurotoxins such as lead, mercury and pesticides. Brain development is a lifelong process starting with fertilization and continuing throughout life. In contrast to many other tissues, there are critical periods of brain sensitivity during pregnancy, the post-natal period and the first 3-6 years of life where synaptic plasticity, neurogenesis, neuronal differentiation, myelination, migration, synaptogenesis and apoptosis occur in a tightly-controlled schedule [START_REF] Rice | Critical periods of vulnerability for the developing nervous system: evidence from humans and animal models[END_REF]. In particular, during neurogenesis, neuronal differentiation and synaptogenesis take place before birth, myelination and synaptic plasticity continue from childhood to adulthood [START_REF] Lenroot | A review of semi-volatile organic compounds (SVOCs) in the indoor environment: occurrence in consumer products, indoor air and dust[END_REF]. As a consequence, pregnant women, neonates, infants and young children are particularly sensitive to brominated flame-retardants, which may lead to neurodevelopmental disorders such as autism spectrum disorders and attention-deficit hyperactivity disorder [START_REF] Grova | Epigenetic and neurological impairments associated with early life exposure to persistent organic pollutants[END_REF]. Pollutant-induced alteration of foetal germ cells in utero and gametes in later life may also induce transmission of the latent disease to the next generation. Thus, even though correlations between brain disorders and exposure to brominated flame-retardants are difficult to assess with absolute certainty, notably due to the presence of other pollutants, research should decipher the mechanisms of neurodevelopmental disorders induced by brominated flame-retardants.

Autism spectrum disorders

Neurodevelopmental diseases such as the autism spectrum disorder have dramatically increased recently, of 38.3% from 1997 to 2017 [START_REF] Zablotsky | Prevalence and trends of developmental disabilities among children in the United States: 2009-2017[END_REF]. This disorder induces a deficit in communication, struggles with social interactions, poor interests, and repetitive behaviours that appear early in life [START_REF] Cattane | Prenatal exposure to environmental insults and enhanced risk of developing schizophrenia and autism spectrum disorder: focus on biological pathways and epigenetic mechanisms[END_REF][START_REF] Roux | The cerebellum: from motor diseases to autism[END_REF]. One per cent of individuals are affected by autism spectrum disorder in France [START_REF] Ha | roubles du spectre de l'autisme en France. Estimation de la prévalence à partir du recours aux soins dans le Système national des données de santé (SNDS)[END_REF]). In the USA, the disorder for children rose from 1 out of 166 in 2004 to 1 out of 59 in 2018 [START_REF] Baio | Prevalence of autism spectrum disorder among children aged 8 years -autism and developmental disabilities monitoring network, 11 sites, United States, 2014[END_REF]). The disorder is probably due to a disruption in brain development, with a prevalence 3-4 times higher in boys than in girls. Research on hereditary and genetic components has disclosed more than 400 genes associated with autism spectrum disorders [START_REF] Bourgeron | From the genetic architecture to synaptic plasticity in autism spectrum disorder[END_REF][START_REF] Chaste | Autism risk factors: genes, environment, and gene-environment interactions[END_REF][START_REF] Roux | The cerebellum: from motor diseases to autism[END_REF]. Epigenomic analyses suggest that methylation-dependent transcriptional regulation is a driver in the aetiology of autism spectrum disorders [START_REF] Tremblay | DNA Methylation and susceptibility to autism spectrum disorder[END_REF]. There is now a clear and pressing requirement to define the precise role of gene-environment interactions in autism spectrum disorders.

Exposure to chemical pollutants contributes to the aetiology of autism spectrum disorders [START_REF] Grova | Epigenetic and neurological impairments associated with early life exposure to persistent organic pollutants[END_REF][START_REF] Kalkbrenner | Environmental chemical exposures and autism spectrum disorders: a review of the epidemiological evidence[END_REF]. Brominated flame-retardants are suspected to disrupt the thyroid axis, the oxidative stress system, immune response, and neuroinflammation [START_REF] Hertz-Picciotto | Polybrominated diphenyl ethers in relation to autism and developmental delay: a case-control study[END_REF][START_REF] Messer | Mini-review: polybrominated diphenyl ether (PBDE) flame retardants as potential autism risk factors[END_REF]. Hexabromocyclododecane disrupts the thyroid system in humans and other animals, and is a substance of concern for foetuses, new-borns exposed by breastfeeding, and children exposed via food and dust ingestion (Chain et al. 2021;ECHA 2019;[START_REF] Kim | Tetrabromobisphenol a and hexabromocyclododecane flame retardants in infant-mother paired serum samples, and their relationships with thyroid hormones and environmental factors[END_REF]. Pre-clinical early-life models confirm the toxicity of hexabromocyclododecane for the developing brain [START_REF] Fernandes | N6-Methyladenine in Eukaryotic DNA: tissue distribution, early embryo development, and neuronal toxicity[END_REF][START_REF] Maurice | Short-term effects of a perinatal exposure to the HBCDD alpha-isomer in rats: assessment of early motor and sensory development, spontaneous locomotor activity and anxiety in pups[END_REF]. Epidemiological data suggest that in utero exposure to hexabromocyclododecane induces modifications in motor skills, attention span and metabolism [START_REF] Roze | Prenatal exposure to organohalogens, including brominated flame retardants, influences motor, cognitive, and behavioral performance at school age[END_REF]. Nonetheless, a recent study failed to validate this hypothesis for children aged 18 months, born in mothers with a median blood hexabromocyclododecane level of 0.82 ng/g of lipids [START_REF] Goodman | Epidemiology of PCBs and neurodevelopment: Systematic assessment of multiplicity and completeness of reporting[END_REF].

Perinatal exposure to hexabromocyclododecane at realistic doses induced behavioural deficiencies including anxiety, locomotion and sexual activity in rats, from the age of 4 weeks until adulthood [START_REF] Maurice | Short-term effects of a perinatal exposure to the HBCDD alpha-isomer in rats: assessment of early motor and sensory development, spontaneous locomotor activity and anxiety in pups[END_REF]. Hexabromocyclododecane exposure may induce neurodevelopmental disorders through the regulation of DNA methylation. For instance, 6-methyl adenine modifications have been linked to phenotypic diseases such as tumorigenesis [START_REF] Xiao | /sup>-Methyladenine DNA modification in the human genome[END_REF]. This methylation increases with chronic stress in mice's prefrontal cortex [START_REF] Yao | DNA N6-methyladenine is dynamically regulated in the mouse brain following environmental stress[END_REF]. Another study suggests a potential overlap between genes that show dynamic changes in 6-methyladenine expression and genes that are involved in the onset of neuropsychiatric disorders [START_REF] Armstrong | Diverse and dynamic DNA modifications in brain and diseases[END_REF].

Demonstrating the mechanisms underlying the developmental neurotoxicity induced by hexabromocyclododecane, especially the role of DNA methylation and other epigenetic processes, should help in understanding the pathophysiology of autism spectrum disorders and identify future therapeutic research lines. Further research is required to understand the mechanisms by which certain brominated flame-retardants could act as a factor of susceptibility in the aetiology of autism spectrum disorders. This will help in developing safer alternative products.

Alternative flame retardants

In 2017, 120,000 people died and 9 million were injured in heat-related incidents, with higher death tolls of children and the elderly [START_REF] James | Epidemiology of injuries from fire, heat and hot substances: global, regional and national morbidity and mortality estimates from the Global Burden of Disease 2017 study[END_REF]. Moreover, the proportion of flammable materials in new products has increased and, in turn, has reduced the time available to escape a blaze to 3-6 min, and the time to flashover, i.e. the time when a fire is no longer controllable, to less than 20 min. Flame retardants therefore remain essential for building materials and furniture. Historically, organochlorines were used until evidence of their persistence, accumulation and toxicity. In the 1970s organochlorines were replaced by brominated compounds such as polybrominated diphenyl ethers, tetrabromobisphenol A and hexabromocyclododecane, which are still currently incorporated into furniture, textile, children's products, building materials and electronics. Nonetheless, from the early 2000s, the use of all organohalogenated flame retardants has declined due to mounting scientific evidence on their toxicity. Three polybrominated diphenyl ethers were banned in 2009 and 2017 by the Stockholm convention, and hexabromocyclododecane is is currently under scrutiny.

Organophosphate flame retardants have started to replace polybrominated diphenyl ethers, but recent research has disclosed toxicity at exposure levels currently reported in biota and humans [START_REF] Blum | Organophosphate ester flame retardants: are they a regrettable substitution for polybrominated diphenyl ethers?[END_REF]. The incorporation of a brominated styrene-butadiene copolymer into polystyrene has been developed as an alternative to hexabromocyclododecane. This copolymer is more stable, inert and shows little to no bioavailability in environmental matrices, yet the sole published toxicological data are that of the manufacturer and the US-EPA report, which indicates a lack of acute toxicity genotoxicity, sensitization and irritation, and ecotoxicology after only 28 days of exposure [START_REF] Beach | New class of brominated polymeric flame retardants for use in polystyrene foams[END_REF]. Hints of possible toxicity come from the fact that the brominated styrene-butadiene copolymer is synthesized from styrene and butadiene, which are known for their neurotoxic and developmental toxicity [START_REF] Khalil | Clinical evaluation of 1,3-butadiene neurotoxicity in humans[END_REF].

Sustainable alternatives include glass or mineral fibres of very low organic content, which therefore do not release toxic fumes. Sandwiching a polystyrene or polyurethane layer with steel, aluminium, or mineral layers is an alternative to limit contamination and maintain insulation. The use of non-combustible interliners in aircraft seats is also very effective in reducing post-ignition fire spread and development in upholstered furniture. Last, intumescent coating systems are deposited on steel or timber surfaces to inhibit ignition and flame propagation. These coatings are made by incorporation of carbon, ammonium polyphosphate and melamine in a paint that forms a blown char when heated [START_REF] Camino | Intumescent fire-retardant systems[END_REF]).

Acknowledgements

We acknowledge the support of the Ministère de l'Enseignement supérieur et de la Recherche in France, the Luxembourg Institute of Health and the Ministry of Higher Education and Research of Luxembourg for their continued support. The work of NG, HS and CM on neurodevelopmental toxicity of HBCDD was funded by the A2F young research grant-Lorraine University and the "Ministère de l'Enseignement supérieur et de la Recherche". The work of JDT on the long-term effects of early live environmental exposure was funded by the Fonds National de la Recherche, Luxembourg, Grant numbers: C12/BM/3985792 "EpiPath", INTER/ ANR/16/11568350 "MADAM", C16/BM/11342695 "MetCOEPs", and C20/BM/14766620 "ImmunoTwin".

Declarations

Conflict of interests All the authors declare that they have no known conflict of interest.

Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


N Roelandt
email: nicolas.roelandt@univ-eiffel.fr

F Bahoken

G Le Campion

L Jégou
email: jegou@univ-tlse2.fr

M Maisonobe
email: marion.maisonobe@cnrs.fr

B Mericskay
email: boris.mericskay@univ-rennes2.fr

E Côme

ONE ARABESQUE IN THE SMALL WORLD OF OD WEBMAPS

Keywords: Open Geospatial Software, Origin-Destination Matrix, Spatial Network, Geoweb, Flow Mapping

Arabesque is an application for the exploration and geovisualisation of origin-destination flows (or spatial networks), developed within the framework of the Univ. Gustave Eiffel (ex. IFSTTAR)-funded research project geographic flow visualisation (gflowiz) geoflowiz, in collaboration with the CNRS. It allows both the exploration and the filtering of OD data and their representation, with a strong emphasis on geographic information layering and features' semiology. The key-objective is to propose an easy way to produce a modern cartography (a geovisualisation) of thematic flows (e.g. bilateral flow volume), at several geographic scales, even from your own datasets. The objective of this article is to position Arabesque in the range of geoweb applications for producing flow maps, by comparing its functionalities with those of similar web applications-Magrit, Kepler.gl, flowmap.blue -pointing out their respective advantages and limitations. The analysis of its functionalities is compared on the same flow dataset-MOBSCO, i.e. a dataset describing the school mobility of French pupils and students on a given year -for a practical and empirical "validation" of its contributions. We demonstrate that the configurations and appearances of these tools' visual output depend largely on the culture of their developers, and on the use and audiences for which they have been developed. Flow mapping web tools FOSS4G compliance open free

INTRODUCTION

Arabesque is a Web application for the exploration and geovisualisation of origin-destination (OD) flows. Its ongoing development since 2018 comes at the same time as several other new online flow mapping tools. These new online tools are part of the same family within the wider universe of existing IT solutions for mapping relational and origindestination data. Within this family of online tools, there is a certain variety: some are aimed at a wide range of users, and others are more specific and aimed at more specialised audiences. Most of them share the fact that they rely on common and open data-visualisation, mapping and geoprocessing libraries of functions. However, only some of them are entirely free and open-source, as is Arabesque. The objective of this article is to position Arabesque in the range of geoweb applications for producing flow maps, by comparing its functionalities with those of comparable applications, pointing out their respective advantages and limitations.

To provide a practical and empirical "validation" of Arabesque contribution and specificity, we first select a small number of flow mapping tools sharing common characteristics with Arabesque and second, compare these tools using the same flow dataset: MOBSCO, i.e. a dataset describing the school mobility of French pupils and students on a given year. In doing so, we highlight the complementary effects between these tools which, for the same set of data, make it possible to obtain different types of representations. Those configurations and appearances depend largely on the culture of the developers of these tools, and on the use and audiences for which they have been developed. The differences between these tools bring out two different cultures: that of thematic mapping and statistical analysis of information and the more 'dataviz' oriented culture of information exploration. We think that this dividing line is somehow reminiscent and related to the one described and analysed by Carmichael and Marron between data science and statistics (2018).

THE SMALL WORLD OF ORIGIN -DESTINATION MATRIX MAPPING

The development of online web mapping tools

There are currently a growing number of computer solutions for mapping flows and origin-destination (OD) matrix with one's own data. According to their environment, these tools can be classified into 3 main families: 1) desktop GIS softwares (e.g. QGIS, ArcGIS); 2) programming languages with (geo)data analysis capabilities (e.g. Python/R); 3) online tools with user-friendly interfaces.

As demonstrated in a recently published paper [START_REF] Bahoken | Typologie d'un geoweb des flux et réseaux / Typology of a flow and network geoweb[END_REF], the latter category has tended to develop further in recent years. This dynamics is concomitant with and part of a wider phenomenon: the rise of the geoweb (Hackley et al., 2008;Mericksay and Roche, 2011). But among online mapping tools, flow mapping tools tended to emerge in greater numbers from the mid-2010s onwards, taking advantage of the opportunities offered by the release of feature libraries such as D3 (Botstock et al., 2011) and new technologies such as WebGL or vector tiles that opened up a range of new possibilities for online graphics and mapping. The growing attraction to open source tools and replicable approaches in the world of mapping and geomatics also fostered this dynamic [START_REF] Lambert | Reproducible Cartography[END_REF].

In a recent contribution, Bahoken (2020) provided a review of about 80 flow-mapping web applications resulting from a digital curation. Among all these applications, only a minority are offering users the possibility of importing, analysing, exploring and mapping their own dataset. The majority of retrieved applications only allows the exploration of a preselected dataset on a certain thematic (i.e. global trade, urban mobilities, migration flows, scientific collaboration).

Flow mapping tools

Among the 80 web applications listed in that review of flow map, 8 have the particularity of allowing users to import their own dataset, as does Arabesque. (developed between 2017 and 2019). Thus, the main criterion that allowed us to choose the applications finally retained for the test is their accessibility.

While it also allows users to map their own data, NETSCITY has the particularity of being more specifically designed for mapping science data (i.e. scientific collaborations). Moreover, being still in beta version, the code sharing operation of this application is not yet finalized; thus justifying excluding it from the following comparison with Arabesque.

The MOBSCO dataset

To perform our comparison of Kepler.gl, Magrit and Flowmap.blue with Arabesque, we selected a flow dataset that will be visualized in each software. For this purpose, we choose a dataset which remains little known. It is the detailed file of the French population census describing the school mobility of individuals and, more specifically, commuting to and from the place of study (MOBSCO). In spite of its great thematic interest for mobility researchers in France, it has not been the object of much exploitation and valorisation until now.

It is a product of the French National Institute for Statistics and Economic Studies (INSEE) and is available online. It comprises two sorting variables: the municipality of residence and that of the place of study (including in bordering countries). Its scope encompasses individuals enrolled in an education establishment and aged 2 years or over. The source file contains 4,730,521 records and 22 variables. For each statistical individual, it lists its municipality of residence, its place of study, its age group, and highest diploma, the socioprofessional category of the households he belongs to, the number of persons and cars of the households.

The size of the dataset being too large for online flow mapping tools, we preformatted and aggregated it at the departmental level (higher administrative level than municipalities). We also filtered the data to only keep records for students only (higher diploma is at least the baccalaureat), because they are more likely to move outside their municipality of residence to study. The final dataset contains 27090 records. In addition, we used the ADMINEXPRESS database of 24-09-2019 provided by the French National Geographic Institute (IGN) for the departments' georeferencing. We limited our analysis to the France metropolitan area. The nodes were created by calculating the centroids of the departments.

BENCHMARKING/ USE TEST

In this section, we are testing 4 flowmap geoweb tools for OD flows that are interfaced on the web, with the same dataset, in order to compare them, both in terms of ergonomics and rendering. The objective is to position Arabesque in the range of geoweb applications for producing flow maps, by comparing its functionalities with those of comparable applications, pointing out their respective advantages and limitations.

Another interest of this comparative approach is to show the dependence of the rendering: from the type and style of the map to the tool. This dependence is also linked to the audience for which the application is intended, depending on whether its interest will be focused on the exploration and analysis of cartographic data or on cartographic editing. In the first subsection, we introduce the 4 applications, their specificities, the audience and use type for which they were developed. In the second subsection, we present the result of the comparative test we performed with the MOBSCO dataset, and in the third subsection we present a summary diagram showing the specificities of each application at each step of the data processing chain, from data pre-formatting to exporting, including the different analysis options available.

Presentation of the flow mapping web applications

3.1.1 Arabesque: Arabesque is an application for the exploration and geovisualisation of origin-destination flows (or spatial networks), developed within the framework of the Univ. Gustave Eiffel (ex.IFSTTAR)-funded research project geographic flow visualisation (gflowiz) geoflowiz, in collaboration with the CNRS. Its development benefits from several well known open-source data-visualisation, mapping and geo-processing libraries (OpenLayers, D3, TurfJS) and build also upon well known open data-sets (OpenStreetMap, NaturalEarthData).

It allows both the exploration and the filtering of OD data and their representation, with a strong emphasis on geographic information layering and features' semiology. The keyobjective is to propose an easy way to produce a modern cartography (a geovisualisation) of thematic flows (e.g. bilateral flow volume), at several geographic scales, even from your own datasets. Arabesque main public is academia (researcher and students) but with the objective to be simple to use for anyone. Arabesque was launched in 2019 by Côme E. ( 2019) under a BSD 2-clause licence.

3.1.2 Kepler.gl: launched in 2018, Kepler.gl is an open source geovisualisation tool designed for complex datasets (large, spatio-temporal, 3D). Built with Deck.gl [START_REF] Wang | Deck. gl: Large-scale web-based visual analytics made easy[END_REF], Kepler.gl uses WebGL to display large data sets in multiple forms quickly and efficiently. For example, it provides dot maps, choropleth maps, heatmaps, clusters or even 3D grids [START_REF] He | Data-Driven Urbanism: The Balance Between Spatial Intelligence and Design Craftsmanship[END_REF]. Developed by the technology company Uber, this application allows both exploring and mapping large and complex geographic datasets thanks to client-side rendering. All layer geometry calculations are GPU accelerated, smoothly rendering millions of points and making kepler.gl a much more powerful web tool than traditional mapping software and other web mapping solutions. The target audience is that of data journalists and data scientists whose objective is above all to explore large data sets and not to correctly represent flow logics. Kepler.gl is available under a MIT licence. [START_REF] Boyandin | Flowmap.blue, Geographic flows visualisation tool for the people[END_REF]. Only developed in JavaScript using flowmap.gl, deck.gl, mapbox, d3, blueprint and CARTOColors, the tool is designed to explore and create flow maps from Google Sheets or tabular data in a web browser. Additional tools are also available such as geocoding, matrix converter (wide / long formats). Flowmap.blue allows automatic clustering, provides global data filtering and automatically adapts the scales of the flow thickness and colors. The tool is however limited in its style, the type of flow map that can be made and the projection proposed. It's prime objective seems to allow the production of a nice flow map easily and very quickly, in response to the requirements of data journalists and data scientists. Flowmap.blue is also available under a MIT licence.

Flowmap.blue: this web application was firstly launched in 2019 by

3.1.4. Magrit: this thematic web mapping application was launched in 2017 [START_REF] Viry | Magrit: a new thematic cartography tool[END_REF]. It is free under the CeCILL licence and compatible with the GNU GPL. Mainly developed in Python and modern JS (ES6) Javascript using notably the d3.js library, its code is already open. Magrit is mainly intended for social scientists and thematic cartographers as it is firstly designed to teach statistical cartography, respecting the rules of cartographic semiology [START_REF] Bertin | Sémiologie graphique. Les diagrammes. Les réseaux. Les cartes[END_REF]. To do so, it offers 12 types of maps (e.g. choropleth and smoothed choropleth, proportional symbols, waffle, cartogram, discontinuity) according to different projections. One of them allows to represent flows, but in a reduced version: only in the form of hub lines with weighted symmetrical links.

Use test of the flow mapping web applications

As said before, we loaded the dataset we subsetted from the MOBSCO dataset. Then we perform some manipulations in order to test the filtering and rendering capabilities of each software. The figures from 1 to 7 show, for each application, its interface and the final result obtained. In order to highlight the capabilities of each software, we have chosen not to show the default displays, which are generally quite unreadable with OD data and have preferred to highlight maps that are more elaborate.

Arabesque:

Arabesque starts by loading the links data, and allows the user to specify ID columns, the flow (called volume) column and, if needed, an aggregation function. The next step consists in loading location data or to use a preset location dataset. When joining the two datasets, it will display a message indicating how many links were removed if the location of a node is not available (i.e. missing data). The layer panel offers functionalities to add, remove and organize layers. From there, you can change the map projection (default is webmercator but others are available) or the title of the map. For each layer, if available, you can access customisation panels (see Figure 2) to change the colour scale, the links shape and size, etc.

The right panel is dedicated to the data. By default, a histogram represents the distribution of the flows. It is also a graph query, i.e. a sliding window that can be adjusted to display only a portion of the distribution. This is shown in Figure 3, where only the flows greater than 100 are displayed. Figure 3 shows another graph query on distance. Arabesque allows the user to define his own filters on the loaded data but also on the automatically created indicators, like, for example, the distance (link size) shown in Figure 3. In the 4 applications reviewed here, Arabesque is the only one to propose that. Figure 3. Exploration with Arabesque. Finally, in the center, the map displays the layers with the chosen symbology according to the filters defined in the data panel.

Kepler:

In terms of flow mapping, Kepler.gl offers two rendering modes for data origins destinations, arcs and lines. On the one hand, the arc mode draws an arc between two points to represent a link in 3D. The higher arc represents the greatest distance. Line layers are the 2D version of arc layers (flat on the map). The symbology of arcs and lines can be customized in terms of line stroke and colour and is based on a variable (Figure 4). This method of cartographic design involves applying an angle to the line to turn it into a curve, giving it a more aesthetic appearance [START_REF] Gu | Revisiting flow maps: a classification and a 3D alternative to visual clutter[END_REF]. With 3D, this curvature of the line can be combined with an elevation, which allows the flow to be represented differently [START_REF] Dorling | The visualisation of spatial social structure[END_REF]. In terms of interactivity, this mode of representation is accompanied by interesting features, such as the selection of a subset of data relating to a departure or arrival area by hovering over the map (brushing), as shown on Figure 4.

3.2.3

Flowmap.blue: The optimal import of data into flowmap.blue is done with a googlesheet composed of 3 sheets (map properties, locations, flows), a minimal template is proposed. Flowmap only allows you to make a flow map. In the interface (Fig. 5.), it is possible to filter by origin or destination as well as to modify the style of the flows and in particular to animate and cluster them. Flowmap.blue can be efficient for data exploration thanks to its interactivity and the display of toolboxes on locations and flows.

Other customization options are possible but require a mastery of Mapbox, and are to be integrated in the "properties" page of the googlesheet (background modification, filtering by feed directions...). Flowmap.blue also offers a "simplified" approach with a data import page focusing only on locations and flows, via the website.

Magrit:

Magrit requires an upload in two parts: the geographical points dataset, and the links data. With OD data, Magrit allows to create a single type of map, links between points, but with the option to filter the small links and to group the lines into classes.

After the initial configuration of the map, the user can adjust the style of the representation: colour and size of the links, overall design of the page (titles, mentions, scale bar…).

Figure 6. Magrit interface

As shown in Fig. 6, Magrit provides some advanced features to build the map: several discretization modes, number of classes and their limits.

3.3

Summary diagram

We tried to present in a synthetic way the different steps involved by the flow mapping web-tools to explore a flow dataset. These steps are organized in five main tasks: data preprocessing, data processing, mapping, exploration and export. The diagram (Figure 7) therefore shows the workflow of a flow map from data to a finalized map and the functionalities offered by each application at each step. To complement this synthetic view, we detail in the next paragraphs the main differences between the functionalities offered by the tools to perform these five main tasks

Data pre-processing

The pre-processing step does not take into account the data preparation that has to be done by the user, in order to load them in the application. It concerns the processing of the uploaded data by the application to make it conform to its internal data model.

In most applications the data must be provided in the form of two data sets: -location data (here called geodata), which correspond to the starting and ending points of the flows, i.e. nodes.

-the link data, which concerns the flows between the nodes.

Note that, with the exception of Kepler.gl which takes only one data source, most tools use two different files or tables (one for the nodes and the other for the links).

To spatialize the nodes, there are two cases. The most common is to provide geographic coordinates (latitude and longitude) in the WGS84 coordinate reference system. Another case allows an automatic geocoding of the nodes if in the link file, the identifiers correspond to a known nomenclature already present in the application (e.g. ISO2 UN code for countries). It is then possible to load only the link data and to indicate to which nomenclature the data belong. The links data are then loaded into the application. The Arabesque application allows you to choose an aggregation function that will be used if several links share the same Origin and Destination.

Data processing

Once integrated, the flows and the spatial data are joined in an Origin-Destination matrix to be processed. Depending on the data, the resulting matrix can be asymmetric, the flows will be directed. If it is symmetrical, the flows will not be directed. Arabesque manages both types of flows. Flowmap.blue handles only directed flows while Magrit and Kepler.gl considers that the flows are not directed. This is an issue because only the most important outgoing flow between two nodes is represented. This limitation will prevent a fine analysis of bilateral flows. Some tools calculate new indicators. Arabesque automatically calculates the flow distances and common graph statistics for the nodes: inbound and outbound degrees and balance (weighted by the flows or not). Flowmap.blue automatically calculates a new indicator that is used for its clustering function, but these new values are not accessible to the user. Magrit may calculate new values using an expression entered by the user. Arabesque and Magrit can also compute descriptive statistics (mean, median, etc.) and a distribution histogram of all the features (computed or imported).

Mapping

Once the Origin-Destination matrix has been created and the possible indicators calculated, the software will produce the representation. It will then be customizable or not. Arabesque and Kepler.gl allow for further styling of the map whereas Flowmap.blue allows only one type of representation. Magrit proposes several representation modes but only allows the link map for OD data.

Arabesque and Magrit offer advanced graphic styles, i.e. it is possible with these tools to modify the style of the links (shape, colour, orientation). Background information can also be added to the map to provide context, either from web resources (Arabesque, Flowmap.blue, Kepler.gl) or from vector files provided by the user (Arabesque, Kepler.gl, Magrit).

Note that only Arabesque and Magrit allow to change the map projection. Flowmap.blue and Kepler.gl are limited and can only produce maps in Web Mercator projection. This may be useful for flow data analysis with global datasets but for local datasets a local projection might be wanted for mapping.

Exploration

The data exploration possibilities are, along with the representation/styling possibilities, where the softwares differ the most. The four applications studied in this article offer data sorting and ordering functions. But some features are specific to certain applications. Flowmap.blue, Kepler.gl and Magrit offer functions to display labels and tooltips to provide information about the objects studied. This is a feature missing in Arabesque. Arabesque and Kepler.gl offer an interesting feature for data exploration: the graph query. By manipulating a window on a graph, this feature allows to hide part of the data in order to highlight the data included in the window.

Kepler.gl also offers a brushing function (brush), which displays only the links present in a radius around the cursor, the other data being hidden. Again, this feature can be useful in an exploratory context or during a demonstration. Finally, Flowmap.blue offers an original feature of spatial aggregation (clustering), that allows to easily explore the flow at different spatial scales. This aggregation is possible thanks to the calculation of an indicator, not accessible to the user, which allows to aggregate/disaggregate the flows when zooming. Everything is done automatically by the application, the user can activate or deactivate the functionality or choose one of the available aggregation levels. The number of aggregation levels is calculated according to the data by the application.

Export

The export functionalities are also very diverse. The studied applications, with the exception of Flowmap.blue, offer the possibility to export the map in raster (PNG, JPEG) or vector (SVG) image formats. Kepler.gl and Flowmap.blue also offer an online hosting service in order to publish the map on the Web. Arabesque, Kepler.gl and Magrit allow you to save and export your project data and settings in order to be able to reload them afterwards. This is useful to be able to continue the analysis later or share it with a third party and is a great benefit for reproducibility.

DISCUSSION

This comparison shows that these four solutions have different functionalities, flow maps forms and styles. Overall, each tool meet specific needs according to the profiles (technical skills, mapping knowledge) and objectives (communication, exploration, mapping) of the different users (geographer, cartographer, GIS specialist, data scientist, data journalist, urban planner, etc.). We would like to highlight some elements that might help the user to choose between these tools for a specific task.

Rigor vs design

From the same type of data (OD), these 4 applications based on offer quite varied representation modes (hub lines, arcs, arrows, curved lines). The cartographic "rigor" is not supported by all the tools in the same way and from the same perspective. Arabesque and Magrit, developed by cartographers, are intended for "informed" audiences who are more "rigorous" in terms of graphic semiology. Flowmap.blue and Kepler.gl offer less rigorous but more aesthetic modes of representation, whose design is in line with current trends in online cartography and infoviz.

Analysis, exploration or representation?

Beyond the types of maps, these online applications mainly refer to three types of approaches around flow data: analysis, representation and exploration. Arabesque is the only application offering some flow data analysis functionalities (multivariate filtering, aggregation, selection on distances, network analysis indicators).

The exploration dimension through dynamic data interaction (filtering, brushing, hovering) is at the heart of Kepler, Arabesque and Flowmap.blue. In these three interfaces, the user can move around, zoom in, and query elements. Magrit, on the other hand, is more focused on the production of static maps but do not allow navigation and exploration.

Performance

These four applications based on different technologies (D3, DeckGL, MapboxGL) do not offer the same performance in terms of data integration and representation. Because of the use of WebGL and vector tiles, Kepler.gl and flowmap.blue can handle large volumes of data. On the other hand, Magrit and Arabesque are based on D3 javascript technology and have limitations with datasets of several thousand objects.

Interoperability and reproducibility

About the data preparation, 3 out of 4 tools are using data formatted as two separate tables: one for the nodes (geographical points) and one for the links (thematic information). Kepler.gl is apart, it uses a single data Depending on the tools, the dissemination possibilities are very different. While some tools (Kepler.gl, Flowmap.blue) offer to host the results online, others (Arabesque, Magrit) offer to export the map as an image. The choice of the tool can therefore be based on the intended purpose of its use: a dynamic web map or a static illustration for a scientific article or report.The capability to export the parameters of a project can then constitute a real bonus to transmit the whole work to collaborators and contribute to ease the reproducibility of the analyses.

In 2016, an experiment reproducibility issue has been identified [START_REF] Baker | 1,500 scientists lift the lid on reproducibility[END_REF]. If it is sometimes very difficult to reproduce exactly the conditions of an experiment. Using open source tools can help to attain Computational Reproductibility [START_REF] Leonelli | Including a Symposium on Mary Morgan: Curiosity, Imagination, and Surprise[END_REF], as when data are shared, anyone can reproduce the results. Arabesque, Kepler.gl and Magrit are interesting in this regard. The export function of these tools integrates both the data and the project parameters, making it possible to reproduce the map.All the tools are open source and their code is auditable. Their processing and data visualization algorithms can therefore be inspected.

Purpose

Among the four tools tested:

1. Kepler and Flowmap.blue are intended for an audience wishing to explore data without the desire to perform statistical analysis and the intention to produce and export a thematic map.

2. Magrit is intended for the realization and production of thematic maps and allows statistical processing to be carried out beforehand.

3. Arabesque is at the interface of these two typical uses: it offers powerful exploration functions, visual parameterization of figures, and statistical analysis possibilities. However, the export and captioning functions needed to produce thematic maps in Arabesque are still limited.

4. Arabesque, like Flowmap.blue, is designed for flow maps. Kepler.gl and Magrit are more generalist and offer other representation modes for other datasets.

5. Arabesque and Magrit target the same audience of researchers and students and come from the same school of French cartography (in the line of Jacques Bertin). Magrit's more general approach allows it to attract a wider audience.

6. Flowmap.blue and Kepler aim more at producing something quickly communicable and are very web publishing oriented.

CONCLUSION

This article highlights the specificities of Arabesque within the small ecosystem of web applications for mapping flows. Rather than highlighting the flaws and qualities of each tool tested, we observe that these tools meet distinct user needs.

The style and type of map produced, the target audience, and the technical skills required to use these applications differ interestingly. Having tested these software on the same dataset, this complementarity is clearly visible.

In the end, the business culture, the user's skills and the final use of the map influence the choice of the tool because it will influence the chosen approaches (exploration and/or analysis) and the expected purposes. Our contribution points out the strengths and limitations of each in order to make an informed choice.

This demonstration also shows that the data does not influence the choice of the tool since all of them worked with the same data set. Only the formatting will change, which requires the user to be familiar with the format required by the target application and the means necessary to format them.

Figure 1 .

 1 Figure 1. Arabesque Interface

Figure 2 .

 2 Figure 2. Styling in Arabesque.

Figure 4 .

 4 Figure 4. Kepler.gl interface

Figure 5 .

 5 Figure 5. Flowmap.blue interface.

Figure 7 .

 7 Figure 7. Applications workflows diagram

  There are Kepler.gl, Magrit, Flowmap.blue, Tableau, CARTO, Flourish, ArcGIS Online, and NETSCITY. Of course, this list of online flow mapping applications might not be exhaustive and is open to suggestions to enrich it.To this purpose, the fruits of the above-mentioned review of web applications for flow mapping are shared in the form of a collaborative dashboard open to any contribution. Possible contributions to this collaborative work of digital curation can be modifications or enrichment of metadata, addition of new applications and of course reuse in the context of other research (the paper:[START_REF] Bahoken | Typologie d'un geoweb des flux et réseaux / Typology of a flow and network geoweb[END_REF] the dashboard: Le Campion and Nguyen, 2019).

	Table 1. Software licensing of a selection of online flow
	mapping tools
	In the perspective of comparing Arabesque to tools in the same
	category, and in the logic of presenting in this article research
	results that are themselves as open and reproducible as
	possible, we have decided to operate our benchmark on those
	4 applications

While many of the tools identified are based on open-source data-visualisation, mapping and geo-processing libraries, they do not all have the same degree of accessibility, as shown by Table

1

. Among these 8 flow mapping web applications we identified for the study, only 4 are entirely free and open: Arabesque, Kepler.gl, Magrit, and Flowmap.blue.

  table. All the 4 tools can read data with an open format, CSV or GeoJSON. FlowMap.blue imports data hosted in a Google Docs table. It is possible to paste CSV formatted data into the browser tab, but this option renders the project not hostable in the Flowmap.blue website.

The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLVI-4/W2-2021 FOSS4G 2021 -Academic Track, 27 September-2 October 2021, Buenos Aires, ArgentinaThis contribution has been peer-reviewed. https://doi.org/10.5194/isprs-archives-XLVI-4-W2-2021-147-2021 | © Author(s) 2021. CC BY 4.0 License.

This contribution has been peer-reviewed. https://doi.org/10.5194/isprs-archives-XLVI-4-W2-2021-147-2021 | © Author(s) 2021. CC BY 4.0 License.

We note that these applications have been developed in the space of a few years, taking advantage of the rise of geospatial technologies in the web browser. A whole range of needs and possibilities are covered, for the benefits of the community.


Alassane Sidibe
email: alassane.sidibe@laas.fr

Gael Loubet
email: 2gael.loubet@laas.fr

Alexandru Takacs
email: alexandru.takacs@laas.fr

Daniela Dragomirescu
email: 4daniela.dragomirescu@laas.fr

Gaël Loubet #

Energy Harvesting for Battery-Free Wireless Sensors Network Embedded in a Reinforced Concrete Beam

Keywords: Energy Harvesting, Wireless Power Transmission, Rectenna, Wireless Sensor Networks

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.

I. INTRODUCTION

With the huge democratization of the use of connected and communicating objects for long term applications, conventional power supply methods are showing their limitations: primary batteries need to be changed periodically, secondary batteries need to be recharge; and capacitors and supercapacitors have very limited energy storage capacities. The harvesting of ambient energies (EH) could be a solution to increase the lifetime of objects but ambient sources are not always available. In the case of a Wireless Sensors Network (WSN) in harsh environments as embedded in Reinforced Concrete (RC) -thus fully inaccessible once deployed-, light, thermal or mechanical energy sources could be insufficient or non-existent to be used as main power supply [START_REF] Han | Smart and Multifunctional Concrete Toward Sustainable Infrastructures[END_REF]. In this context, the use of far field Radio Frequency (RF) Wireless Power Transfer (WPT) approach makes sense [START_REF]Europe and the Future for WPT: European Contributions to Wireless power Transfer Technology[END_REF]. In this paper, we will introduce an innovative rectenna (rectifying antenna) specifically designed in order to scavenge RF energy generated by a dedicated source and supply a wireless sensing node embedded in a reinforced concrete beam.

In the past, many antenna and rectifier was proposed and connected separately to for a rectenna as presented in [START_REF] Loubet | LoRaWAN Battery-Free Wireless Sensors Network Designed for Structural Health Monitoring in the Construction Domain[END_REF][START_REF] Takacs | Compact Planar Integrated Rectenna for Batteryless IoT Applications[END_REF]. In this paper is proposed a single Printed Circuit Board (PCB) three-dimensional (3D) rectenna allowing to reach a efficiency up to 67%.This rectenna is tuned to work in the European Industrial, Scientific and Medical (ISM) 868 MHz frequency band, is ultra-compact (56 mm x 32 mm x 8 mm, only of 0.00035•λ 3 ) In the following part, the targeted application: a communicating reinforced concrete made of an embedded battery-free WSN; and its implementation are introduced. Then, the design and the characterization of the manufactured ultracompact and efficient rectenna are developed. Finally, some tests and results of the use of the full system are presented as a full proof-of-concept never presented before in literature.

II. WIRELESS POWER TRANSMISSION SYSTEM

The smart wireless system presented in Fig. 1 is proposed in this paper. The aim is to wirelessly power a sensor node through a WPT interface including a rectenna in order to transmit data through LoRaWAN (Long Range Wide-Area Network) frames to a gateway [START_REF]LoRaWAN 1.1 Specification[END_REF]. From an electromagnetic (EM) waves radiated by a dedicated RF source, the rectenna transfers the harvested energy to a power management unit (PMU) (Texas Instruments bq25504) which stores this latter in a 22-mF supercapacitor (AVX BZ01CA223ZSB). Once enough energy is available (defined by an activation threshold of 5.25 V), this one is used to supply the active part through a dc-to-dc regulator (Texas Instruments TPS6303x). The supercapacitor is discharged until a deactivation threshold of 2.30 V. The active part is composed of a LoRaWAN communication module (Murata CMWX1ZZABZ-091) embedding a microcontroller unit (MCU) and a LoRa transmitter; and a temperature and humidity sensor (Texas Instruments HDC2010). For each discharge of the supercapacitor, the MCU is started and initialized, a temperature and humidity measurement is achieved and a LoRaWAN frame with the collected data is transmitted.

III. RECTENNA: DESIGN, SIMULATION AND MEASUREMENT

All the substrates used in this work are FR4 (thickness: 0.8 mm, 𝜀 : 4.3 and 𝑡𝑎𝑛𝛿: 0.02). The metallization is a thin layer of cooper of 35 µm. Simulations of the rectifier and the proposed antennas were carried out with Keysight Advanced Design System (ADS) and Ansys HFSS software, respectively.

A. Rectifier design

In this section, the conception of an optimized rectifier for a 10 kΩ load (which emulates the input impedance of the bq25504 [START_REF]Texas Instrument -bq25504 Ultra-Low Power Boost Converter with Battery Management for Energy Harvesting Applications[END_REF]) and operating in the European ISM 868 MHz frequency band is described. It consists of a doubler rectifier composed by a series pair of Schottky diodes (Skyworks SMS7630_005LF) [START_REF]Skyworks-Surface Mount Mixer and Detector Schottky Diodes (SMS7630-005LF)[END_REF]. The rectifier circuit is designed in coupled microstrip transmission lines. An LC matching network was designed and optimized. It is composed by a wirewound SMD inductor (Murata series LQW15AN -33nH) serially connected and a shunt SMD capacitor (Murata series GRM15 -4pF). For accurate simulation results, the Murata libraries, the spice model of the diodes and the parasitic model of the SOT-23 plastic diode package [START_REF]Linear Model for Diode Surface Mount Packages[END_REF] were included in ADS simulation. More details about the rectifier design and experimental results are presented in [START_REF] Sidibe | Compact 3D Rectenna for Low-Power Wireless Transmission[END_REF].

B. Antennas design and measurement

Two antennas, namely DA for the 3D dipole antenna and DARP for the DA with a 14 cm x 9 cm rectangular reflector plane (RP) are presented Fig. 2. The DA is composed of a planar dipole with a short circuit stub commonly known as T-match antenna [START_REF] Zamora | Design and Synthesis Methodology for UHF-RFID Tags Based on the T-Match Network[END_REF]. As the dimensions of a typical dipole antenna at 868 MHz are important, we reduced in this work the size of the dipole antenna by connecting vertically two short metallic arms (32 mm x 10 mm). Theses induce a capacitive effect and downshift the resonant frequency of the antenna. The geometries of DA are summarized in Fig. 2.

Knowing that the rectenna will be implemented with the sensing node, its grounded bottom side was considered as a reflector plane. So that, it will increase the maximum gain of DA and thus the harvested power by the rectenna but reduce the beamwidth. Additional simulations were performed in order to verify the impedance matching of DARP and the optimal distance of DA from the RP. The largest side of the RP is placed in the direction parallel to the co-polarization plane of the antenna at a distance of 6 cm to focus the maximum gain in a single E-plane lobe and reduce the backlobe. Moreover, the width of the metallic arms was tuned to 8 mm (instead of 10 mm for DA). From the S11 plots in Fig. 3 the two different antennas are well matched as expected in simulation with a fabrication tolerance (imperfect shape of the metallic arms and connection with the planar dipole) which induces a down-shift of 15 MHz. As presented in Fig. 4, the measured gain in the E-plane for DA is +1.54 dBi and +5 dBi close to the simulated gain of +5.68 dBi for DARP.

C. Rectenna measurements with and without reflector

A rectenna with the DA antenna (R1) and a second connected with a nylon stem to the sensing node (R2) were finally prototyped. The experimental setup in an anechoic chamber (as presented in [START_REF] Takacs | Compact Planar Integrated Rectenna for Batteryless IoT Applications[END_REF]) is composed of a patch antenna (gain Gt = +9.4 dBi) connected to a RF signal generator (Anristu MG3694B) which injects a RF power Pt. The insertion losses introduced by the cable were subtracted in the following calculations. The rectenna under test (which consists of DARP (gain Gr) antenna and a rectifier) was placed at a distance d of 1.5 m from the transmitting antenna in its far field region with a linear polarized E-field. Then, a multimeter connected to the rectenna output measures the harvested dc voltage across the load. The setup is automatized by using a homemade data acquisition interface implemented in LabVIEW.

The performed measurements depicted in Fig. 5 show that the rectenna is well efficient at the desired frequency (868 MHz). It can harvest a maximum dc voltage of 1.9 V and 2.8 V for R1 and R2, respectively.

𝜂 = 𝑃 𝑃 = 4 • 𝜋 • 𝑃 𝑆 • 𝐺 • 𝜆 • 100 ; 𝑆 = 𝑃 • 𝐺 𝑑 • 4 • 𝜋 (1)
The RF-to-dc efficiency versus power density is computed thanks to the formula (1) and is presented in Fig. 6. The measured dc voltage obtained for R2 increases by 1V compared to R1 when the rectenna is illuminated by a power density of 3.9 µW/cm². However, above 2 µW/cm², R2 exhibits lower efficiency than for R1 due to the Schottky diodes saturation when receiving too much power (with a higher gain antenna). The maximum RF-to-dc conversion efficiency is about 71 % for R1 and 67 % for R2 for S = 3.9 µW/cm².

IV. BATTERY-LESS SYSTEM PERFORMANCE MEASUREMENTS

The interesting performances obtained by R2 allows embedding the full rectenna in a cavity of the reinforced concrete beam manufactured to perform experiments and whose dimensions are presented [START_REF] Loubet | LoRaWAN Battery-Free Wireless Sensors Network Designed for Structural Health Monitoring in the Construction Domain[END_REF]. In this paper, H is the distance between the bottom side of the cavity and the top side of the uncovered concrete beam (25 cm) and D the elevation position of the sensor node (R2) relative to the bottom side.

The optimal position of the rectenna placed inside the covered concrete cavity (15 cm x 15 cm x 15 cm) was experimentally verified. Thus, a characterization of R2 by measuring the harvested dc voltage across a 10 kΩ load was performed as a function of the rectenna position (fully embedded in the concrete beam) illuminated by a transmitting antenna (at a distance of 156 cm relative to the top of the concrete beam.). As seen in Fig. 7, the maximum dc power obtained inside the cavity was obtained for D = 5 cm. In this position, the rectenna exhibits a maximum dc voltage of 1.46 V with +32 dBm EIRP power (Fig. 8) generated by the RF source positioned at 170 cm (156 cm in air and 14 cm in the concrete) far away from the rectenna embedded in the concrete. This optimal position was chosen for the next steps.

After having characterized the rectenna in the anechoic chamber and embedded in the covered concrete cavity, a full sensing node prototype was characterized. Then, the output of the rectenna was connected no more to the 10 kΩ load but to the input pins of sensing node as shown in Fig. 10 (b). In all experiments, a signal power generator connected to a patch antenna is used as the RF source with a transmitting EIRP power fixed at +32 dBm at 868 MHz. The voltage measurements were performed using a LeCroy WaveRunner 6100 oscilloscope. The operational behaviour of the sensing node and its functionality are fully described in [START_REF]0014 McBIM project (Communicating Material at the disposal of the Building Information Modeling)[END_REF]. As seen in Fig. 9, the PMU (bq25504) stores the available power provides by the rectenna until the voltage across the supercapacitor exceeds 5.25V and then supplies the active components until the voltage goes down under 2.30V. During each discharge a temperature and humidity measurement and a LoRaWAN frame transmission are achieved. According to Fig. 9 and Fig. 10 V. CONCLUSION For the first time, a wirelessly powered and battery-free LoRaWAN wireless sensing node fully embedded into a concrete beam was presented. Its RF energy harvesting system is composed by a compact (0.00035•λ ) and optimized rectenna. By using the grounded plane of the sensing node as a reflector, the gain of the antenna/rectenna increases from +1.54 dBi to +5 dBi. Thus, the rectenna exhibits an efficiency of 67% at 868 MHz for a power density of 3.9 µW/cm². The sensing node embedded into reinforced concrete is able to collect and transmit temperature and humidity data though a LoRaWAN frame to a gateway each 36 minutes by harvesting the RF energy specifically generated by a RF source located at 170 cm. Thus, this system is for SHM applications in the construction domain.

Figure 1 .

 1 Figure 1. Schematic diagram of the targeted application.

Figure 2 .

 2 Figure 2. (a) Simulation model and geometries of the DARP antenna; (b) Manufactured DA antenna; (c) Manufactured DARP antenna.

Figure 3 .

 3 Figure 3. Comparison of the simulated and measured reflection coefficient of DA and DARP antennas.

Figure 4 .

 4 Figure 4. Simulated and measured radiation patterns (gain in dBi) of the DA and DARP antennas in the E-plane at 868 MHz.

Figure 5 .

 5 Figure 5. Measured dc voltage for the two manufactured rectennas over the frequency in anechoic chamber at a power density of 3.4 µW/cm².

Figure 6 .

 6 Figure 6. Computed RF to dc conversion efficiency (continuous line) and the measured dc voltage (dashed line) over the power densities at 868 MHz.

Figure 7 .

 7 Figure 7. dc power as function of the position of the rectenna (R2) inside the covered concrete cavity (top green part is the cover).

Figure 8 .

 8 Figure 8. Harvested dc voltage of the DARP over the EIRP of the RF source positioned at 170 cm far away from the rectenna fully embedded in the covered concrete cavity at 868 MHz.

Figure 9 .Figure 10 .

 910 Figure 9. Voltage waveforms at the ports of the supercapacitor (blue upper curve) and rectenna output voltage (red lower curve) for a +32 dBm EIRP power.

  (d), nearly 36 minutes are needed for a complete charge of the supercapacitor and the transmission of the temperature and humidity data to a local LoRaWAN gateway.

ACKNOWLEDGMENT

The authors would like to acknowledge all their partners in ANR McBIM project [11], especially FINAO SAS/260SmartConnect for providing well defined use cases and the reinforced concrete beam.


Problem-Specific Analysis of Molecular Dynamics

Trajectories for Biomolecules

Konrad Hinsen

My name is Konrad Hinsen, and I am a researcher at the Centre de Biophysique Moléculaire in Orléans, France. My field of research is molecular biophysics, and in particular the study of the flexibility and dynamics of proteins. All of my work is based on computational approaches, of which the most important ones are elastic network models and Molecular Dynamics (MD) simulations. Moreover, most of my work concerns the development of computational methods rather than the application of already established methods.

This case study is about the extraction of information from MD simulation trajectories, a very common type of work in my field. MD simulations themselves are relatively standard procedures, performed using one of a handful of wellknown software packages. They take a few days to a few weeks on a small parallel computer with a few tens of processors, and produce a so-called trajectory file that is one to ten GB in size. Analyzing these trajectories in order to actually learn something about the system that was simulated is a separate step that is much less standardized, meaning that there is a lot of problem-specific code involved. This code is as much a result of the workflow as the plots of the computed quantities.

For reproducible and publishable workflows, there are three specific challenges in this situation:

1. The size of the trajectory files, which are difficult to publish in a citeable way, often being larger than the current upper limits of Zenodo or figshare.

2. There are CPU-intensive tasks that are typically run on a parallel computing cluster in batch mode, and explorative tasks that are done interactively or near-interactively (running short scripts that take about a second) on a desktop machine. Dependency tracking across machines is not supported by most workflow management systems. It doesn't help that computing clusters often have limited network connectivity.

3. The distinction between "software packages" and "workflows" is not useful when most of the code being executed is problem-specific. A more appropriate code structure is "well-established techniques implemented in libraries", "problem-specific scripts" and at the top level "coordination of a small number of scripts". It's the last two levels that must be captured for reproducibility. A published example of the workflow described below can be consulted in the form of two code/data packages (package 1, package 2) and the article describing the study.

Workflow

The workflow diagram is actually a dataflow graph with attached workflow information. Compared to most approaches to workflow, which place the tools (workflow manager, software packages, Web services, . . . ) in the center of attention, the approach I describe here focuses on the data and on the way scientists interact with the data. The workflow below is not about "getting a job done" but about "developping and fine-tuning a scientific model".

The dataflow graph shows code in rectangles, and "passive" data in rounded boxes. Code consists of a small number of Python scripts, of which four are shown in the diagram. Data flows from top to bottom, as shown by the arrows, starting with the MD trajectory that is the overall input, and ending in plots showing computed quantities. The three rounded boxes labelled "computed results" are intermediate results, computed by Python scripts 1 and 2 and consumed by Python scripts 3 and 4.

From the point of view of workflow management and reproducibility, the most important distinction among the data items is "human input" (solid outline) vs. "computed data" (dotted outline). It's the human input that represents the scientific model, and thus the main output of this workflow. It consists of code (Python scripts 1 to 4) and numerical parameters (a single one in the diagram), though that distinction is rather arbitrary: every parameter could be turned into a line in a script. Computed data includes the plots that go into the journal article, but also intermediate results. In a fully reproducible workflow, the computed data need not be stored, because it can be recomputed at any time. Nevertheless, it is often preferable to store it explicitly, in particular if recomputation takes a long time. Stored computed data is also more readily available for exploration by scientists who want to gain a better understanding of the method.

The workflow consists of the iterative refinement of the models and methods.

The two key tools in processing the workflow are:

• a version control system such as git for keeping track of the changes • the ActivePapers dependency manager for coordinating the computations Correspondingly, the state of the project consists of • a repository under version control, which tracks the changes to the "human input" items as the project advances The fourth step recomputes all data that is affected by the changes made in step 1. The recomputation is steered by the dependency graph, which is obtained from the data flow graph by redirecting arrows that point into a script to point instead to the outputs of the script. The ActivePapers dependency manager computes the dependency graph automatically during the execution of the scripts. Users do not have to deal with (or even know about) either graph explicitly. They write and run scripts as they did before reproducibility became an issue. Similar approaches are used in Sumatra and noWorkflow, but most workflow managers adopt the opposite strategy of letting the user construct a workflow explicitly and then execute it.

A project can be transferred from one computer to another by copying the ActivePaper file and the version control repository. For the common situation in molecular simulations that lengthy computations are off-loaded to a cluster, step 4 in the above procedure is slightly modified: The ActivePaper is sent to the cluster, the "run new script" or "update" operation is performed on the cluster, and the modified ActivePaper file is transferred back to the user's desktop machine. All the tools have a command-line interface, making it easy to use them over an ssh connection.

Method-development projects tend to be small, involving a handful of people.

The pitfalls of coordinating modifications to files can easily be avoided by having a single person perform each refinement step, or even all of the refinement steps.

Other participants can of course contribute ideas, and inspect the current state of the project for analysis.

At the end of the project, the ActivePaper file(s) can be published, making all of the code and data available to other researchers. The ActivePaper file contains the complete final state of the project (though not its history), meaning that anyone can continue from that state. An ActivePaper file for a new project can re-use items from already published ActivePaper files through a DOI (Digital Object Identifier), allowing other researchers to build on published computational work. The DOI can also be used for citations in journal articles.

Pain points

The main practical difficulty is that most of today's computational scientists grew up with tools and practices that are not compatible with reproducibility. This is particularly true for the field of molecular simulations, where reproducibly published studies are still rare. Working reproducibly requires adopting new tools and habits, and modifying existing software for integration with reprooducble workflows. There is a permanent temptation to give up reproducibility for faster scientific progress.

The immaturity of current workflow tools for reproducible research adds another layer of cognitive overhead. In the workflow described above, this is mainly the use of separate tools for tracking history and dependencies. Today's version control systems, designed for software development rather than computational science, cannot easily be extended by the kind of dependency management required for research. On the other hand, writing new version control software integrated with depenency management represents an effort that is hard to justify at this time.

A major constraint imposed by the ActivePapers system is that all code must be written in Python and all data must be stored in HDF5 datasets. While Python is popular enough for molecular simulation to make the first constraint very acceptable, HDF5 is still a rare choice for data storage, although this is changing thanks to initiatives such as H5MD.

The use of specific tools is rarely sufficient to ensure reproducibility. Tools can only take care of replicability, i.e. the technical aspect of tracking all computational dependencies such that a computation can be re-run identically.

Reproducibility at the scientific level requires that all steps can easily be understood and verified by fellow scientists. Best practices for reaching this goal remain to be developed. One observation from the applications of the above workflow is the importance of access to intermediate results for human inspection. This suggests an overall structure of many small scripts that each do a well-defined job and communicate via explicitly stored datasets.

Key benefits

The traditional workflow of changing scripts and running the interactively in a shell is extremely prone to mistakes. The most frequent one is forgetting to re-run a script after its input data has changed because of an update to another script. Before I adopted reproducibility support tools, I regularly found myself looking at a data file and wondering which exact sequence of script executions had produced it. The question typically comes up when writing a paper. Even for today's minimal "materials & methods" sections, it is typically necessary to look up parameters and other choices in the scripts when writing the documentation, and that's often the moment when one discovers what a mess they are. This is no longer an issue when the complete final project state is available for inspection, and guaranteed to be complete and coherent by software tools.

Key tools

The key tool in my workflow is the ActivePapers toolset, which was in fact designed specifically for supporting reproducibility in the context of atomistic and molecular simulations. It supports in particular

• computations on large datasets by storing them efficiently in HDF5 files with the dependency information stored as HDF5 metadata

• dependency tracking across machines by storing all datasets and their dependency graph in a single HDF5 file that can be copied easily from one machine to another

The only other reproducibility-enabling tool in the workflow is a version control system.

Questions

What does "reproducibility" mean to you?

Given that my work is 100% computational, my long-term goal is full reproducibility, starting from a specification of the simulation and ending with the plots that go into a journal article. This goal is unrealistic at the moment because the simulation software packages do not support reproducibility. One problem is the accumulation of numerical roundoff errors, which are insufficiently standardized across processors and compilers to be reproducible. Another problem is the widespread use of random number generators without user control over the random seed.

For this reason, I have been setting myself a more modest goal for this case study: reproducibility of the trajectory analysis step, using the MD simulation trajectories as input as if they were experimental data outside of my control. This is a useful compromise because the development of trajectory analysis techniques is the central scientific topic of this work.

Why do you think that reproducibility in your domain is important?

Most MD simulation studies are so complex that in the absence of reproducibility, it is impossible to be sure what was really computed. Most mistakes do not lead to a recognizably wrong result, but to a somewhat different one that could well be correct.

How or where did you learn about reproducibility?

I developed them myself, having found nothing suitable for the specific needs of molecular simulations after a careful survey of existing technology and practices.

What do you see as the major challenges to doing reproducible research in your domain, and do you have any suggestions?

The main challenges are human and social. Most of my colleagues have experienced the problems that non-reproducibility creates, but few are willing to invest the extra effort to do a better job, and many remain unaware of the tools and practices for reproducibility that already exist. Journals in my field generally don't require the publication of software or data, and do not in any way encourage reproducibility. Technical challenges exist in that the most popular software packages do not support reproducibility, but the technical challenges could be met with little effort if there were sufficient motivation in the community.

What do you view as the major incentives for doing reproducible research?

• Feeling more confident about the correctness of my results.

• Being able to build safely on earlier work (by myself or others)

Are there any best practices that you'd recommend for researchers in your field?

I'd already be happy if publishing software and data became the norm in my field. It's hard to recommend any more elaborate practices before the basics become standard.

Stage III: Data Analysis plot 1

	MD trajectory	parameter
		Python
	Python	library	Python
	script 1		script 2
	computed	computed	computed
	result 1	result 2	result 3
	Python	Python
	script 3	script 4
		plot 2

  • an ActivePaper file, which stores the current state of all data items and the dependency graph There are two variants of a refinement step: adding a new script or parameter, and modifying existing scripts and parameters. The first kind, which extends the data flow graph, consists of the following user actions:

	2. Commit the changes to version control.
	3. Check in the modified versions to the ActivePaper.
	4. Update the ActivePaper.
	1. Write the new script.
	2. Commit it to version control.
	3. Check in the script to the ActivePaper.
	4. Run the script via the ActivePapers dependency manager.
	The second kind, which preserves the data flow graph, differs only slightly:
	1. Edit scripts and parameters.