Matjaž Ogrinc

Ildar Farkhatdinov

Rich Walker

Etienne Burdet
email: e.burdet@imperial.ac.uk

Sensory integration of apparent motion speed and vibration magnitude

Keywords: vibrotactile, tactons, apparent motion, JND, wrist, wearable !

Tactile apparent motion can display directional information in an intuitive way. It can for example be used to give directions to visually impaired individuals, or for waypoint navigation while cycling on busy streets, when vision or audition should not be loaded further. However, although humans can detect very short tactile patterns, discriminating between similar motion speeds has been shown to be difficult. Here we develop and investigate a method where the speed of tactile apparent motion around the user's wrist is coupled with vibration magnitude. This redundant coupling is used to produce tactile patterns from slow&weak to fast&strong. We compared the just noticeable difference (JND) of the coupled and the individual variables. The results show that the perception of the coupled variable can be characterised by JND smaller than JNDs of the individual variables. This allowed us to create short tactile pattens (tactons) for display of direction and speed, which can be distinguished significantly better than tactons based on motion alone. Additionally, most subjects were also able to identify the coupled-variable tactons better than the magnitude-based tactons.

INTRODUCTION

Tactons (also called haptic icons [START_REF] Maclean | Perceptual design of haptic icons[END_REF] or tactile melodies [START_REF] Van Erp | Distilling the underlying dimensions of tactile melodies[END_REF]) are abstract haptic messages that can employ variation of different physical parameters of vibrotactile stimulation, including waveform, magnitude, frequency, rhythm and location [START_REF] Brewster | Tactons: structured tactile messages for non-visual information display[END_REF]. The simplest example of tactons can be found in our everyday lives; a smartphone can buzz in different temporal patterns to let us distinguish between a calendar event and an incoming text message. Conversely, attempts have been made to deploy sets as large as 84 tactons [START_REF] Ternes | Designing large sets of haptic icons with rhythm[END_REF], challenging the human perceptual and associative capacity. Therefore, a tacton should encode the message in an intuitive way to ease the learning of associations. For instance, [START_REF] Brunet | invitation to the voyage: The design of tactile metaphors to fulfill occasional travelers' needs in transportation networks[END_REF] employed metaphors based on user requirements to design tactile cues for spatial navigation.

Tactons commonly employ arrays of spatially distributed tactile stimulators and are used in spatial navigation and orientation tasks [START_REF] Brunet | invitation to the voyage: The design of tactile metaphors to fulfill occasional travelers' needs in transportation networks[END_REF]- [START_REF]Deaf-blind can practise horse riding with the help of haptics[END_REF]. Tactile arrays can be activated in spatio-temporal patterns to create an illusion of apparent tactile motion. Apparent motion can suggest the user a change of direction to reach a point of interest and also provide information about various vector quantities such as motion, force and torque.

In the case of tactile apparent motion, the presented information can be encoded by the motion speed and direction of travel. To maximise the information transfer of a set of such tactons, the changes in speed must be clearly distinguishable and the vibration patterns short. Though using slow motion speeds may increase the number of distinct tactons, it also increases the minimum stimulation time. Therefore, our aim is to design motion based tactons that are fast (thus only require a short stimulation duration) but clearly distinguishable.

Previous work has shown that humans have difficulties in absolute ranking of the speed of apparent motion [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF]. To overcome this limitation, we propose a novel stimulation method where the speed of apparent motion around the wrist is coupled with vibration magnitude. This coupling produces tactile motions that range from slow and weak to fast and strong. As the central nervous system seems to combine sensory information in a fashion that is similar to the maximum-likelihood integrator [START_REF] Ernst | Humans integrate visual and haptic information in a statistically optimal fashion[END_REF], we hypothesised that the proposed coupling will improve the users' ability to discriminate between similar motion speeds, which is investigated in the present work. First, we evaluated the maximum speed of apparent motion that can be achieved for the case of forearm stimulation using inertial (eccentric mass) motors. Then, we experimentally estimated the just noticeable difference (JND) of apparent motion speed, vibration magnitude, and the coupled modulation. Finally, we designed sets of tactons for each modulation type based on the estimated JND and evaluated the participants' ability to distinguish them using a rating task.

In the next section we present the relevant background and describe the proposed stimulation technique. We explain our interface design choices with respect to previous work on similar feedback approaches. Sections 3 and 4 report on two two-alternative forced choice experiments to determine the maximum realisable speed of apparent motion and to estimate the JND of the proposed stimulation approach. Finally, the rating task experiment, carried out in order to investigate the absolute discriminability, is presented and discussed in section 5.

DESIGN OF TACTILE STIMULATION

Just noticeable difference in tactile perception

The just noticeable difference I JND is the change of intensity of some physical variable I, such as vibration frequency, acceleration, duration, or motion speed, that can be detected at least half the time. For many sensory modalities, the perception follows Weber's law according to which w = IJND I (Weber fraction) is independent on I [START_REF] Gescheider | Psychophysics: the fundamentals[END_REF]. It has been shown that the law applies to vibration magnitude [START_REF] Choi | Vibrotactile display: Perception, technology, and applications[END_REF], [START_REF] Francisco | Vibrotactile amplitude discrimination capacity parallels magnitude changes in somatosensory cortex and follows webers law[END_REF], frequency [START_REF] Pongrac | Vibrotactile perception: Differential effects of frequency, amplitude, and acceleration[END_REF], [START_REF] Mahns | Vibrotactile frequency discrimination in human hairy skin[END_REF] and speed of apparent motion [START_REF] Kirman | Tactile apparent movement: The effects of interstimulus onset interval and stimulus duration[END_REF]. In 1860, Weber's law has been extended by Fechner to explain the relationship between the stimulus intensity and perceived magnitude of a stimulus. Steven's law (1957) has since been found to better quantify the intensity-perceived magnitude curves for a number of sensory phenomena [START_REF] Nutter | The role of psychophysics in phytopathology: The weber-fechner law revisited[END_REF]. However, in this work we are not concerned with perceived absolute magnitude, but with the discrimination and ranking of similar intensities, in particular apparent motion speeds. In our design, the intensities of tactile stimuli, whether it is vibration magnitude or motion speed, are separated by the JND. Choi et al. suggested that at least 20%-30% difference in vibration amplitude or frequency is necessary for robust discrimination between vibrotactile stimuli [START_REF] Choi | Vibrotactile display: Perception, technology, and applications[END_REF]. However, humans cannot discriminate between the changes in frequency and magnitude well. In fact, it has been shown that a change in either frequency or amplitude of stimulation can cause an equivalent change in perceived intensity [START_REF] Hwang | Vibrotactile perceived intensity for mobile devices as a function of direction, amplitude, and frequency[END_REF], [START_REF] Ryu | Psychophysical model for vibrotactile rendering in mobile devices[END_REF]. This limitation should be considered when designing tactile patterns based on independent control of frequency and vibration amplitude. The most common DC inertial actuators used for tactile stimulation, a sub-type of which is also known as pancake or coin motors, do not allow their independent control. Instead, both frequency and amplitude increase with the voltage applied to the motor. {However, as we are not concerned with absolute frequency and magnitude identification in this work, we used DC motors due to their simplicity of use and low cost. The coding based on coupled vibration frequency and amplitude is simply referred to as magnitude modulation, as shown in Fig. 1.

The temporal sensitivity of the skin is very high, close to that of the auditory system and better than that of the visual system [START_REF] Jones | Human hand function[END_REF], which can be exploited in tactile patterns.

An example of temporal encoding is pulse train modulation, where short vibration bursts are a function of some physical value. For example, such mapping has been demonstrated for the purpose of prosthetic hand feedback [START_REF] Chatterjee | Testing a prosthetic haptic feedback simulator with an interactive force matching task[END_REF], though pulse width modulation was reported to be inferior to amplitude modulation [START_REF] Stepp | Vibrotactile sensory substitution for object manipulation: amplitude versus pulse train frequency modulation[END_REF].

Sensory integration of apparent motion speed and vibration magnitude

Combining the spatial and temporal modulations can produce tactile apparent motion [START_REF] Kirman | Tactile apparent movement: The effects of interstimulus onset interval and stimulus duration[END_REF], [START_REF] Tedesco | The cutaneous" rabbit": a perceptual illusion[END_REF], [START_REF] Ninu | A novel wearable vibro-tactile haptic device[END_REF] as shown in the bottom of Fig. 1. Tactile apparent motion can provide an intuitive way of presenting spatial information, but discriminating between different motion speeds has been shown to be difficult [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF]. We propose to address this issue by coupling the motion speed with vibration magnitude to emphasize the difference in speed and potentially improve the ability to discriminate between similar stimuli. The magnitude (M ), motion speed (S) and our proposed modulation approach (S&M ) are depicted in Fig. 1.

Fig. 2b depicts the proposed coupling of motion speed and magnitude for the case of L tactons. The i-th and (i + 1)th tactons differ by the JND both in their speed of motion S, defined as the rotation frequency around the forearm, and the magnitude of vibrations M , controlled by motor voltage. This can be described using the Weber fraction as:

M i = M max (1 + w M ) i-L S i = S max (1 + w S ) i-L , (1)
where w M and w S are Weber fractions of magnitude and frequency mapping respectively, and M max and S max are the maximums at i = L. Inherently, the Stimulation Onset Asynchrony SOA i = 1/(k S i ) (where k is the array size) also changes according to the Weber's law. Fig. 2a illustrates the temporal and magnitude parameters for a three-actuator array. As the changes of stimulus properties S and M are coupled, the delivery of information by the S&M stimulus is redundant. The nervous system seems to combine redundant sensory information in a fashion that is similar to a maximum-likelihood estimate (MLE), according to which the combined estimates should have lower variance, and therefore lower discrimination thresholds, than either estimate alone [START_REF] Ernst | Humans integrate visual and haptic information in a statistically optimal fashion[END_REF], [START_REF] Ernst | Merging the senses into a robust percept[END_REF]. Assuming this rule applies to the sensory integration of S and M , we can express the detection threshold as Weber fraction of the coupled variable as

w2 S&M = w 2 S w 2 M w 2 S + w 2 M . (2)
Therefore we hypothesise that the Weber fraction of the S&M modulation will be smaller or at least equal to that of the individual M and S modulation.

Apparent motion around the wrist

Vibrotactile displays are commonly designed for stimulation of the torso [START_REF] Van Erp | Waypoint navigation with a vibrotactile waist belt[END_REF], [START_REF] Hao | Design of a wearable vibrotactile display waist belt[END_REF], [START_REF] Piateski | Vibrotactile pattern recognition on the arm and torso[END_REF] and the forearm [START_REF] Piateski | Vibrotactile pattern recognition on the arm and torso[END_REF]- [START_REF] Chen | Tactor localization at the wrist[END_REF]. Identification of the vibrotactile patterns can be superior on the torso [START_REF] Piateski | Vibrotactile pattern recognition on the arm and torso[END_REF], and the information display can be more intuitive in the case of navigation tasks due to the intuitive correspondence to the user's egocentric direction [START_REF] Barber | Toward a tactile language for human-robot interaction: Two studies of tacton learning and performance[END_REF]. However, in tasks that include manual interaction, feedback displayed on the forearm is more intuitive. The wrist also offers the possibility of combining tactile feedback with co-located visual display and touch or gesture input. Such interactions are becoming increasingly interesting in the growing market of smart watches and similar personal wearable interfaces. Therefore, we chose to study our stimulation approach on the wrist.

In a preliminary study, we investigated the required actuator density to achieve a good illusion of circular motion around the forearm. In a forced-choice task the subjects were presented with stimuli induced by an array of either 3, 4 or 6 motors and had to respond whether they feel clockwise or counter-clockwise motion. The results showed that three motors are not sufficient to yield a circular motion illusion, but the rate of correctly identified direction between four and six actuators was not statistically different. This result agrees with the study [START_REF] Niwa | Determining appropriate parameters to elicit linear and circular apparent motion using vibrotactile cues[END_REF], that reported no significant advantage of using more than four actuators to display apparent motion around the upper arm. However, the subjective feedback from the users in our study revealed that six motors created a continuous sensation but four did not. Therefore we decided to use a six-actuator array in the present study.

Section 3 presents the evaluation of the highest perceivable apparent motion frequency around the forearm. Choosing fast apparent motions is key to achieve short tactons which in turn leads to shorter perception delays. Next, in section 4 we estimate the JND of S&M coding and compare it to the JND of the individual S and M coding. In section 5 we compile sets of tactons for S&M , M and S codings following Eq. 1. We examine the absolute discriminability of the tactons in a rating task and discuss the information transfer.

EXPERIMENT 1: MAXIMAL SPEED OF MOTION

First we evaluated the maximum motion speed at which the subjects were able to correctly identify the direction of motion. Based on the preliminary results we designed a wearable interface consisting of six actuators and evaluated the highest motion frequency that can be induced by such array. In a force choice task, the participants experienced apparent motion at different speeds and reported whether they felt a clockwise or anti-clockwise motion.

Apparatus

The wrist interface is shown in Fig. 2c. Shafted vibration motors were inserted into 3D printed housings to enable attachment to a velcro strap and provide sufficient contact area with the skin. Precision Microdrives model 307-100 (UK) motors were used, which have a rise time of 34 ms and a stop time of 73 ms (according to the data sheet). The motors were powered using haptic controllers DRV2603 (Texas Instruments, USA) which utilise feedback control to minimise the rise and stop times of a vibration motor by applying over-voltage and braking-voltage accordingly. The main module contains an Arduino microcontroller and haptic drivers, and communicated with the host PC via serial protocol.

Methods

Nine 21-30 years old subjects, recruited among graduate students and postdoctoral researchers at Imperial College, participated in the study. None of them reported any abnormalities in their tactile perception. All subjects had previous experience with vibrotactile stimulation. They were seated in front of a computer while wearing the device on the forearm of their right hand. To eliminate auditory cues from the motor noise, subjects wore noise cancellation headphones. Five different S values were tested from 1 to 5 s -1 . There were 10 trials for each S split between clockwise and counter-clockwise direction, resulting in 50 trials per subject. Each stimulus lasted two seconds. As we were only interested in the ability to identify the direction of motion, the magnitude of vibrations was kept constant; the voltage applied to the motors was equal to the nominal voltage of 3 V for any S. After each stimulus, the participants indicated whether they felt a clockwise or counter-clockwise motion around the forearm by clicking the respective buttons on the screen using a touchpad. The subjects were allowed to familiarise themselves with the stimuli before the experiment began.

Results

Fig. 3 shows the distribution of percentages of correctly identified direction of motion within subjects. For S lower than 3 s -1 , all subjects identified the direction of motion in 100% of trials. The majority of subjects were 100% correct in the case of 3 s -1 , while the overall success rate across subjects was 90%. In the case of S = 4 s -1 , the overall success rate was reduced to 70%.

EXPERIMENT 2: SENSORY FUSION AND JND

In this experiment we investigated the effects of combining the speed and magnitude modulations in the proposed apparent motion stimulus. We experimentally evaluated the Weber fraction of apparent motion speed S, vibration magnitude M and the coupling S&M . We fit psychometric functions to the experimental results and discuss the estimated Weber fractions with respect to the multimodal sensory integration concept summarised by Eq. 2.

Methods

EXPERIMENTAL PROTOCOL. Ten subjects with age between 22 and 33 years old without any self reported abnormalities in tactile sensitivity took part in the force choice experiment.

They were recruited among researchers at Imperial College.

All were familiar with vibrotactile stimulation and three subjects also participated in the previous experiment. The same experimental setup was used as in Experiment 1.

The subjects sat at a desk facing a computer screen, used earplugs and noise cancellation headphones. They wore the interface on their right forearm and used the left hand to input the answers. The method of constant stimuli [START_REF] Shore | Spatial modulation of tactile temporal-order judgments[END_REF] was used to determine the JND. In each trial, two stimuli of 500 ms were presented in succession separated by a 500 ms pause as in [START_REF] Pongrac | Vibrotactile perception: Differential effects of frequency, amplitude, and acceleration[END_REF]. The subjects entered their answer same or different using a PC keyboard by pressing the S or D key, respectively. Between the trials, there was a pause of three seconds with the countdown shown on the screen, but the subjects could pause the countdown and rest at any time.

The experiment consisted of three phases, one for each modulation condition. Table 1 shows the experimental conditions for each block. Five I/I values were tested from 0 to 0.4 for the case of low I and high I, which were 33% and 66% of the nominal motor voltage M max , and 33% and 66% of the maximum S max for M and {S, S&M } modulations respectively. Each condition was repeated five times. There were 150 trials in total per subject, which lasted roughly 20-30 minutes to complete. In the case of the M modulation, the actuator located on the volar side of the forearm produced tactile stimulation. We chose to use only one actuator instead of the entire array as in this way the mean stimulated area was equal for all modulations.

WEBER FRACTION ESTIMATION. The fitting of psychometric functions p(x) allowed to predict the probability of perceiving a pair of stimuli as different for an arbitrary x = I/I:

p(x) = 1 + exp - x -a b -1 (3)
where a shifts the psychometric function horizontally and represents the 50% detection threshold, while b determines its slope [START_REF] Wichmann | The psychometric function: I. fitting, sampling, and goodness of fit[END_REF]. We used the fit to estimate the Weber fractions of all three modulations.

Results

For each subject, the rates of stimuli perceived as different were computed for each condition (modulation, I and I/I). as different in at least half of trials for I/I ≥ 0.3 in case of M (low I) and I/I ≥ 0.2 for S&M (either I). In the case of S modulation, the mean rate of stimuli perceived different never reached 50%. Only three subjects identified the S stimuli as different above the 50% threshold at I/I = 0.4 for low and/or high I.

BETWEEN SUBJECT ANALYSIS OF RESPONSES. A 3-way (modulation, I, I/I) ANOVA was performed which found no effect of low and high I on the responses, nor any interaction of I with other conditions. Further twosample tests were carried out for the effect of modulation on response rates per each I/I. For I/I ≥ 0.2 the mean answer rate was found significantly higher for S&M w.r.t. the other two modulations (p < 0.005 in all 12 comparisons).

PSYCHOMETRIC FUNCTION AND JND. Psychometric functions defined by Eq. 3 were fit to the experimental data. The resulting curves are overlaid in Fig 4 . Table 2 shows parameters a which equal the w = IJND I where the curve crosses 50% rate, and the R 2 values which represent the quality of fit. Due to a large bias towards identifying small or zero I/I as different in case of S modulation, a second function was fit considering only points I/I > 0.1. These fits are represented with dashed curves and S* column.

Based on the fits, the estimates of the Weber fractions could be obtained (see

EXPERIMENT 3: TACTON IDENTIFICATION

In general, absolute identification of stimuli is far more difficult than relative identification [START_REF] Brewster | Tactons: structured tactile messages for non-visual information display[END_REF], [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF], [START_REF] Geldard | Some neglected possibilities of communication[END_REF]. We compiled sets of tactons based on M , S and S&M modulations and compared the participants' ability to discriminate between them.

Methods

For each modulation type (M , S and S&M ) a set of L stimuli (tactons) and L responses were compiled with oneto-one association. Participants were presented with the stimuli in random order. On each presentation, they picked a response. The collected responses were tabulated in the form of a stimulus-response confusion matrix. TACTON SET. Given the previously estimated maximum speed of apparent motion and Weber fraction, an arbitrary number of tactons can be compiled. Enlarging the tacton set produces slower motion, which in turn requires longer time to complete the cycle around the forearm. Therefore, enlarging L comes at the cost of longer tacton duration T . In this experiment, we choose L such that the ratio L/T is maximal.

L/T = L f L (1 + w) 1-L (4)
Fig. 5 vious experiment where w for both M and S&M was ≤ 0.3, we chose to compare all three modulations at 0.3. From the analysis depicted in Fig. 5, the particular Weber fraction yields L = 4. Additionally, S&M was also tested for w = 0.2 and L = 5 (condition S&M 5). Tacton duration was chosen such that the slowest tacton completed one cycle. The conditions that characterise the tactons are listed in Table 3. PARTICIPANTS AND PROTOCOL. Thirteen subjects between 23 and 34 years old, all researchers at Imperial College, participated in the study. None reported any abnormalities in their tactile perception. All subjects were familiar with vibrotactile stimulation. Although three of these subject also participated in the previous experiment, it is unlikely this prior experience gave them any advantage as several months passed between the experiments. The apparatus and experimental setup were as in previous experiments. At the beginning of the experiment, instructions were shown on the screen. The subjects were informed that the tactons may differ in speed and/or magnitude, but were not told what modulation was used in each of the four blocks. Table 3 describes the conditions of each block. The blocks are shown in the order as they appeared in the experiment. In each block, the test phase was preceded by the exploration and practice phase as in [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF]. In the exploration phase the subjects first familiarized themselves with the tactons. By pressing the number keys (1 to L) on a standard computer keyboard they triggered the corresponding tacton. Their choice was also indicated on the computer screen (see Fig. 6 for the screen-shot of the graphic display). They explored different tactons until they felt they could distinguish between them and chose to conclude the exploration. Next was the practice phase, where the stimuli of all levels were presented in random order. Each condition was repeated twice and the stimulation was performed once without the possibility to repeat. The subjects rated the stimulus by pressing the corresponding number key.

Their selection and the correct answer were displayed on the screen. In the test phase, the process was same, but the correct answer was not shown. Each condition was repeated 8 times during the test phase. At the end of the experiment the subjects were asked whether they realised the speed of motion was changing in the first two phases (S&M 5 and S&M ). DATA ANALYSIS. For each modulation we examined the rate of correctly identified stimulation levels (tactons) and the variance of error. Analysis of error is suitable for comparing results between equally sized sets of tactons. However, when comparing S&M , M and S to S&M 5, another method may provide a better interpretation of the results. Static information transfer rate quantifies the amount of information bits transferred to the user [START_REF] Tan | Optimum information transfer rates for communication through haptic and other sensory modalities[END_REF], [START_REF] Garner | Uncertainty and structure as psychological concepts[END_REF]. It considers the distribution of error. The maximum likelihood estimate of data transfer β, derived from the stimulus-response matrix, is computed as

β = L l=1 L m=1 n lm n log 2 n lm n n l n m , ( 5
)
where n is the total number of trials, n lm the number of correct responses, n l and n m are the row and column sums [START_REF] Tan | Optimum information transfer rates for communication through haptic and other sensory modalities[END_REF].

Results

CONFUSION MATRICES. The number of correct answers was compared over all subjects. The stimulus-response confusion matrices are shown in Fig. 7. For the case of L = 4, we can compare the overall rate of correct answers for all three modulations. Subjects performed best in the case of S&M with 71% correct compared to 66% and 34% for the case of M and S respectively. Four of the 13 subjects reported that they did not realise the speed of motion was varied in the S&M blocks. In average these four subject identified M tactons more accurately than S&M tactons (69% versus 57%), but performed better in S&M than S (57% versus 28%). ATTENTION TO MOTION SPEED. Based on the observation that four subjects did not detect speed variations in S&M and S&M 5 trial blocks, we examined the effect of this self-reported factor that we call awareness. A twoway ANOVA was performed to test for the interaction between the conditions block and awareness. Significant effects were found for both conditions and their interaction (p < 0.005, p < 0.005 and p < 0.020). Follow-up multiple comparison tests confirmed the interaction in case of the S&M block (p < 0.012), but not in others (p ≥ 0.55).

INFORMATION TRANSFER. The results of information transfer estimation β computed using Eq. 5 are shown in Fig. 7b for all subjects (left) and the subjects aware of speed changes (subset; right). One-way ANOVA tests confirmed the effect of modulation on β for both subject sets. Further two-sample T-tests were carried out to test which differences in means of β are significantly different. The horizontal lines link the significantly different groups. In the case of circle arrowheads the significance was p < 0.031, whereas the square arrowheads denote p < 0.005 significance. TIME TO ANSWER. Between subjects mean time required to answer was 2.21, 1.62, 1.47 and 1.82 seconds for S&M 5, S&M , M and S respectively. Pairwise two-sample T-tests showed the mean time for S&M 5 was significantly longer than S&M and M (p < 0.01).

DISCUSSION

In this work we presented a concept of design of tactons based on apparent motion. To achieve fast information transfer, we aimed to design short yet distinct tactons. We developed a tactile interface that creates vibrotactile apparent motion around the forearm. To improve the ability to discriminate between motions of different speeds, we proposed to couple the speed with vibration magnitude, resulting in stimuli that range from slow and weak to fast and strong. We evaluated the fastest perceivable motion speed and investigated the relative and absolute discrimination of the individual and coupled modulations by estimating the JND and information transfer.

MAXIMAL SPEED AND HARDWARE LIMITATIONS. We found that most subjects were able to perceive the direction of the motion correctly at 3 rotations per second. At S = 4 s -1 the SOA equals 41.7 ms, but humans can detect vibration pulses and gaps of 10 ms [START_REF] Petrosino | Temporal resolution of the aging tactile sensory system[END_REF], [START_REF] Gescheider | Temporal relations in cutaneous stimulation[END_REF]. The said SOA is very close to the rise time of the actuator used in this work, therefore we conclude that the ability to perceive the direction of motion correctly was limited by the motor dynamics, rather than by the human tactile perception. Higher motion speed may be achievable using voice coils (linear actuators), such as the Haptuator (Tactile Labs, USA). These are typically quicker and offer independent control of vibration frequency and amplitude, but the cost of these actuators is considerably higher, and the actuators require more complex AC driver circuits.

JND OF MAGNITUDE AND APPARENT MOTION SPEED. By fitting the psychometric function we obtained an estimate of Weber fractions for each modulation. w for M modulation was estimated between 0.22 and 0.26, which agrees with [START_REF] Choi | Vibrotactile display: Perception, technology, and applications[END_REF]. We found that in the case of S modulation it is very difficult to discriminate between similar motion speeds even for I/I as high as 0.4. By fitting psychometric functions to the experimental results, we were able to obtain estimates of Weber fraction between 0.422 and 0.629. These estimates are higher than the results in [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF], which reported w of 0.25. However, in the said study the stimuli were six times longer (lasting 3 seconds), which could explain the difference as the participants were given more time to sample the motion speed during each stimulus presentation. Thus, a further evaluation of JND at different stimulus durations is required. MLE SENSORY INTEGRATION. The comparison of answer rates and the estimated JND show superior performance of the coupled S&M modulation, which confirms our hypothesis that the discrimination ability of the coupled variable is equal or better than that of the individual variables. However, the detection thresholds for the coupled modulation predicted by the MLE (Eq. 2) shown in Table 2 are substantially larger than those experimentally obtained. Explaining this result with probability summation was also dismissed, as it could only explain the case where the detection threshold of the coupled variable ranks between those of individual ones. Perhaps this difference between the predicted and measured threshold is caused by the tactile spatial summation phenomenon [START_REF] Gescheider | A four-channel analysis of the tactile sensitivity of the fingertip: frequency selectivity, spatial summation, and temporal summation[END_REF]; although the total stimulated area of the skin was constant and equal for all three modulations, in case of M the total number of stimulated tactile receptors during the stimulus duration was smaller as only one actuator was used. Therefore, future experiments should compare the JND of magnitude changes for the case of M modulation and S&M modulation with constant motion speed. ABSOLUTE DISCRIMINATION AND INFORMATION TRANSFER. Between-subject analysis showed that compared to M and S, S&M enabled more correct answers and higher information transfer β. Although the estimates of β for S&M and S&M 5 (see Fig. 7b) were not substantially higher than those obtained in [START_REF] Kohli | Towards effective information display using vibrotactile apparent motion[END_REF] for apparent motion (1.02 bits), the tacton duration in our study was roughly four times shorter. Therefore, thanks to coupling of the sensory cues, our approach allows to achieve higher β values for much shorter tactons and thus higher information transfer rate β/T (bits/second). The within-subjects average time to answer was smallest in case of M modulation, followed by S&M with 9% longer reaction times, however, no significant difference was found between these two. Finally, as the subjects were not explicitly told what cues to pay attention to, four of them did not realise that the speed was changing in case of S&M . These subjects performed best in M tacton identification as they did not pay attention to speed changes and thus could not take advantage of redundant cue coupling. It remains unclear whether instructing them explicitly would yield better performance, but prolonging the tacton duration may enable them to realise the coupling during training and facilitate better identification performance.

To summarise, the results show that the relative discrimination of the coupled variable can be characterised by JND smaller than JNDs of the individual variables. This allowed us to create short tactons for display of direction and speed, which can be distinguished significantly better than tactons based on motion alone. Most subjects were also able to identify the coupled-variable tactons better than the magnitudebased tactons. As our experiments were conducted using stimulation of the wrist, the results are directly applicable to personal wearable devices such as smart watches. The proposed coupled modulation could for example be employed to render different angles of a turn in spatial navigation tasks. Additionally, the coupled variable could present the priority of an incoming message, while the direction of motion could suggest the context (work or private message). Thanks to the lower JND of the proposed coupled variable, this feedback approach could also be employed for real time feedback of vector quantities in manual interactions. For instance, the coupled variable could encode the torque exerted on the wrist of a teleoperated robot. However, the results highlight the need for further studies to investigate the dependency of apparent motion perception on stimulus duration, the potential effect of spatial summation on the perception of magnitude of S&M stimuli, and the role of user's attention to motion speed in absolute identification.

Fig. 2 :

 2 Fig. 2: Vibrotactile apparent motion. (a) Three motors in a closed loop array are sequentially activated to create continuous apparent motion. (b) The magnitude and stimulus frequency increase with l. The coupling results in stimuli ranging from slow & weak to fast & strong. (c) Photo of the tactile interface worn on the wrist.

Fig. 3 :

 3 Fig. 3: Result of Experiment 1: distributions of correct answer percentage across subjects for the six wave frequencies.

Fig. 4 :

 4 Fig. 4: Result of Experiment 2: Rates of stimuli perceived as different between subjects. Psychometric functions fitted to experimental data are overlaid. The dashed line represents the fits that only considered the data points for I/I > 0.1.

Fig. 5 :Fig. 6 :

 56 Fig. 5: Tactons per duration as functions of L and their maxima (dots) for different Weber fraction values. The tick curves represent w obtained from the same-different judgements, while the thin curves are for w derived from fitted models.

Fig. 7 :

 7 Fig. 7: Result of Experiment 3: (a) Confusion matrices depict the frequency of responses in % across all subjects. (b) Distributions of static information transfer rate between all (left) and the 9-subject subset (right).

  Types of tactile stimulation provided to the forearm depicted as vibration amplitude against time. Using a single point

	single point	magnitude (M) modulation	pulse width modulation
	stimulation		(frequency, amplitude)	(temporal)
			weak	strong	short	long
	spatio-temporal		motion speed (S)	motion speed & magnitude
	stimulation		modulation	(S&M) modulation
	stimulators form a closed loop around the forearm	ampl.	slow	fast	slow &	fast &
		time t			weak	strong
	Fig. 1:

stimulation (top), vibration frequency, amplitude and temporal parameters can be varied to create a set of cues. A tactile array (bottom) can employ combination of encodings to produce spatio-temporal and magnitude-spatio-temporal cues.

TABLE 1 :

 1 Fig 4 shows the means and standard deviations of the rates between subjects. Participants identified the stimuli Experiment

	MODULATION	M	S	S&M
	REFERENCE INTENSITY I
	LOW	M	33% M max	const.	33% M max
		S	/	33% S max	33% S max
	HIGH	M	66% M max	const.	66% M max
		S	/	66% S max	66% S max
	COMPARISON INTENSITY I + I
	I/I		0.0, 0.1, 0.2, 0.3, 0.4

2 consisted of three blocks of trials, one for each modulation. In each block the JND was estimated around low and high I. Mmax is the nominal motor voltage, and Smax is the maximal speed determined in Experiment 1.

Table 2 )

 2 . The last two columns are the MLE estimates following Eq. 2.

	I	M	S	S*	S&M MLE MLE*
	LOW	0.256	0.629	0.422	0.136	0.237 0.219
		(0.286)	(0.041)	(0.131)	(0.743)
	HIGH	0.221	0.653	0.544	0.144	0.209 0.205
		(0.192)	(0.084)	(0.090)	(0.617)
	BOTH	0.240	0.652	0.473	0.141	0.225 0.214
		(0.375)	(0.105)	(0.196)	(0.759)

TABLE 2 :

 2 Results

of Experiment 2: Columns 2-5 show Weber fraction estimates derived from the psychometric function fits (R 2 shown in brackets). Columns 6 and 7 show Weber fractions predicted by MLE.

  depicts L/T (tactons/second) as a function of the number of levels L for multiple values of the Weber fractions that we considered. Based on the results of the pre-

	block	S&M 5 S&M	M	S
	CONDITIONS
	modulation	S&M	S&M	M	S
	Weber fr./tactons	0.2 / 5 0.3 / 4 0.3 / 4 0.3 / 4
	tacton dur. T (ms)	691	732	732	732
	TRIALS
	exploring		variable (1-3 minutes)
	practice	10	8	8	8
	testing	40	32	32	32

TABLE 3 :

 3 Summary

of experimental conditions. Four blocks tested different stimulation conditions resulting in 170 trials per subject.

ACKNOWLEDGEMENT

This work is supported by the European Commission under EU-FP7 grants PEOPLE ITN-GA-2012-317488-CONTEST, ICT-601003 BALANCE, ICT-2013-10 SYMBITRON and H2020 ICT 644727 COGIMON.


Ingrid Girardeau

Aline Coftier

Using bioaccessibility testing in health risk assessment as a decision support tool for appropriate redevelopment of former mining sites: a French case study

Keywords: arsenic, bioaccessibility, human health risk assessment, cassiterite, redevelopment

In many countries, remediation is required when a site poses a risk to human health and/or the environment. In this context, risk assessment methodologies usually rely on the total contaminant concentrations measured on site without considering chemicals bioavailability. Such approach is actually irrelevant when considering former or abandoned mining sites characterised with high concentrations of heavy metals in soils as it often results in an overestimation of risks, leading to heavy, lengthy and costly remediation works.

The case study was the site of a cassiterite mining exploitation that ended in 1957. At the present time, 3.7 million cubic meter of tailings and waste rocks, with rather high arsenic and other heavy metals concentrations, remain on the site. Beside these residues, and issues associated with the acid mining drainage, there is still great concern with the frequentation by the public of this site. Effectively, because of its astonishing shape, it is highly visited by a wide range of population (local and national). It offers a unique opportunity for sport and leisure activities (e.g.: mountain bike, walking, trail, horse riding) in a relatively flat region. Therefore, this site represents an opportunity of tourism and economic development for the municipality.

The aim of this study was to assess the compatibility of these uses with the soils quality, given that they are especially highly concentrated in arsenic. Ingestion is one of the major routes of soil exposure, by children especially. However, data from literature indicate that the bioaccessible fraction of arsenic for human soil ingestion can vary from 2 to 90%. Therefore, it is relevant to carry out site specific tests.

Bioaccessibility tests were carried out on 10 soil samples collected on site, following the Unified BARGE (BioAccessibility Research Groupe of Europe) Method (UBM). This in vitro method simulates the human digestive procedure using synthetic digestive fluids. Results indicate that the bioaccessibility of arsenic on this site ranges between 5 and 50 %, averaging out at 17%. The human health risk assessment at this specific site integrated these bioacessibility results so that health risk quantifications are more relevant, accurate and closer to reality. In this case study, we show that using standard highly conservative approach and more accurate approaches lead to rather different results in terms of possible uses.

Using this approach, appropriate solution for redeveloping the site balancing health and environment protection requirements with tourism and economic issues will be available.

Introduction

In many countries, remediation is required when a site poses a risk to human health and/or the environment. If most of threshold or regulatory values are available on (drinking) water or air quality, no threshold value is available on soil quality in France. In order to assess if soil quality is compatible with the uses, human health risk assessment (HHRA) is the only available tool. On former or abandoned mining sites, ingestion is one of the major routes of soil exposure to heavy metals contamination, by children especially. Current approaches are usually largely conservative and exposure is currently assessed using the total soil concentration of individual contaminants.

Such approaches lead to important safety margins, and thus could be considered as less appropriate when considering former or abandoned mining sites characterised with moderate to high concentrations of heavy metals in soils resulting of both natural and anthropogenic activities. Moreover, several in vivo studies, using diverse animals, such as monkeys, juvenile swine, rabbits, and rodents, have demonstrated that only a fraction of a contaminant, the bioavailable fraction, is absorbed following oral administration [START_REF] Denys | In vivo validation of the unified BARGE method to assess the bioaccessibility of arsenic, antimony, cadmium, and lead in Soils[END_REF].

The BioAccessibility Research Groupe of Europe (BARGE) has developed an in vitro test, the Unified BARGE Method (UBM), to measure the bioaccessibility of soil contaminants. This method was subjected to an invivo validation and provides a robust tool for use in risk assessment of As, Cd and Pb [START_REF] Denys | In vivo validation of the unified BARGE method to assess the bioaccessibility of arsenic, antimony, cadmium, and lead in Soils[END_REF].

In this paper, we present a case study of an old mining site where remain tailings and waste rocks with moderate to high arsenic concentrations. This site is widely frequented by the population and the aim of the study was to assess the compatibility of the uses with the soil quality. The bioaccessibility testing were used in health risk assessment.

The aim is to obtain a quantification more relevant, accurate and closer to reality in order to define appropriate solution for redeveloping the site balancing health and environment protection requirements with tourism and economic issues.

Methodology

Field site description

The studied site is an old mining exploitation of cassiterite (SnO2) located to the west of France. The main exploitation was conducted between 1952 and 1957 by open pit mining. The cassiterite deposit is related to quartz veins in schists. It contains also sulphide minerals as arsenopyrite. At the present time, 3.7 million cubic meter of tailings and waste rocks remain on the site with high arsenic concentrations, as shown on Figure 1. Because of its astonishing shape, the site is highly visited by a wide range of population (local and national). It offers unique opportunity for sport and leisure activities in a relatively flat region. Several uses have been noted, some developed by the municipality, others that have developed over time, such as mountain bike, horse riding, walking, trail, picnic. Associations often organise trail, mountain bike and BMX competitions on the site. It represents a good opportunity of tourism and economic development for the municipality.

However, arsenic concentrations in soils constitute a potential risk to human health and therefore require assessment.

Soils characterization

On site investigations were carried out in order to characterize arsenic concentrations in subsoils. The depth sampled were determined according the identified uses. At the tailings disposal site, the main route of exposure identified is the ingestion of soil and dust from superficial horizons. Thus, investigations concerned the horizon from 0 to 5 cm deep.

177 measures using an X-Ray Fluorescence portable (pXRF) analyser were carried out. Due to the relative homogeneity of materials, the sampling plan followed a systematic grid of 50 m. This allows a statistical analyse of results and a good representativeness of average exposure concentrations and arsenic occurrence.

10 samples were collected on site to analyse metallic trace elements in laboratory for soil characterization and site-specific linear correlation with the field analyser. The samples were selected to ensure a spatial representativeness of the site as well as a representativeness of encountered concentrations with the field analyser (selection from the lower, middle and upper range of concentrations measured at the site). Bioaccessibility testing were performed on these samples following the Unified BARGE Method (in accordance with ISO/DIS 17924).

UBM procedure

Unified BARGE Method is an in vitro method for simulating the human digestive procedure. Digestive fluids are synthetic fluids used in the UBM test to simulate the fluids present in human digestive system: saliva, gastric fluid, duodenal fluid and bile. The current procedure describes a method for simulating the human gastro-intestinal tract through 3 different compartments: mouth (5 minutes), stomach (1 hour), and small intestine (4 hours). Figure 3 shows the schematic diagram of the UBM methodology. Each sample follows two separate tests:

• The gastric phase is a digestive extract collected after 1 hour agitation with saliva and gastric fluids.

• The gastro-intestinal phase is a digestive extract collected after 1 hour agitation with saliva and gastric fluid followed by 4 hours agitation with duodenal fluid and bile. This method was applied on the 10 collected samples in duplicate. Blank and reference materials (NIST 2710a) also followed the complete procedure.

Human Health Risk Assessment (HHRA)

Human health risk assessment, regarding exposure to chemical substances, is performed following these different steps:

• Hazard identification -Hazard Identification is the process of determining whether exposure to a substance can cause an increase in the incidence of specific adverse health effects (e.g., cancer, birth defects) and whether the adverse health effect is likely to occur in humans.

• Dose-Response assessment -A dose-response relationship describes how the likelihood and severity of adverse health effects (the responses) evolve with the amount and condition of exposure to an agent (the administered dose).

• Exposure assessment -Exposure assessment is the process of measuring or estimating the magnitude, frequency, and duration of human exposure to a substance in the environment.

• Risk characterization -Risk characterization is the result of the risk assessment process. It aggregates the data collected in previous steps in order to quantify risk levels for human health and their acceptability with regard to standard comparison thresholds. This step also includes the evaluation of the assumptions and uncertainties that may still exist in the assessment. This paper will not describe "Hazard identification" and "Dose-response assessment" steps.

Exposure assessment

Based on observations made and developments noted on the site, several areas of homogeneous use were defined, as shown in Figure 4 and Table 1. The observations supporting these scenarios were made during many site visits occurring within or out of school holiday periods during the last 3 years. It is assumed that only adults between 15 and 50 years old engaged in "extreme" sport activities.

Risk characterisation

Potential non-cancer risk for individual contaminants are expressed as Hazard Quotient (HQ). HQs are calculated as the ratio of the estimated daily intake of each contaminant to the corresponding route-specific reference dose (RfD -tolerable daily intake or tolerable concentration).

HQ = ADD / RfD

(1)

where:

HQ: hazard quotient ADD: average daily dose RfD: reference dose When the average daily dose estimated from site-associated soil contaminants exceeds the protective RfD, the HQ exceeds one. Such risk levels are therefore considered as inacceptable.

Risk characterization of carcinogenic substances consists in calculating the Incremental Lifetime Cancer Risk (ILCR). ILCR is expressed as an estimated upper-bound probability of additional lifetime cancer risk due to exposure to site-related contaminants. The upper-bound estimate of excess risk related to each contaminant is calculated by multiplying the lifetime average daily dose (LADD) estimated for that contaminant by its corresponding route-specific cancer slope factor.

ILCR = LADD x CSF (2)

where:

ILCR: Incremental Lifetime Cancer Risk LADD: Lifetime average daily dose CSF: Cancer slope factor For non-threshold contaminants, acceptable incremental risk levels in Europe are defined that vary from 1.10 -4 (The Netherlands) to 1.10 -6 (Italy), while 10 -5 is adopted by the large majority of country including France [START_REF] Carlon | Derivation methods of soil screening values in Europe. A review and evaluation of national procedures towards harmonisation[END_REF]. In this case study, ILCR has to be under 10 -5 to be acceptable (ILCR < 10 -5 ).

Integration of bioaccessibility into risk characterisation

Absolute bioavaibility is the fraction of a substance present in ingested soil that reaches the systemic circulation (blood stream). But determine bioavaibility is not easily achievable due to both experimental issues linked to blood sampling and to analytical limitations.

Whereas, absolute bioaccessibility is the fraction of substance in soil or soil material that is liberated in (human) gastrointestinal juices and thus available for absorption (ISO/DIS 17924, 2016). So, bioaccessibility is the maximal fraction of a substance that can reach the systemic circulation.

The ingested matrix containing the toxic substance has an influence on the dose-response relationship of the substance. In order to integrate this parameter in risk calculation, the corrective factor is the relative bioavailability [START_REF] Ineris | Quantité de terres et poussières ingérées par un enfant de moins de 6 ans et bioaccessibilité des polluants: état des connaissances er proposition[END_REF]. This variable is defined as the ratio of absolute bioavailability of the substance in the soil matrix to absolute bioavailability of the element in the matrix used to develop the Reference Toxicity Value (RTV as RfD or CSF). [START_REF] Caboche | Validation d'un test de mesure de bioaccessibilité. Application à quatre éléments traces métalliques dans les sols: As, Cd, Pb et Sb[END_REF][START_REF] Ineris | Quantité de terres et poussières ingérées par un enfant de moins de 6 ans et bioaccessibilité des polluants: état des connaissances er proposition[END_REF][START_REF] Ineris | Quantité de terres et poussières ingérées par un enfant de moins de 6 ans et bioaccessibilité des polluants: état des connaissances er proposition[END_REF] retain the equality of the absorbed fraction of As present in contaminated soil and the absorbed fraction of As present in drinking water (reference matrix for the establishment of the RTVs). In addition, [START_REF] Oomen | How can information on oral bioavaibility improve human health risk assessment for lead-contaminated soils? Implementation and scientific basis[END_REF] indicate that the metabolism of absorbed arsenic does not depend on the matrix in which the element was in the gastrointestinal tract. Finally, the absolute bioavailability of arsenic in water, the reference matrix for RTVs, is close to 100% [START_REF] Ineris | Quantité de terres et poussières ingérées par un enfant de moins de 6 ans et bioaccessibilité des polluants: état des connaissances er proposition[END_REF]. Then, the relative bioavaibility of arsenic is set equal to the absolute bioaccessibility of arsenic in soil.

Soil characterisation

Following EPA Method 6200 (EPA, 2007), a strong correlation (r > 0.8) must be established between the in situ XRF measurements and the arsenic concentrations measured in the laboratory to be used as a definitive characterisation tool. This method also specifies that one of every 20 XRF samples should be collected and submitted for laboratory analysis. 10 samples were collected and analysed out of the 177-pXRF measures.

Figure 5 shows the correlation between in situ XRF arsenic and laboratory arsenic concentrations. Figure 6 shows the arsenic concentrations repartition on the site. Arsenic concentrations in tailings and waste rocks on site range between 20 and 1,400 mg/kg, with 67 % of concentrations between 100 and 600 mg/kg, while the local geochemical background is around 100 mg/kg. The highest concentrations are encountered on the tailings deposit, while they are close to the local geochemical background (100 mg/kg) on the north part of the site Given the large amount of data, chosen concentrations for HHRA is the average concentration calculated in each zone, as shown in Table 2. In a conservative approach, if the amount of data is not relevant, the maximal encountered concentration is selected leading to an overestimation of risk.

Zone Number of analyses

Range of encountered concentrations

Selected concentration (average) Standard deviation

Zone

Bioaccessibility

Results on the blank allow validating the absence of contamination related to the procedure that could affect the estimation of arsenic bioaccessibility in the samples.

Results on the reference material (NIST 2710a) reflect a good repeatability of the results and are consistent with all previous results obtained by the laboratory.

Figure 7 shows the results of absolute, gastric and gastrointestinal bioaccessibility in the 10 samples analysed (average, minimum and maximum of the 2 replicates per sample).

Figure 7 Bioaccessibility testing results

This figure shows a low variability of the results between the replicates of each sample.

Absolute gastric bioaccessibility is superior to gastrointestinal bioaccessibility for 6 samples and relatively similar for the other 4 samples. The gastric bioaccessibility measured on a waste rocks sample (51 % on ABNit154 for 537 mg/kg in soil) is higher than that measured on all the other samples (6 to 27 %). Despite this high value, gastric bioaccessibility in waste rocks is not significantly different than those in tailings (nonparametric Mann-Whitney test: p=0.11; significance of difference for p<0.05). Therefore, waste rock and tailings were considered to have similar arsenic gastric bioaccessibility, averaging at 17.2 %.

Results

Calculations of risk characterisation were performed with site specific parameters when they were available. Potential risks, calculated on the basis of the average concentrations and bioaccessibility of soils, are summarised in the Table 3 below.

Table 3 Results of risks characterisation

Results above show that HQ is less than 1 for all the considered zones and scenarios. But ILCR is higher than the threshold value (10 -5 ) on zone 2, 3 and 4 for all the considered scenarios.

For the scenario "walking and running" on zone 2, the contribution of risk is the high arsenic concentration, where for others scenarios (mountain bike, BMX, etc.) on zones 2, 3 and 4, the major contribution of risk is the high ingestion rate selected due to these activities that generate a lot of dust.

Conclusion

Results show that one zone of the site is compatible with a tourist and leisure use despite the high arsenic concentrations encountered. It would not have been possible to draw such conclusion without a good screening of arsenic occurrence and bioaccessibility testings. Indeed, this method allow a better representativeness of human exposure by a risk quantification more accurate and closer to the reality while remaining conservative.

Site improvements will be proposed to the municipality in order to better control the uses on the site, and to ensure health and environment protection:

• In zone 1, since risk levels are close to the acceptability threshold, it could be wise for example to reduce direct contact of the walkers with soil by consolidating the paths and areas with specific attractive structures (bench, viewpoint indicators) with non-erodible safe soil materials. We could also recommend the removal of the picnic area, where As concentration is higher than the average concentration of the area, or the substitution of the topsoil with safe previously controlled soil. These simple dispositions should be sufficient to enable a safe leisure development and public frequentation of zone 1. • However, specific dispositions should be implemented to prevent public access to zones 2 to 4.

Moreover, the improvement of the trail marking could also help to limit the access to these zones.

Figure 1

 1 Figure 1 Aerial view of the site (tailings (left) and waste rocks (right) deposits, water-filled open pit)

Figure 2

 2 Figure 2 Examples of uses identified on the site

Figure 3

 3 Figure 3 Schematic diagram of the UBM methodology (http://www.bgs.ac.uk/barge/ubm.html)

Figure 4

 4 Figure 4 Localisation of selected data for the different scenarios of exposure

  It is thus possible to integrate absolute bioaccessibility of arsenic from the UBM test in risk calculation for exposure by ingestion as shown in equations below: HQ = ADD x BAAs absolue sol / average daily dose BAAs absolute soil: Absolute bioaccessibility of arsenic (= relative biodisponibility) CSF: Cancer slope factor 3 Data

Figure 5

 5 Figure 5 In situ XRF total concentration versus laboratory total arsenic concentrationA strong correlation (R² = 0.92) is shown between the pXRF arsenic measurements and the laboratory total arsenic concentrations. The equation mentioned on the Figure5was used to convert the pXRF measurements to laboratory equivalent arsenic concentrations at the site.

Figure 6

 6 Figure 6 Total arsenic concentrations map

Table 1 Exposure scenarios defined for HHRA

 1

	Zone	Uses	Receptors	Exposure routes
	Zone 1	Walking, running	Child, Adult, Child becoming adult	Soil/dust ingestion Dust inhalation
		Walking, running	Child, Adult, Child becoming adult
	Zone 2	Moutain bike, horse riding	Adult (6 to 70)	Soil/dust ingestion Dust inhalation
		BMX	Adult (15 to 50)
	Zone 3	BMX	Adult (15 to 50)	Soil/dust ingestion Dust inhalation
	Zone 4	Mountain bike	Adult (15 to 50)	Soil/dust ingestion Dust inhalation

Table 2 Selected arsenic concentrations on the different zones of exposure

 2

Standard ISO/DIS 17924 2016, 'Soil quality -Assessment of human exposure from ingestion of soil and soil material -Procedure for the estimation of the human bioaccessibility/bioavaibility of metals in soil', Project of revision. US Environmental Protection Agency (EPA) (2007) Field portable XRF spectrometry for the determination of elemental concentrations in soil and sediment, Washington, DC: EPA.


Xavier Bourrat
email: xavier.bourrat@univ-orleans.fr

Loic Francke

Evelyne Lopez

Marthe Rousseau

Philippe Stempflé

Martine Angellier

Patrick Albéric

Nacre biocrystal thermal behavior

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

The thermal stability of nacre is of potential interest in different fields as crystallography, biomineralisation, geochemistry, exobiology [START_REF] Reitner | Origins[END_REF] but also in biomaterials, biomimetism and chemistry. [START_REF] Xu | [END_REF] The reason is that nacre is not a pure mineral but a biomineral or biogenic aragonite; the nacre crystal contains organic compounds long believed to be organic inclusions. 3 It was recently shown that a nacre biocrystal is in fact, made up of aragonite nanograins encapsulated into the intracrystalline organic matrix. [START_REF] Rousseau | [END_REF] Despite this nano-composite structure the biocrystal (tablet) presents a perfect coherent diffraction pattern. 5,6 This was asserted by AFM (atomic force microscopy) in tapping mode (Fig. 1) evidencing the nanograins and TEM SAD single crystal-like pattern at the same scale (transmission electron microscopy selected area diffraction).

So the questions raised in this paper are: how nacre behaves during thermal treatment? What is the evolution of the organic matrix with regards to the mineral phase in such a composite crystal? And what is the thermal stability of aragonite under the form of nanograins intimately encapsulated within the intracrystalline organic matrix? Some of these questions were investigated in oxidative conditions in the case of the same nacre by our group 7 as well as in coral by Dauphin et al. 8 These papers have shown that the organic matrix is not fully destroyed after an oxidation heat-treatment up to more than 500 °C. At the same time aragonite had already transformed into calcite.

Results

The thermal gravimetric analysis (TGA) plots of Pinctada maxima nacre showed a good thermal stability up to 230 °C or showed a faint mass loss in the case of fresh nacre due to dehydration (less than 1%). Blocks contained more humidity and lost more weight. Above this temperature, nacre degraded with a two-step mass loss. The first one is minor and corresponds to the degradation of the most labile organic matter between 250 and 450 °C (500 °C for blocks, Fig. 2). The second, between 585 °C and 915 °C (550 uC and 890 °C for the powder), is an important mass loss which occurred due to the transformation of calcite into lime. Finally, at 1000 °C, the mass loss is stabilized (45% of the initial mass).

TMA records any volumetric change with temperature under argon. Two situations were systematically compared: elongation of nacre block parallel (1) and perpendicular (2) to the nacre tablets (see schematic in Fig. 3). First runs were conducted below 200 °C.

Expansion of both samples was systematically reversible (not shown). It was possible to measure the two coefficients of thermal expansion: ε = 5 x 10 -8 K -1 in the plane and ε = 24.5

x 10 -8 K -1 perpendicular to the planes. Thus, as expected, nacre exhibits a good reversible anisotropy below 200 °C: expansion is 5 times higher perpendicularly to the tablets.

As a summary, from room temperature up to 230 °C nacre was stable when heated under argon. A small mass loss was attributed to dehydration. No hydrocarbon nor CO 2 were detected by means of the Rock-Eval technique. A first stage can be pointed out as that of 'nacre reversible expansion from room temperature up to 230 °C'.

TMA was also conducted up to 1000 °C under argon. An example is given in Fig. 3. A small expansion of 3.5% was seen to occur at 250 °C, solely visible in the stacking direction.

It is attributed to a 'puffing' due to the onset of gas departure. This is asserted by the temperature-programmed desorption technique Rock-Eval which is used for detecting hydrocarbons, CO and CO 2 . The TGA 'puffing' at 250 °C occurs as hydrocarbons (Fig. 4(a)) but also CO 2 (Fig. 5(a)) start being released from the structure. So, a second stage can be defined which extends between 250 and 450 °C when the TGA curve gets back to zero characterized by the 'organic matrix cracking under the form of hydrocarbons'. Indeed, most of the hydrocarbons are released during this second stage. At the same time, the CO 2 signal exhibits a broad massif between 250 and 500 °C with exclusively C and O of organic origin. The abiotic aragonite crystal-purely mineral-only shows a negligible CO 2 departure below 500 °C. At the same time, decalcified nacre demonstrates that 75% of the organic CO 2 is still to come at the end of the second stage. It is to note that during this second stage the strong cracking of the organic matrix occurs without allotropic transformation of the bio-aragonite: it is stable up to 470 °C as deduced from TMA.

The nanocomposite structure of the biocrystal does not change the thermal stability of aragonite.

Fig. 2 Nacre thermal gravimetric analysis (under nitrogen).

Dotted line : powder; full line : block. The inset is a zoom of the thermal behaviour from 20 to 600°C with the derivative of the signal for nacre powder.

The third stage started at 470 °C with a large 'swelling' in the TMA plots in both stacking and in plane directions. This expansion is produced by the allotropic transformation of aragonite to calcite. It was occurring at lower temperature under an isothermal oxidation treatment (300 to 400 °C, see ref. 7). The aragonite-calcite lattice expansion is known in the literature to be +2.8% and occurring between 420 and 520 °C for natural aragonite. 9 Our results are coherent in plane (+2.1%) but much higher in the stacking direction (+21.6%). This lattice expansion is supposedly amplified by a 'puffing' effect due to the cracking of the intracrystalline matrix encapsulating the aragonite nanograins (see Fig. 1). Indeed, the Rock-Eval plots exhibit a peak of hydrocarbon and a peak of organic CO 2 at the same time. As the mechanical resistance of nacre is less in the stacking direction, all the additional expansion occurs in this direction. At the same time, the abiotic aragonite did not show any extra degassing (continuous and very faint) during the allotropic transformation. Also these peaks disappear in the case of decalcified nacre (Fig. 4(b) and 5(b)).

Fig. 3 A TMA plot of nacre blocks up to 1000°C under argon : (1) sensor parallel to the plane and (2) perpendicular to the tablet stack.

As a summary, the third stage can be considered as the 'allotropic transformation of bio-aragonite into calcite between 470 and 550 °C'. A comparison of nacre with decalcified nacre showed that most of the organic oxygen functions are still stable at the end of this process.

Finally, a fourth and last stage relates to a strong shrinkage between 550 and 940 °C (910 °C in the plane, Fig. 3) and the main weight loss, 40% as asserted by TGA. The Rock-Eval plots showed at the same time that the organic matrix continues to produce mainly CO 2 and CO molecules up to 650 °C. Above this temperature, all the mass loss is due to the calcination of calcite. The end of this fourth stage occurred at 940 °C beyond which, the sample is supposed to be composed of lime, residual calcite and a small fraction of refractive carbon. This phenomenon happened at lower temperature under isothermal oxidation treatment: a blend of calcite and lime was observed already at 550 °C up to 900 °C where a tiny amount of calcite is still visible in the powder. 7 The mineral CO 2 departure can be calibrated with the abiotic aragonite. It really starts above 630 °C with limited amplitudes anyway. It fits with the TMA shrinkage due to the chemical transformation of calcite into lime by departure of CO 2 . The amplitude is meanwhile very limited and one could think that 'CO 2 released by nacre before 650 °C is produced quasi exclusively by the organic fraction'.

Discussion and conclusions

The presence of organic matrix in nacre was seen to change the well known transformation of abiotic aragonite to calcite and then calcite into lime with temperature. In nacre, the organic matter is stable up to 250 °C. Then cracking of the organic content occurs two times, between 250-550 °C (comprising all the hydrocarbons) and then between 550-650 °C (oxygen functions). What is noticeable here, is the very high temperature for which the insoluble organic fraction is still active at producing CO 2 : the highest peak is centred at approximately 650 °C. Above 650 °C, only a small residue of refractive organic carbon remains. Refractive organic carbon is quantified by a subsequent oxidation heat treatment by means of the Rock-Eval method up to 850 °C (Fig. 6(b)) measuring the related CO and CO 2 at low temperature (Fig. 6(a)): 20% of the total organic carbon remains as refractive carbon in nacre. Refractive carbon increases up to 50% in the decalcified samples (AIM).

The Rock-Eval technique provides also the total organic carbon balance (TOC).

Decalcified nacre (AIM) gave a TOC value of 45 wt% of the mass of the sample. This value was calculated following the standard IFP (Institut Francais du Petrole) method for isolated organic matter, adding high temperature peaks as S3', S3' CO as well as S5. As expected the abiotic aragonite did not give any TOC. Nacre provided 2 sets of values: nacre powders gave a TOC value of 1.2%, while blocks gave 0.9%. The organic matrix content of nacre is thereafter 2.7 wt% if measured in the powder and 2.0% in the blocks. The order of magnitude is good since the estimated value by the chemical extraction protocol gives 2.4%.

The experiments conducted on the powder (TGA or Rock-Eval) have shown that the 4 thermal stages looked the same but shifted at lower temperature. The TGA were also conducted under air. There were no changes with this technique when oxidizing at 5 °C min -1 .

The reason is that nacre has no porosity so the oxygen diffusion inside the material is limited. At 5 °C min -1 under air, oxidation remained limited even for powder and is much similar to pyrolysis. A comparison of the oxidation conditions of our previous work 7 to the present ones also reveals that the 'oxidation' effect is negligible regarding the 'thermal' one.

Fig. 4 Rock-Eval hydrocarbon departure (S2 Hc ) during the pyrolysis: (a) nacre blocks comparison of 3 same samples; (b) insoluble organic matrix (freeze-dry powder of AIM, 4 runs)

The third stage started at 470°C with a large 'swelling' in the TMA plots in both stacking and in-plane directions. This expansion is produced by the allotropic transformation of aragonite to calcite. It was occurring at lower temperature under isothermal oxidation treatment (300 to 400°C, see ref 7). The aragonite/calcite lattice expansion is known in the literature to be +2.8% and occurring in-between 420 and 520°C for natural aragonite [START_REF] Reitner | Origins[END_REF] . Our results are coherent in plane (+2.1%) but much higher in the stacking direction (+21.6%). This lattice expansion is supposedly amplified by a 'puffing' effect due to the cracking of the intracrystalline matrix encapsulating the aragonite nanograins (see Fig. 1). Indeed, Rock-Eval exhibits a peak of hydrocarbon and a peak of organic CO 2 at the same time. As the mechanical resistance of nacre is less in the stacking direction, all the additional expansion occurs in this direction. In the same time, the abiotic aragonite did not show any extra degassing (continuous and very faint) during the allotropic transformation. Also these peaks disappear in the case of decalcified nacre (Figs 4b and5b).

As a summary, third stage can be considered as the 'allotropic transformation of bioaragonite into calcite between 470 up to 550°C'. Comparison of nacre with decalcified nacre showed that most of the organic oxygen functions are still stable at the end of this process.

Fig. 5 Rock-Eval CO 2 pyrolysis plots (S3 and S3'); a) in nacre (3 runs); b) insoluble organic matrix (AIM, 4 runs); c) abiotic aragonite : very weak release (compare the scales, 3 runs).

Finally, a fourth and last stage relates to a strong shrinkage in between 550°C and 940°C (910°C in the plane, Fig. 3) and the main weight loss, 40% as asserted by TGA. Rock-Eval showed in the same time that the organic matrix continues to produce mainly CO 2 and CO molecules up to 650°C. Above, all the mass loss is due to the calcination of calcite. The end of this fourth stage occurred at 940°C. Above, the sample is supposed to be composed of lime, residual calcite and a small fraction of refractive carbon. This phenomena happened at lower temperature under isothermal oxidation treatment : a blend of calcite and lime was observed already at 550°C up to 900°C where a tiny amount of calcite is still visible in the powder 7 .

The mineral CO 2 departure can be calibrated with the abiotic aragonite. It really starts above 630°C with limited amplitudes anyway. It fits with the TMA shrinkage due to the chemical transformation of calcite into lime by departure of CO 2 . The amplitude is meanwhile very limited and one could think that CO 2 released by nacre before 650°C is produced quasi exclusively by the organic fraction.

Discussion -Conclusions

The presence of organic matrix in nacre was seen to change the well known transformation of abiotic aragonite to calcite and then calcite into lime with temperature. In nacre, organic matter is stable up to 250°C. Then cracking of the organic content occurs in two times, 250-550°C (comprising all the hydrocarbons) and then at high temperature 550-650°C (oxygen functions). What is noticeable here, is the very high temperature for which the insoluble organic fraction is still active at producing CO 2 : the highest peak is centred at approximately 650°C. Above 650°C, only a small residue of refractive organic carbon remains. Refractive organic carbon is quantified by a subsequent oxidation heat-treatment by means of Rock-Eval up to 850°C (Fig. 6b) measuring the related CO and CO 2 at low temperature (Fig. 6a) : 20% of the total organic carbon remains as refractive carbon in nacre. Refractive carbon increases up to 50% in the decalcified samples (AIM). Rock Eval provides also the total organic carbon balance (TOC). Decalcified nacre (AIM) gave a TOC value of 45 wt% of the mass of the sample. This value was calculated following the standard IFP method (Institut Francais du Petrole) for isolated organic matter, adding high temperature peaks as S3', S3' CO as well as S5. As expected the abiotic aragonite did not give any TOC. Nacre provided 2 sets of values : nacre powders gave a TOC value of 1.2 %, while blocks gave 0.9%. The organic matrix content of nacre is thereafter 2.7 weight % if measured in the powder and 2.0% in the blocks. The order of magnitude is good since the estimated value by the chemical extraction protocol gives : 2.4%.

The experiments conducted on the powder (TGA or Rock-Eval) have shown that the 4 thermal stages looked the same but shifted at lower temperature. The TGA were also conducted under air. There were no changes with this technique when oxidized at 5°C.mn -1 .

The reason is that nacre has no porosity; so the oxygen diffusion inside the material is limited. At 5°C.mn -1 under air, oxidation remained limited even for powder and is much similar to pyrolysis. Comparing the oxidation conditions of our previous work 7 to the present ones also reveals that the 'oxidation' effect is negligible regarding the 'thermal' one.

Experimental

Densities were obtained on nacre powder with a helium pycnometer (AccuPyc 1330 from Micrometrics) and a mean value of ten successive tests. The Pinctada maxima nacre used in this work was found to have a specific mass of 2.836 + 2.10 -3 g cm -3 by helium picnometry. This value is to be compared with that of geologic crystals which varies between 2.920 and 2.995 g cm -3 or with the calculated value of 2.920 g cm -3 for a reference crystal (JCPDS no. 41-1475). This lower experimental density can be simply related to the fact that nacre is not pure aragonite. The lower density of the organic matrix fraction makes the difference (2.4 wt% as measured by the biochemical extractions or approximately 6% in volume).

Thermogravimetric analyses (TGA) were conducted with a TAG 24 from Setaram with a ramp of 5 °C min -1 on pristine powder and block of nacre under argon (or under air, with no difference).

The thermal behaviour was also characterized by means of thermo-mechanical analysis (TMA) conducted with a Dilatometer Sestys 24 from Setaram, France. The first measure concerns the anisotropy of the reversible expansion of nacre between 20 °C and 230 °C. The ramp used for this purpose is 1 °C min -1 up to 200 °C and back to room temperature at 1 °C min -1 . The second series of tests concerns the thermo-mechanical characterization of nacre in the range of 1000 °C (ramp up to 1000 °C at 5 °C min -1 and back to room temperature at the same speed). For that purpose, two blocks of nacre were cut with a diamond saw (L1 = 4.474 + 0.005 mm and L2 = 3.320 + 0.005 mm). All the runs were performed along the two main directions, say parallel and perpendicular to the anisotropy plane of nacre.

Fig. 1

 1 Fig. 1 Nanostructure of nacre biocrystal : nanograins as seen by AFM and TEM electron diffraction showing a single crystal-like pattern at the same scale (Ref 4).

Fig. 6

 6 Fig. 6 Rock-Eval oxidation CO 2 plots (S5). (a) nacre: mineral CO 2 released by calcination (3 samples); (b) decalcified nacre : organic CO 2 released by oxidation of refractive carbon (4 samples); (c) abiotic aragonite: mineral CO 2 (3 samples).

Acknowledgements

This work was supported by grants from the Agence Nationale de la Recherche Francaise under the awards ANR Blan06-3_135480: Nacre Fluid. D. Keravis is acknowledged for his participation in Rock-Eval analyses.

The Rock-Eval analysis (Vinci Technologies, Rock-Eval 6 Turbo, France) was developed for estimating the petroleum potential of sedimentary rocks (IH, OH and Tmax), the total organic carbon content (TOC), the total mineral carbon content (CMin) by heating samples in an open pyrolysis system under argon. [10][11][12] An analysis takes place in 2 steps. First, during a pyrolysis under nitrogen a desorption of hydrocarbon is maintained at 200 °C for 5 min. Then a cracking occurs between 200 and 650 °C at 30 °C min -1 . Hydrocarbons are monitored by a flame ionization detector (FID), forming the so-called peak S1 (mgHc (g sample ) -1 , 'volatilized hydrocarbons'), S2 (mgHc (g sample ) -1 , 'pyrolyzed hydrocarbons'). At the same time infrared cells detect CO and CO 2 (Rock-Eval gives 2 values: S3 CO (mgCO (g sample ) - 1 ) and S3 CO2 (mgCO 2 (g sample ) -1 ) corresponding to the release of functions linked to organic molecules. At higher temperature it records also the S3' CO which is organic and mineral CO and S3' generally attributed to mineral CO 2 . In the present work, it was evidenced that organic CO 2 can be easily integrated up to 650 °C for nacre. Second, the residue recovered after pyrolysis undergoes a combustion from 400 °C up to 850 °C, under artificial air (N 2 : O 2 , 80 : 20) in a second oven. CO and CO 2 released are monitored on line by means of infrared cells. The Rock-Eval method then gives S4 CO and S4 CO2 (respectively mgCO (g sample ) -1 and mgCO 2 (g sample ) -1 ) of organic origin in this work. Finally, S5 measure the CO 2 provided by the mineral source above 400 °C. The initial weight was measured with a precision over 0.5 mg in order to obtain reliable Rock-Eval parameters (i.e. S2 + 0.5 mg (g rock ) -1 , TOC and C Min + 0.1 wt%).

The organic matrix extracted using the acetic acid protocol was also tested (insoluble fraction: AIM).


Sally Chambers

Piraye Hacigüzeller

Joke Daems

Marco Raciti

Marco Raciti Dariah

DARIAH Beyond Europe: an international exchange of ideas

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.

DESIR has received funding from the EU Horizon 2020 Research and Innovation Programme under Grant Agreement No. 731081 https://www.dariah.eu/activities/projects-and-affiliations/desir/ Chambers, S., Hacigüzeller, P., Daems, J. and Raciti, M. (2018) DARIAH Beyond Europe: an international exchange of ideas -a poster for the DARIAH-EU Annual Event, Paris, France, 2018.

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