A Sivakumar

S S Jude Dhas

Raju Suresh Kumar

Abdulrahman I Almansour

Natarajan Arumugam

Shubhadip Chakraborty

S A Martin

Britto Dhas

S Sahaya

Jude Dhas

Assessment of sustainability on structure-optical properties of prismatic face ADP crystal at dynamic shocked conditions

Keywords: Shock waves, ADP crystal, Prismatic face, Optical transmission, Defects, Structural properties

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

Introduction

In the process of trying to get to the bottom of what is the basis of the observed changes caused by the impact of shock waves on materials, it has been identified that there are a lot of fundamental modifications which enforce drastic characteristic transformations. Much of these experimental works have come about because, in recent times, there have been witnessed a few groundbreaking results that emerge out from research groups owing to their incredible commitment. Space environmental factors can possibly affect the characteristic nature of space materials when they encounter the space condition such as gamma rays, low temperature, shock waves and cosmic rays, etc [START_REF] Amina | Study of gamma-radiation-induced optical effects in Ge-Se-Cd for possible industrial dosimetric applications[END_REF][START_REF] Iannucci1 | High velocity impact and armour design[END_REF][START_REF] Novikov | Radiation Effects on Spacecraft Materials[END_REF][START_REF] Chinke | Surface interactions of transonic shock waves with graphene-like nanoribbons[END_REF]. Optical devices are used in space mission for electronic types of equipment and laser operating systems to determine the efficiency of lifetime of satellites and data communication [START_REF] Alharbi | Gamma Irradiation Effects On The Linear And Nonlinear Optical Properties Of Noncrystalline Sb2S3 Films[END_REF][START_REF] Buchs | Radiation hard mode-locked laser suitable as a spaceborne frequency comb[END_REF]. If the above-mentioned environmental factors could really affect the materials, the optical instruments are prone to undergo critical condition such that the resultant performances of the device tend to decrease resulting in deterioration of their characteristic features. Non-linear optical (NLO) crystals are used as frequency converters and for many other electronic devices such as LIDAR, SONAR, in space, military and defense technological applications [START_REF] Ahlam | Investigation of gamma radiation effect on chemical properties and surface morphology of some nonlinear optical (NLO) single crystals[END_REF][START_REF] Dong | Gamma-ray induced color centers in Yb:YAG crystals grown by Czochralski method[END_REF]. High optical transmission is one of the basic requirements for the applications J o u r n a l P r e -p r o o f of converters because it plays a crucial role in laser frequency converter devices. Hence, the stability of optical transmission has a paramount role in space, military and defense applications.

To overcome the above-mentioned drawbacks and find novel suitable space materials for space applications, shock wave recovery experiment is proven to be one of the key techniques [START_REF] Shamal | Shock wave hydrodynamics of nano-carbons[END_REF][START_REF] Chinke | Graphene-like nanoflakes for shock absorption applications[END_REF].

Because during the shock wave propagation on test material, it can experience a similar situation as that of the space environment exposed on the surface of the test material that can be accomplished in the indoor laboratories itself. The aftermath of shock wave impact can generate a lot of usual and unusual effects on materials. Therefore, experimental investigations on crystalline and non-crystalline materials after the shock wave exposure have been witnessed to be consistently going up such that it is positioned to be a prominent research topic in current years [START_REF] Rita | Structural, optical and magnetic properties of silver oxide (AgO) nanoparticles at shocked conditions[END_REF][START_REF] Sivakumar | Impact of shock waves on vibrational and structural properties of glycine phosphite[END_REF][START_REF] Jayaram | Catalytic Effect of CeO2-Stabilized ZrO2 Ceramics with Strong Shock-Heated Mono-and Di-Atomic Gases[END_REF][START_REF] Sivakumar | Shock Wave Driven Solid State Phase Transformation of Co3O4 to CoO Nanoparticles[END_REF][START_REF] Sivakumar | Phase Transformation of Amorphous to Crystalline of Multiwall Carbon Nanotubes by Shock Waves[END_REF]. At shock wave loaded conditions, micro and macro-level structural defects can be developed on material surfaces. In addition to that, grain boundaries diffusion, re-crystallization and phase transformation can also occur which are highly associated with the nature of material's response to high-pressure and temperature [START_REF] Chen | Effects of copper nanoparticle inclusions on pressure-induced fluid-polynanocrystalline structural transitions in krypton[END_REF][START_REF] Bystrzycki | Static recrystallization of shock-wave (explosively) deformed Fe-40 at% Al-Zr-B intermetallic[END_REF]. It is well authenticated that if crystallographic structural properties are altered, all kinds of dynamic physical properties such as thermal, electrical and optical properties are found to be greatly affected. Hence, research on finding the materials of crystallographic phase stability under harsh environmental conditions has attained the position of imploding a paradigm shift to be termed as one of the top-class research themes in recent years.

Over the years, a lot of NLO materials have been found for high-power laser applications whereas potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals are being regarded as the legendary research materials because of their exceptional J o u r n a l P r e -p r o o f characteristic nature of growth and physical properties [START_REF] Bhagavannarayana | An Interesting Correlation between Crystalline Perfection and Second Harmonic Generation Efficiency on KCl-and Oxalic Acid-Doped ADP Crystals[END_REF][START_REF] Asakuma | A study of growth mechanism of KDP and ADP crystals by means of quantum chemistry[END_REF][START_REF] Balamurugan | Growth and characterization of unidirectional (100) KDP single crystal by Sankaranarayanan-Ramasamy (SR) method[END_REF]. Among the two crystals, KDP's crystal structure [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF] and other physical properties such as surface properties [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF], optical transmission [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF], dielectric constant [START_REF] Sivakumar | Tailoring the dielectric properties of KDP crystals by shock waves for microelectronic and optoelectronic applications[END_REF], thermal diffusivity [START_REF] Sivakumar | Effect of Shock Waves on Thermophysical properties of ADP and KDP crystals Optic[END_REF] and optical band gap energy [START_REF] Sivakumar | Shock wave-induced optical band gap engineering on pure and dye-doped potassium dihydrogen phosphate crystals[END_REF] have been extensively studied by our research group at dynamic shock wave loaded conditions. In the case of ADP crystal, the crystal structure of the pyramidal face and other physical properties have been studied at shocked conditions [START_REF] Sivakumar | Effect of shock waves on structural and dielectric properties of ammonium dihydrogen phosphate crystal[END_REF][START_REF] Sivakumar | Modification of optical properties of ammonium dihydrogen phosphate crystal by employing shock waves[END_REF][START_REF] Joshi | Tailoring the low dielectric constant in glutamic acid doped ammonium dihydrogen phosphate single crystal by virtue of MPa shock waves for microelectronic applications: the complex impedance and modulus formulation studies[END_REF]. Sivakumar et al have demonstrated the significant improvement in the degree of crystalline nature of the pyramidal face of ADP crystal at shocked conditions and ADP's surface properties, dielectric constant [START_REF] Sivakumar | Effect of shock waves on structural and dielectric properties of ammonium dihydrogen phosphate crystal[END_REF],

optical transmission [START_REF] Sivakumar | Modification of optical properties of ammonium dihydrogen phosphate crystal by employing shock waves[END_REF], thermal diffusivity [START_REF] Sivakumar | Effect of Shock Waves on Thermophysical properties of ADP and KDP crystals Optic[END_REF] are found to have enhanced. Joshi et al have conducted the shock wave recovery experiment on Glutamic acid doped ADP crystal and found the reduction of the degree of crystalline nature and dielectric constant at shocked conditions [START_REF] Joshi | Tailoring the low dielectric constant in glutamic acid doped ammonium dihydrogen phosphate single crystal by virtue of MPa shock waves for microelectronic applications: the complex impedance and modulus formulation studies[END_REF]. Furthermore, our research group has examined a few other technologically potential single crystals' optical properties such as benzyl [START_REF] Sivakumar | Shock wave induced enhancement of optical properties of benzyl crystal[END_REF], tri-glycine sulfate [START_REF] Sivakumar | Shock wave-induced defect engineering for investigation on optical properties of triglycine sulfate crystal[END_REF], potassium sulfate [START_REF] Sivakumar | Switchable Phase Transformation (Orthorhombic -Hexagonal) of Potassium Sulfate Single Crystal at Ambient Temperature by Shock Waves[END_REF], copper sulfate pentahydrate [START_REF] Sivakumar | Enhancement of the optical properties of copper sulphate crystal by the influence of shock waves[END_REF] crown ether magnesiuchloride potassium thiocyanate [START_REF] Showrilu | Shock wave-induced defect engineering on structural and optical properties of crown ether magnesium chloride potassium thiocyanate single crystal[END_REF] and found several interesting results. Based on the available literature reports, the prismatic face ADP single crystal's physical properties have not yet been reported at shocked conditions. Therefore, the prismatic ADP crystal is considered for the current investigation on the stability of structural and optical transport properties under the condition of various counts of shock waves exposed in such a way that the observed results could provide the possible key evidence on the applicability of the crystal for space applications.

On those lines, we have analyzed the stability of crystal structure and degree of crystalline nature of the prismatic ADP crystal at shocked conditions by X-ray diffraction and

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Raman spectroscopic techniques. Furthermore, the optical transmittance profile has been analyzed by UV-Vis spectrometer and the stability profiles have been established with respect to the number of shock pulses.

Crystal growth and Shock wave loading methodology

The crystal growth details of ADP crystal have been discussed in our previous publications [START_REF] Sivakumar | Effect of shock waves on structural and dielectric properties of ammonium dihydrogen phosphate crystal[END_REF][START_REF] Sivakumar | Modification of optical properties of ammonium dihydrogen phosphate crystal by employing shock waves[END_REF]. For the present experiment, cut and polished prismatic ADP crystals of 1mm thickness have been used. A pressure-driven tabletop Reddy tube, which was indigenously designed in our laboratory, has been made use of to generate the shock waves and the details of the shock tube has been published elsewhere [START_REF] Sivakumar | Measurement of "Shock Wave Parameters" in a Novel Table-Top Shock Tube Using Microphones[END_REF]. In the present experiment, shock wave of Mach number 1.7 equivalent to the transient pressure of 1.0 MPa has been utilized for the assessment of the stability of crystal structure and optical properties of the prismatic ADP crystal. Five identical crystals have been loaded with shock pulses of 0,1,2,3 and 4, respectively.

After shock wave loading, the respective crystals have been subjected to the studies of stability of crystal structure and optical transmission.

Results and Discussion

X-ray diffraction analysis

In the present experiment, Powder X-ray diffractometry (Rigaku -SmartLab X-ray Diffractometer, Japan) has been utilized to analyze the structural properties of the prismatic ADP crystal at shocked conditions. CuKα as the X-ray source (λ = 1.5407 Å) of radiation and 1D detector have been utilized. Before loading the shock waves on the test sample, the control test sample's XRD pattern has been recorded and the obtained XRD pattern is presented in Fig. 1a. impact of shock waves on the structural properties of the prismatic ADP crystal. As the test sample is a single crystal, the intensity of the recorded XRD pattern must be taken into the account in order to get a clear picture on the impacted changes imposed on the crystal in terms of its structure and degree of crystallinity because of shock waves. As seen in Fig. 1b, the shocked sample shows the retention of (200) crystalline plane and there is no new diffraction peak observed. Hence, at the first screening of the observed XRD pattern, it is obvious that the tetragonal crystal structure has not undergone any crystallographic phase transitions at shocked conditions. KDP family of crystals undergo crystallographic phase transitions at high-pressure and high-temperature conditions [START_REF] Takanorifukami | Crystal Structure and Transitions for Monoclinic KH2PO4 Crystal[END_REF][START_REF] Ren | Ambient-condition growth of highpressure phase centrosymmetric crystalline KDP microstructures for optical second harmonic generation[END_REF]. Takanori Fukami et al have demonstrated the crystallographic phase transition from the tetragonal I-42d to the monoclinic P21c at about 453 K [START_REF] Takanorifukami | Crystal Structure and Transitions for Monoclinic KH2PO4 Crystal[END_REF]. Yan Ren et al have found the monoclinic KDP crystal at1.9 GPa [START_REF] Ren | Ambient-condition growth of highpressure phase centrosymmetric crystalline KDP microstructures for optical second harmonic generation[END_REF]. In addition to that, it is worthwhile to note that potassium sulfate crystal undergoes the changes from the orthorhombic to the hexagonal crystal phase transitions at the first shocked conditions and retain the orthorhombic structure at the 2 nd shocked conditions [START_REF] Sivakumar | Switchable Phase Transformation (Orthorhombic -Hexagonal) of Potassium Sulfate Single Crystal at Ambient Temperature by Shock Waves[END_REF]. But, in the present case, the substantial reduction of diffraction peak intensity is noticed which may be due to the formation of the lattice defects, changes in bonding schemes, misalignment of atomic lattice planes and deformations impacted by the shocks which have been also obtained quite similarly for the prismatic face of KDP crystal in such conditions [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF]. Therefore, substantiation could be arrived at for the stability of the tetragonal crystal structure at post-shock conditions, but the degree of crystallinity is unstable at shocked conditions. Apart from the function of the crystal structure, the degree of crystalline nature also has the significant contribution in application point of view since most of the crystalline properties such as transmittance; thermal conductivity and dielectric constant are significantly altered with respect to the degree of crystalline nature [START_REF] Sivakumar | Effect of Shock Waves on Thermophysical properties of ADP and KDP crystals Optic[END_REF]. Followed J o u r n a l P r e -p r o o f by the X-ray diffraction intensity changes, the diffraction angle position also shows a considerable change at shocked conditions. Initially, the most intense diffraction peak (200) is located at 24.461° and at the 4 th shocked condition; it experiences a quite higher angle diffraction shift that is located at 24.596°. The observed diffraction angle shift of the (200) plane may be due to the compression of hydrogen bonds such as N-H•••O and O-H•••O as well as crystal unit volume by the impact of shock waves [START_REF] Asakuma | A study of growth mechanism of KDP and ADP crystals by means of quantum chemistry[END_REF][START_REF] Sivakumar | Effect of shock waves on structural and dielectric properties of ammonium dihydrogen phosphate crystal[END_REF]. In addition to that, the title crystal belongs to the hydrogen bond family of crystals and the hydrogen bonds have high susceptible nature at hightemperature and high-pressure conditions [START_REF] Subramony | Reevaluation of Raman Spectra for KH2PO4 High-Temperature Phases[END_REF][START_REF] Cai | Structure of the high-pressure phase IV of KH2PO4 J o u r n a l P r e -p r o o f (KDP)[END_REF]. As per the crystal structure of the prismatic face ADP crystal, N-H•••O is connected to (NH4) 4 (PO4) 3 and O-H•••O lies between (PO4) 3 groups and the corresponding crystal packing along with (200) plane is presented in Fig. 2.

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Fig.2 Crystal packing of ADP crystal along the (200) plane

As reflected in Fig. 2, the prismatic face of ADP crystal comprises of alternative array of anions and cations contributing to the growth habit of the crystal as against the pyramidal face J o u r n a l P r e -p r o o f that has two hydrogen bonds which are highly susceptible at high-pressure and high-temperature as well as at dynamic shocked conditions. In addition to that, the total bond strength of the prismatic face ADP crystal (0.7282) is significantly low compared to the pyramidal face ADP crystal (1.6341) [START_REF] Xu | Morphology and structure studies of KDP and ADP crystallites in the water and ethanol solutions[END_REF]. So that, it may not sustain the original crystalline state with specific atomic positions at shocked conditions and as a result it may produce the displacement of anions and cations including bond length changes from the original positions which may enable the loss of crystal symmetry and enhance the deformation levels in the test sample. Due to such changes, the net degree of crystalline nature of the prismatic plane in the crystal has faced a remarkable influence in the physical properties of the crystal.

Raman Spectroscopic analysis

Raman spectroscopic study has been performed to get deep in-sights on the structural properties of the test sample and the obtained Raman spectra of the control and shocked samples are presented in Fig. 3. As it is known, ADP crystal is built by (NH4) + and (PO4) 3- In addition to that, the symmetric Raman band of PO4 at 339cm -1 shows the similar results as that of the asymmetric PO4 at shocked conditions. Hence it is clear that, at shocked conditions, structural distortions have occurred in ADP crystal due to the instability of the tetragonal crystal structure [START_REF] Sivakumar | Phase stability analysis of shocked ammonium dihydrogen phosphate by X-ray and Raman scattering studies[END_REF]. In addition to that, the lattice Raman modes must be analyzed for a better elucidation of the actual crystal structure of ADP crystal and hence, the lattice Raman modes of the control and shocked crystals are presented in Fig. 5. As seen in Fig. 5, the control crystal has well-defined three lattice Raman modes at 70 cm -1 , 126 cm -1 , and 179cm -1 , respectively and the lattice Raman band positions are well corroborated with the results of the previous publications [START_REF] Joshi | electrical studies of pure and L-serine doped ammonium dihydrogen phosphate single crystals: an understanding of defect chemistry in hydrogen bonding[END_REF][START_REF] Zhou | Raman spectral characterization of NH4H2PO4 single crystals: Effect of pH on microstructure[END_REF] such that the Raman modes corresponding to the lattice vibrations of the crystal are due to the absorption or emission of optical phonons. At shocked conditions, the initial signatures such as lattice Raman mode pattern and intensities are destroyed which are the non-debatable proofs for the formation of the distorted tetragonal crystal structure at shocked conditions. On the other hand, there is neither crystallographic phase transition nor mixed phase formation has occurred at shocked conditions.

UV-Visible spectral studies

For non-linear optical materials, wide optical transmittance window and high optical transmittance are the major requirements for the industrial applications and hence the crystal growers have taken significant effort to grow crystals of high optical transmittance. But, while suggesting a particular functional material for industrial applications, the sustainability of the optical properties should also be measured at high-pressure and high-temperature environments for a better understanding of the scale of workability of the material. If the optical transmittance window and optical transmittance percentage are changed by the external parameters such as high-pressure and high-temperature or by sudden dynamic impact, the output of the electronic devices will vary significantly. In the case of the materials of second harmonic generation, the efficiency of the harmonic generation is significantly altered as and when the surface defects and structural defects are induced by shock waves so that we have measured the stability of the optical transmittance at shocked conditions and corresponding optical transmittance profiles are presented in Fig. 6 wherein the observed pattern of optical transmittance is found to be well-J o u r n a l P r e -p r o o f 14 matched with the experimental data of the previous publications [START_REF] Sivakumar | Modification of optical properties of ammonium dihydrogen phosphate crystal by employing shock waves[END_REF]. As seen in Fig. 6, the control sample has 36 % optical transmittance at 500 nm. Fig. 6. The profiles of optical transmittance of the control and shocked prismatic face of ADP crystals At shocked conditions, the optical transmittance has linearly reduced with respect to the number of shock pulses and the corresponding plot is portrayed in Fig. 7. The downward trending of the optical transmittance of the test crystal may be due to the formation of lattice deformations, distortion of crystal structures and surface defects. Similar results have been observed for potassium dihydrogen phosphate [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF], tri-glycine sulfate [START_REF] Sivakumar | Shock wave-induced defect engineering for investigation on optical properties of triglycine sulfate crystal[END_REF] at shocked conditions. For a better understanding of the optical properties of ADP crystal in terms of its J o u r n a l P r e -p r o o f sustainability with respect to the faces, we have made a systematic comparison of the percentage of optical transmittance for the prismatic and the pyramidal faces at shocked conditions and the corresponding plots are depicted in Fig. 7a and7b.

As seen in Fig. 7a, optical transmittance of the (200) plane for the crystal has reduced with respect to the number of shock pulse whereas optical transmittance of the (101) plane has increased which has been previously reported [START_REF] Sivakumar | Modification of optical properties of ammonium dihydrogen phosphate crystal by employing shock waves[END_REF]. The prismatic face of ADP and KDP [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF] crystals' optical transmittance stability profiles are presented in Fig. 7b. As seen in Fig. 7b, both the crystals follow the downward trending percentage of optical transmittance with respect to the number of shock pulses. But KDP crystal shows very fine changes in the optical transmittance while compared to the prismatic face of ADP crystal. For example, the face of the KDP crystal shows the reduction of optical transmittance from 58% to 55.9% at the 4 th shocked condition.

Whereas the prismatic face of ADP crystal shows the reduction of optical transmittance from 36% to 23.9% at the 4 th shocked condition and hence, we consider that the prismatic face of KDP crystal has little higher shock resistance than that of the prismatic ADP crystal.

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Optical Microscopic Images

Furthermore, we have obtained the optical microscopic images to get the possible insights about the surface morphological changes of the prismatic ADP crystal at shocked conditions. As seen in Fig. 8a, the control crystal's surface has a flat and smooth morphology.

But at the 4 th shocked condition, the test crystal's surface shows significant changes in the form of surface defects and the corresponding micrograph is presented in Fig. 8b. The formation of surface defects may also have the equal contribution for the reduction of optical transmittance due to scattering that is followed by the loss of degree of crystalline nature and such surface defects formation and loss of optical transmittance have been witnessed for potassium dihydrogen phosphate and Tri-glycine sulfate crystals at shocked conditions [START_REF] Sivakumar | Shock Wave Induced Defect Engineering on Structural and Optical Properties of Pure and dye doped Potassium Dihydrogen Phosphate Crystals[END_REF][START_REF] Sivakumar | Shock wave-induced defect engineering for investigation on optical properties of triglycine sulfate crystal[END_REF]. show the formation of the distorted tetragonal crystal structure at shocked conditions due to the noteworthy reduction of intensity of the diffraction peaks and complete modification of the lattice Raman modes. The analysis retrieved from UV-Visible spectrometer clearly discloses the optical linearly that decreases in terms of percentage in accordance with the number of shocks because of the formation of defects in the crystal structure and hence the prismatic face ADP crystal might not be a potential material for high-temperature and high-pressure environmental applications. In addition to that, optical microscopic images also provide the possible evidence for the formation of surface defects at shocked conditions. It could be noted that the prismatic face KDP crystal has high shock resistance than that of the prismatic face ADP crystal so that the prismatic face KDP crystal can be used for high-temperature applications.

  corresponding diffraction position belongs to (200) crystallographic plane in the tetragonal crystal structure and the diffraction angle position is well-matched with JCPDS -85-0815. It is noteworthy to mention that the crystal has only one diffraction plane with the specific orientation (200) and no other considerable diffraction planes have been observed.

Fig. 1

 1 Fig.1 XRD pattern of the control and shocked prismatic face of ADP crystals

  Fig.3 Raman spectra of the control and shocked prismatic face of ADP crystal

Fig. 5

 5 Fig.5 Lattice Raman modes of the control and the prismatic face of ADP crystal

Fig. 7 .

 7 Fig.7. The profiles of optical transmittance of the control and shocked crystals (a) (200) and

Fig. 8

 8 Fig.8 Optical micrographs of the prismatic ADP crystal (a) the control (b) the 4 th shocked ADP

Acknowledgements

The authors thank Department of Science and Technology (DST), India for DST-FIST progarmme (SR/FST/College-2017/130 (c)). The project was supported by Researchers Supporting Project number (RSP-2021/231), King Saud University, Riyadh, Saudi Arabia.

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Conflicts of interest

The authors declare that they have no conflict of interest.

Conflict of interest

The authors declare that they have no conflict of interest.

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Ildar Farkhatdinov
email: i.farkhatdinov@qmul.ac.uk

Arnaud Garnier

Tomoki Arichi

Hannes Bleuler

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

Evaluation of a Portable fMRI Compatible Robotic Wrist Interface

This paper presents evaluation of a portable fMRI compatible haptic interface to study the brain correlates of sensorimotor control during wrist motion. The interface is actuated by a shielded DC motor located more than 2 m away from the 3T MR scanner's bore. The achievable wrist torque of the interface is up to 2 Nm, and the interface provides sufficient bandwidth for human motor control experiments. Ergonomic and fMRI compatibility testing with a 3T MR scanner showed that the interface is MR safe, compatible with a strong static magnetic field and radio frequency emission, and its operation does not affect the quality of the acquired images.

Clinical relevance-We present and evaluate an fMRI compatible robotic interface to study human wrist joint motor function.

I. INTRODUCTION

Haptic interfaces that could be used in conjunction with functional magnetic resonance imaging (fMRI) enable neuroscientists and clinicians to investigate the brain mechanisms involved in performing tasks with arbitrary dynamics [START_REF] Gassert | Opportunities and challenges in mr-compatible robotics[END_REF]. The application of novel materials and techniques, as well as advances of MR technology, have enabled deployment of mechatronic systems in MR environments [START_REF] Gassert | Actuation methods for applications in MR environments[END_REF], [START_REF] Gassert | Sensors for Applications in Magnetic Resonance Environments[END_REF], [START_REF] Hartwig | Systematic review of fmri compatible devices: design and testing criteria[END_REF], [START_REF] Fleury | A survey on the use of haptic feedback for brain-computer interfaces and neurofeedback[END_REF].

fMRI compatible haptic interfaces with different actuation principles and design configuration for human motor control experiments were introduced mostly with upper limbs movements. Investigating the control of movements with several degrees of freedom (DoF) can provide important information about how the nervous system coordinates movements involving multiple joints and deals with coupled and nonlinear dynamics [START_REF] Shadmehr | Adaptive representation of dynamics during learning of a motor task[END_REF], [START_REF] Burdet | The central nervous system stabilizes unstable dynamics by learning optimal impedance[END_REF]. However, dynamic interactions between limb segments often cause head movements and thus result in motion artifacts on brain MR images [START_REF] Hollerbach | Dynamic interactions between limb segments during planar arm movement[END_REF], [START_REF] Lemmin | Model-based attenuation of movement artifacts in fMRI[END_REF], [START_REF] Kühne | An mr-compatible haptic interface with seven degrees of freedom[END_REF]. Furthermore, the analysis of movements and muscle activity becomes significantly more complex with every additional DoF. This suggests investigating multi-joint movements only when the targeted neural processes require them [START_REF] Erwin | Kinesthetic feedback during 2dof wrist movements via a novel mr-compatible robot[END_REF], [START_REF] Menon | A novel haptic fmri interface for five-axis force and motion neuroimaging experiments[END_REF].

While brain imaging is one of the very few non-invasive windows available to observe the neural processes of sensorimotor control across the whole brain, it yields a noisy signal. Traditionally, due to safety and costs restrictions in the technology, brain imaging is better suited for answering relatively simple questions that can in most cases be equally well addressed on one joint rather than on multiple joint movements. For instance, using a wrist flexion/extension interface [START_REF] Gassert | MRI/fMRIcompatible robotic system with force feedback for interaction with human motion[END_REF] in conjunction with fMRI and muscle electromyography (EMG), allowed us to investigate the neural basis of force vs. impedance control [START_REF] Haruno | Differential neural correlates of reciprocal activation and cocontraction control in dorsal and ventral premotor cortices[END_REF]. A simple tiny wrist flexion/extension 1-DoF interface for neonates [START_REF] Allievi | An fMRI compatible wrist robotic interface to study brain development in neonates[END_REF] enabled us to describe the evolution of sensorimotor activity in the brain of preterm infants, from their birth to term corrected age [START_REF] Allievi | Maturation of Sensori-Motor Functional Responses in the Preterm Brain[END_REF].

These successful experiences motivated us to develop a portable MR compatible haptic interface to wrist flexion/extension movements, that can be used to study human motor control by combining haptic interaction, EMG and MRI. These movements are very suitable for MR imaging studies, as they can be carried out easily by subjects placed in the bore of the scanner, and are likely to generate little head motion. Furthermore, the motor commands for these movements can be investigated precisely using EMG by recording the activity of one group of antagonist muscles, e.g. flexor carpi radialis and extensor carpi radialis longus [START_REF] Melendez-Calderon | Hi5: A versatile dual-wrist device to study human-human interaction and bimanual control[END_REF]. Our interface should be compatible with magnetic fields of up to 3T (both from a safety and image artifact perspective), and be adaptable to scanners with different geometries. The overall system should be easily transportable and easily removable if any interruption of the experiment is required.

In this paper, we evaluate with a human-participant a portable MRI compatible wrist interface capable of rendering high dynamic programmable torque output. The design of the interface was previously presented in [START_REF] Farkhatdinov | Development and evaluation of a portable MR compatible haptic interface for human motor control[END_REF]. The overall view of the developed interface is shown in Fig. 1A-C. The portable wrist interface is installed on the left side of the scanner examination table, so that a patient can be easily placed or removed from the scanner in case of emergency. Fig. 1D demonstrates the portability of the system.

II. DESIGN OVERVIEW

Design requirements. The following key design, ergonomics and safety factors for the fMRI compatible robotic interfaces were considered: portability, easiness of installation, user safety and possibility of fast evacuation, control capabilities.

Portability and easiness of installation are particularly important for haptic systems to be used in fMRI environments, as good portability characteristics reduce the time required to transport and install the devices and therefore the cost for expensive and often limited imaging services decreases significantly. However, not many previous studies Fig. 1. Overview of the fMRI compatible haptic wrist interface design. A, a user during wrist interaction experiment in MR room using the designed interface (CAD); B, side view of the interface and its frame (CAD); C, a user during interface ergonomic tests; D, a photo of the disassembled interface and its travel trolley. The interface can be installed/uninstalled within 15 minutes; its total weight is 15 kg. The interface can be transported by one person and its installation requires two people.

describe the portability of the presented interfaces. Some existing systems involve big frames which makes them nonportable [START_REF] Moser | An MR compatible robot technology[END_REF], [START_REF] Gassert | MRI/fMRIcompatible robotic system with force feedback for interaction with human motion[END_REF], [START_REF] Li | Design of a new fMRI compatible haptic interface[END_REF], [START_REF] Menon | Haptic fMRI: combining functional neuroimaging with haptics for studying the brain's motor control representation[END_REF]. Other interfaces require rigid attachment to the scanner's bed which is not desirable for portability [START_REF] Flueckiger | fMRI compatible haptic interface actuated with traveling wave ultrasonic motor[END_REF], [START_REF] Ergin | Design of a new MR-compatible haptic interface with six actuated degrees of freedom[END_REF], [START_REF] Sergi | Interaction Control Capabilities of an MR-Compatible Compliant Actuator for Wrist Sensorimotor Protocols During fMRI[END_REF], [START_REF] Erwin | Development, control, and MRI-compatibility of the MR-SoftWrist[END_REF]. Many of these devices use aluminium frames which (1) may heat due to electric current induced by the strong magnetic field and (2) require time and effort to assemble and move the devices in the scanning room [START_REF] Li | Design of a new fMRI compatible haptic interface[END_REF], [START_REF] Hribar | Phantom haptic device upgrade for use in fMRI[END_REF]. The same limitations apply to interfaces with cable transmission which require special adjustments during installation in the scanning room [START_REF] Chapuis | Investigation of a cable transmission for the actuation of MR compatible haptic interfaces[END_REF], [START_REF] Vigaru | Design and evaluation of a cable-driven fmri-compatible haptic interface to investigate precision grip control[END_REF]. A critical safety factor related to the design is the possibility to perform safe and fast patient evacuation from the scanner in emergency cases which is not possible for interfaces with big frames [START_REF] Moser | An MR compatible robot technology[END_REF], [START_REF] Gassert | MRI/fMRIcompatible robotic system with force feedback for interaction with human motion[END_REF], [START_REF] Li | Design of a new fMRI compatible haptic interface[END_REF], [START_REF] Hribar | Phantom haptic device upgrade for use in fMRI[END_REF], [START_REF] Vigaru | Design and evaluation of a cable-driven fmri-compatible haptic interface to investigate precision grip control[END_REF].

Haptic interfaces to study human movements should have sufficient dynamic performance such as high bandwidth, low friction, and a simple to tune and robust controller. However, fMRI compatibility requirements generally limit the control performance. For instance, some interfaces are capable of applying forces only in one direction [START_REF] Khanicheh | fMRI-compatible rehabilitation hand device[END_REF], [START_REF] Di Diodato | A haptic force feedback device for virtual reality-fMRI experiments[END_REF], [START_REF] Khanicheh | MR_CHIROD v.2: magnetic resonance compatible smart hand rehabilitation device for brain imaging[END_REF] or the force levels are too low when compared to human force capabilities [START_REF] Hara | Design and compatibility of a high-performance actuation system for fMRI-based neuroscience studies[END_REF]. Depending on the type of actuation the control quality and requirements can create additional application limitations. Using ultrasonic motors can yield compact interfaces, but such motors can hardly be used for slow movements and high holding torques limiting haptic transparency [START_REF] Flueckiger | fMRI compatible haptic interface actuated with traveling wave ultrasonic motor[END_REF]. Interfaces with electro-rheological fluid actuators require high voltage power sources [START_REF] Khanicheh | MR_CHIROD v.2: magnetic resonance compatible smart hand rehabilitation device for brain imaging[END_REF]. Devices with ultrasonic motors are not efficient for low velocity movements [START_REF] Sergi | Interaction Control Capabilities of an MR-Compatible Compliant Actuator for Wrist Sensorimotor Protocols During fMRI[END_REF], [START_REF] Erwin | Development, control, and MRI-compatibility of the MR-SoftWrist[END_REF], while interfaces with pneumatic actuation [START_REF] Yu | An fMRI compatible haptic interface with pneumatic actuation[END_REF], [START_REF] Allievi | An fMRI compatible wrist robotic interface to study brain development in neonates[END_REF], [START_REF] Sharini | Novel fmri-compatible wrist robotic device for brain activation assessment during rehabilitation exercise[END_REF] are capable of reproducing only simple movement patterns, characterised by slow response time and require a compressed air source. While many of the described systems are potentially portable, few were designed to be installed fast and easily. Few of existing systems use conventional electromagnetic actuation enabling flexible control for larger range of velocities and frequencies [START_REF] Hara | Design and compatibility of a high-performance actuation system for fMRI-based neuroscience studies[END_REF]. Furthermore, few of the existing devices are designed for one DoF movements. However, the ability to draw clear conclusions from human motor studies with fMRI is at odds with the complexity of the involved movements: complex movements will be more difficult to investigate and correlate muscle to brain activity, and are prone to creating motion artifacts [START_REF] Hribar | Phantom haptic device upgrade for use in fMRI[END_REF], [START_REF] Menon | Haptic fMRI: combining functional neuroimaging with haptics for studying the brain's motor control representation[END_REF], [START_REF] Ergin | Design of a new MR-compatible haptic interface with six actuated degrees of freedom[END_REF]. To overcome these potential shortfalls we developed a portable fMRI compatible wrist interface which can be used for a wide range of human motor control studies.

Design overview. The requirements described above were considered for the design of the fMRI compatible wrist interface is shown in Fig. 1. The frame, designed to support the actuation mechanism, is made adjustable, easily removable (is positioned on one side of the bed only). The components of the frame and transmission do not contain any ferromagnetic material to prevent any projectile risk with the main static magnetic field. The design enables fast assembly, disassembly and easy transportation. The actuation system consists of a DC motor with an optical encoder, a 2.1 m long cable transmission, and a a handle unit. Detailed description of the design can be found in [START_REF] Farkhatdinov | Development and evaluation of a portable MR compatible haptic interface for human motor control[END_REF].

The ergonomic design of the wrist handle unit allows comfortable, adjustable and safe wrist positioning and support facilitating natural and painless movement. Operation of the interfaces should not cause any significant movements of other body parts, which is critical for brain imaging studies [START_REF] Li | Coupling between wrist flexion-extension and radial-ulnar deviation[END_REF]. To fulfil these ergonomic requirements an option to adjust inclination of the handle's rotational axis in the vertical plane was added (Fig. 1C).

The interface's system identification tests showed that the haptic interface provides sufficient dynamic bandwidth for human neuro-motor control experiments [START_REF] Farkhatdinov | Development and evaluation of a portable MR compatible haptic interface for human motor control[END_REF]. The closedloop position control bandwidth was 0-3 Hz and the bandwidth for torque control was 0-30 Hz.

III. EVALUATING ERGONOMICS

To evaluate the ergonomics of the designed interface we asked five subjects to use the interface for 10 minutes of pre-programmed various motor control tests such as tracking position control, interaction with virtual wall, etc. The Fig. 2. Experimental results of movement artifacts for five subjects S 1 -S 5 for index (f 1 ) and middle (f 2 ) fingers, wrist joint (w), elbow (e) and head (h). The displacements of each body part were expressed as root mean square deviations of the measured positions during using with the interface. The standard deviation of head movements was less than 0.6 mm. interface was tested in two configurations: arm support with inclination (as shown in Fig. 1B) and flat arm support (handle is leveled with the main horizontal frame). All subjects preferred the forearm support with adjusted inclination. No discomfort or pain was reported. All tests were conducted in respect with the institutional ethics regulations (reference 12/LO/1247) and involved healthy right handed subjects who were members of our research group.

The validity of motor control studies with fMRI critically depends on avoiding motion artifacts. First, any head movements should be avoided or minimised. Second, to analyse brain activation specifically related to the movement of interest, i.e. wrist flexion/extension, movements in other parts of the body should be negligible.

To evaluate these two factors, we used a visual motion tracking system (VICON Motion Tracking System, VICON, Oxford, UK) acquiring data at 100 Hz to measure movement of the five subjects' arm and head when interacting with the wrist interface. Five visual markers were placed on the right hand of a subject at the wrist joint (near the distal end of the radius bone), elbow joint (near the medial epicondyle of the humerus bone), head (centre of the frontal bone), and index and middle fingers (between distal and middle phalanges). One more marker was attached to the handle of the interface to measure the actual handle motion. The hand of each subject was fixed to the handle with the help of adjustable straps.

In the test the motor generated 1 Hz periodic torque flexing and extending the handle and the subject's wrist with a magnitude of approximately 40 • , for about 2 minutes. The three dimensional position of each marker was recorded and root mean square deviation values were calculated. The results are shown in Fig. 2. For the calculation of the standard deviations for the index and middle fingers the position measurements of the finger markers were subtracted from the handle marker position measurements to obtain the displacements relative to the handle motion, which should be zero in the ideal case when the hand motion follows the handle's motion perfectly. As reported, for most of the subjects the relative finger movements with respect to the handle were about 6 mm, for the wrist joint the movement was about 4 mm, for the elbow joint -less than 2 mm, which are much smaller than the actual handle motion that exceeded 90 mm. The head motion for all subjects did not In summary, the possibility to adjust the interface configuration for more comfortable hand positioning in the arm support provides sufficient user-interface interaction quality for the subjects, and makes the interface effective for fMRI motor control studies.

IV. MRI EVALUATION RF compatibility. Prior to the tests at the MR imaging facility we checked whether the built Faraday cage was filtering out the RF so that that RF emission from the scanner did not influence the interface. For this test we used radio FM receiver set to receive signals at 105 MHz outside and inside the Faraday cage. Received signals were recorded for both cases and their plots are shown in Fig. 3. Clearly the RF emission was not detected when the receiver was inside the cage.

MR compatibility. Compatibility tests were carried out with a Philips Achieva 3.0 T TX scanner (Best, Netherlands) located at St Thomas hospital in London, UK. First we performed MR compatibility tests to verify that the interface is safe to be used inside the magnetic environment and does not deteriorate the image.

In the first recording during MR compatibility tests the motor was controlled by a micro-controller located inside the Faraday cage. Current steps of ±2 A equivalent to 1.2 Nm were applied to the motor. During the test 24 slices of data were acquired using a gradient-echo EPI (echo planar imaging) sequence with an in-plane acquisition matrix of 256×256 and a cylindrical water-filled phantom, under two conditions: interface running and interface OFF. No distortion of the images was observed linked to either field inhomogeneity or speckles/strips image artifacts related to RF interference. The signal-to-noise ratio (SNR) for the two recordings was 28.3 dB (interface OFF) and 28.6 dB (interface ON). Statistical comparison of the images did not detect any significant differences.

Then, an MR compatibility test was performed with a healthy subject (male, right handed, age 28). T 2-weighted images were acquired using a turbo spin-echo (TSE) sequence, with the haptic interface controlled from outside the scanner room using the optical fiber USB cable and specially designed software. Examples of the acquired images (at z=26) for the cases when the wrist interface was ON (active movements of the right wrist) and OFF (no movement) are presented in Fig. 4A. Visual inspection of the images did not show any differences. The difference between the images is shown on the right panel of the Fig. 4A. The difference image was obtained by subtracting the corresponding pixel intensities from the anatomical brain image when the interface was OFF and from the cases when the interface was activated. The normalized mean pixel intensity of the difference image was 0.012 (where 0 corresponds to black and 1 to white pixels) while the original brain images' intensity was about 0.14 or more than ten times larger. fMRI test. To complete the evaluation, an fMRI compatibility test was performed to verify that expected brain responses are adequate for active condition (subject voluntary wrist movements while the interface if OFF) and for passive condition (subject's wrist movements controlled by the running interface). In the passive condition the controller of the DC motor was used in the torque control mode to track the reference 1 Hz periodic sinusoidal torque, as it was used in the ergonomics validation test.

The tests were performed in the same environment as the MR compatibility test with the same subject. The fMRI data was acquired using a gradient-echo EPI (GRE-EPI) sequence with parameters: TR: 1500 ms; TE: 30 ms; FA: 90 • ; resolution (x×y×z): 3.5×3.5×5 mm; 22 slices; SENSE factor: 2. Images were acquired during alternating periods of 30 s rest and 30 s right wrist extension/flexion. The total recording time was 300 s. Each state was repeated five times, as a result 100 data points were acquired for each of the conditions (rest and movement). In the passive condition (the interface was ON), the handle pushed the wrist to flex and extend and the subject was instructed to follow the handle's movements. We verified that in the passive condition the subject did not cause significant unintentional resistance torques to the handle which could cause differences with respect to the brain images acquired in the active condition. 1The General Linear Model (GLM) as implemented in FEAT (fMRI Expert Analysis Tool v5.0.8, FSL image processing package) was used to carry out fMRI analysis [START_REF] Smith | Advances in functional and structural MR image analysis and implementation as FSL[END_REF]. Data were preprocessed using standard steps comprising motion correction, slice-timing correction, non-brain tissue removal, spatial smoothing (Gaussian kernel 5 mm FWHM), global intensity normalization, and high-pass temporal filtering (cut-off 60 s). The results of the fMRI analysis are presented in Fig. 4B. The brain images on the left show the identified activation rendered as red-yellow gradient (z=65 and z=15) representing the fMRI data when the interface was OFF. The remaining images on the right of Fig. 4B show the identified patterns of activation in blue gradient when the interface was ON. In both experimental conditions, characteristic and significant clusters of brain activity were identified in the contralateral (left) peri-rolandic region (comprising the primary motor and somatosensory cortices), the midline supplementary motor area, and ipsilateral perirolandic region and cerebellum. Fig. 4C presents the time series measurements of Blood oxygen level (BOLD) dependent signal in the test when the wrist interface was activated for 30 s for five times. The wrist movements are associated with significant increases in the measured BOLD signal in the brain area marked with a red arrow on the right panel of Fig. 4C.

Statistical analysis of fMRI data and visual inspection of the images showed that in both cases activation was nearly identical and that usage of the wrist interface did not alter or adversely affect the ability of fMRI to identify significant clusters of brain activity.

V. CONCLUSION

The presented fMRI-compatbile robotic wrist interface is easy to transport and set up (weight ≤15 kg, length ≤90 cm, ≤15 minutes installation time), and can be adapted to different scanners with magnetic fields up to 3T. It can be used to provide a high quality haptic environment at 200 Hz with simultaneous recording of muscle and brain activity. An fMRI evaluation with a participant demonstrated that the device provides a comfortable interaction with a bandwidth over the capabilities of wrist movements, does not cause image artifacts, and can be safely used to induce robust patterns of brain activity relating to sensorimotor processing.

Fig. 3 .

 3 Fig. 3. Time history of the recorded radio signal outside and inside the Faraday cage. FM radio signal at 105 MHz was used as a source, and signal obtained after frequency demodulation (sound signal) was recorded. The signals were normalised with respect to the maximal magnitude.

Fig. 4 .

 4 Fig.4. A, anatomical T2 scans of the brain (at z=26) for the cases when the interface was OFF (left), ON (middle) and the image difference (right). Normalized image intensity I is reported for each scan. B, recorded brain activity (at z=65 and z=15) for the case when the human subject moved the wrist and the interface was OFF (left) and for the case when wrist was moved together with running haptic interface (right). C, the sampled Blood oxygen level (BOLD) dependent signal (red) can closely fit the Hemodynamic response function model (black).

Based on the error between the recorded reference and measured DC motor currents during the test, we calculated the root mean square of the resistance torque. The mean absolute resistance torque at the handle in the active condition was about

mNm, which is of the same order as the mean levels of identified interface friction.

ACKNOWLEDGMENT

This project was supported by the EU FP7 grant BAL-ANCE (ICT-601003) and EU H2020 COGIMON (ICT-644727), and in-part by Royal Society grant IEC\R2\202089 and EPSRC grant EP/T027746/1.