Soft Robotics for Space Applications: Cryogenic Performance of Modular Metallic Cable Structures

Abstract

Soft robotic systems are promising for diverse space applications due to their embedded compliance, promising locomotion methods, and efficient use of mass and volume. Space environments are harsher and more varied than those on Earth; extreme temperature, pressure, and radiation may impact the performance and robustness of soft robots. Cryogenic temperatures on celestial bodies such as the Moon or Europa pose significant challenges to the flexibility and actuation performance of conventional soft systems. We present a soft robotic design methodology using novel metallic-based soft robotic structures specifically tailored to extreme space environments. Structures are presented as tunable, reconfigurable modules for soft systems. Module behavior under compression is characterized while submerged in liquid nitrogen, and structural changes are investigated using scanning electron microscopy (SEM). The structures retained flexibility at −196 °C, with a limited 5% increase in peak stiffness over 100 cycles while maintaining a full range of motion. A soft robotic limb was constructed from these modules and demonstrated successful 2D manipulation and grasping of objects at −196 °C. SEM analysis showed no physical signs of microfracture or deformation after cryogenic cycling, indicating changes to the underlying grain structure consistent with properties observed in cold-working stainless steels at cryogenic temperatures in the literature. Our findings demonstrate that metallic soft robotic structures maintain flexibility and exhibit promising performance in cryogenic, analogue space environments. This metal-based cable structure design approach provides a foundation for the development of functional, robust, and reconfigurable soft robots capable of operating in extreme space environments.

Keywords

space robotics cryogenic environment modularity metallic soft structures continuum robotics

Introduction

The growing demand for a robotic presence in space, including satellite servicing, asteroid mining, exploration, and settlement, necessitates the development of safe, robust, and adaptable robotic systems. The unpredictable nature of space environments demands robotic systems capable of robust performance amid unexpected obstacles and changing conditions. Soft robots offer significant advantages over rigid robots due to their inherent compliance and adaptability, allowing them to more gracefully deal with uncertainty. Specific advantages include navigating unstructured and unpredictable terrain, and safe interactions with high-value assets in space environments such as human operators or material of scientific or agricultural value. Characteristics such as space efficiency minimize launch volume and payload mass, aligning neatly with launch constraints.

Space environments experience wider ranges of environmental conditions compared to those on Earth. A selection of environments currently relevant to space research is presented in Table 1. These include the Moon, Europa (an ice/ocean moon of Jupiter), and 101955 Bennu (an asteroid that was the target of a successful sample return by OSIRIS-REx). A review of challenges in space for soft robotics¹⁰ highlighted radiation, temperature, gravity, and atmospheric pressure, specifically vacuum, as key factors that may affect the performance of soft robotic systems in these environments. The behavior of traditional elastomeric materials in cryogenic environments is well understood, with the reduction in temperature reducing their flexibility.^11,12 In a soft robotic context, this would reduce the available compliance of systems, impact effectiveness and reducing the benefits of soft contact.

Table 1.

Table Outlining the Environmental Conditions Present in Space Environments of Interest, Earth,¹ the Moon,^2–4 Europa,^5–7 and Bennu.^8,9

Condition	Earth	Moon	Europa	Bennu
Surface radiation (µSv/day)	0.87^a	1369	5.4 × 10⁶	—^b
Temperature (°C)	−90 to +60	−233 to +123	−173 to −141	−123 to 126
Surface gravity (m/s²)	9.81	1.62	1.31	6.15 × 10⁻⁵
Atmospheric (ATM) pressure (Bar)	1.0	3 × 10⁻¹⁵	2 × 10⁻¹⁰	—^c

Average of world radiation exposure from terrestrial and cosmic sources.

No measurements taken but likely similar to the Moon.

No significant atmosphere-vacuum.

These environmental challenges are not exclusive to soft robots. Typically, rigid robotics can localize these environmental effects to specific dynamic regions, such as joints, which allow for tighter tolerances and specific design decisions. Rigid robotic systems operating at low or near cryogenic temperatures must utilize heaters to ensure the correct functionality of dynamic components.^13–15 Systems have been proposed that use specialized materials for mechanical assemblies, such as the COLDArm system,¹⁴ a sampling arm for the lunar surface that can operate in cryogenic environments without heaters.

Soft robotic systems have been proven for operation in extreme environments on Earth, such as the deep sea,¹⁶ search and rescue,¹⁷ and nuclear radiation.¹⁸ However, space environments and their conditions vary greatly depending on the intended target. Few soft robots have been proposed for space, these being limited to robotic systems for locomotion on asteroids,¹⁹ for manipulation on spacecraft²⁰ and for systems operating in orbit.²¹ These systems encompass a wide range of actuation methodologies and are composed of various materials; however, the performance of the systems in the desired space conditions has not been demonstrated. In general, soft systems are presented without consideration that the extreme environments will quickly degrade flexibility and the performance of many of the novel actuation technologies that soft robotics relies on.

Metallic-based soft robots are a promising solution to the wide range of environmental conditions present in space due to their flight heritage and material stability. Liquid metals have found many applications in soft robotics,²² however, these systems have yet to be realized as untethered, remotely operated systems. Highly flexible systems can be constructed from mostly rigid linkages by leveraging structural design principles. One example is morphing cylindrical lattices^23,24 which have been proposed for extendable boom systems and antennas in space. Cylindrical lattices are interesting as they are easily constrained but allow large deformations in a single axis.²³ The same principles applied to soft robotic structures could remove dependence on traditionally soft materials, allowing space-compliant materials to be used to achieve similar levels of flexibility and deformation.

This article explores a methodology to construct soft robotic limbs, arms, or legs from repeatable, configurable modules named cable structures, shown in Figure 1. Characterizing the behavior of these modules and their long-term performance in analogue space conditions is desirable to demonstrate their applicability. We focus on the compression characteristics of the structures as these show the largest range of deformation compared to bending and torsion, giving us the highest confidence in their performance. By validating these structures against select space conditions, we show that they retain flexibility under cryogenic conditions, supporting previous literature on material behavior and demonstrating their applicability to space applications. We present a soft robotic limb designed using this methodology in Figure 2(A), demonstrating the ability for these structures to be integrated into systems that can perform tasks mimicking those that may occur in space environments. Specifically, here we demonstrate manipulation and grasping of objects in 2D, representing such cases as satellite grappling, on-orbit servicing, or space debris removal.

FIG. 1.

A cable structure with 8 cables and an offset of 360°. The structure is shown as neutral, compressed, and bending. A single cable is highlighted in blue to show the path in each configuration.

FIG. 2.

(A) Cryogenic soft robotic limb actuated by three cable tendons and submerged in LN2 within a 3D-printed vessel (300 × 300 mm). (B) Limb bending angle, differential left–right tendon and gripping tendon tensions during a representative grasping experiment. Key frames and corresponding experimental images are shown, illustrating the limb’s movement and grasping action. LN2, liquid nitrogen.

Materials and Methods

A modular design approach is employed, where traditional continuum systems are discretized into interchangeable flexible modules. These benefits are prototyping and design of large multipurpose systems. The method allows reconfiguration of modules and adjustments depending on operating conditions to achieve a wider functional space compared to monolithic soft robots. Maintenance of these systems is also improved as individual modules can be serviced independently of the larger structure.

Each module is designed as a single component of a larger rod system, with symmetry along two bending planes and a primary axis along the rod direction, as shown in Figure 1. The module is desired to have configurable stiffness in each axis, with a main axis, labeled z, supporting large-scale deformation and torsion, and the other two axes supporting bending.

Samples are assembled into larger continuum structures by chaining many modules together. Actuation can be provided by any method that allows compression of the structure, such as cable tendons drive systems, shown in Figure 2(A), shape memory alloys, or twisted coiled actuators. Similarly, module groups can be combined into larger robotic systems, such as those developed for applications as a locomotion platform.²⁵

A key design goal of these modules is to enable a large range of compressions throughout the rod systems. This is the primary difference between these structures and those present in other continuum robots with a “backbone.” The modules are designed to be highly configurable, such that desired stiffness and flexibility can be easily achieved.

Cable lattices

Structures were investigated that would minimize the material stress experienced when subject to high ranges of deformation. By arranging beam elements helically between two rigid rings, we can form spring-type structures that meet the identified requirements. Helical cable lattice configurations are a promising approach as they provide stiffness in all axes but allow large-scale deformations along the rod axis.

Helical spring structures couple axial displacement and rotation, as seen in typical compression springs found in mechanics. By braiding counter-rotating cable helix pairs, we create a cylindrical metallic lattice structure that removes coupling between these terms and increases the stiffness and robustness of the overall structure.

The ends of the cable structure are constrained with a ring, holding the helices in place and acting as an interface for connections between individual modules. Previous works that have explored helical structures,^23,24,26 allow the cylindrical lattice to expand when compressed, coupling the axial displacement and the structure’s diameter. Our approach aims to better facilitate modularization through a fixed interface, this constrains the ends of the helical lattice, directing us from a structure that deforms evenly to one with non-linear compression characteristics.

Two design parameters are selected for these structures, the number of cables n and the angle made by the helix around the central axis, called the angular offset, $θ$ . An example structure with $n = 8$ and an angular offset of $θ = 360^{°}$ can be seen in Figure 1, noting that an angular offset of $360^{°}$ means the helix makes one full revolution for the sample height. The structure’s height and diameter for this study are fixed to explore the effects of angular offset and the resulting lattice on the structure’s behavior.

Cable structures

The design methodology prioritizes structural morphology over material selection, exploring how traditionally non-soft materials can be arranged to form soft or highly flexible structures. It favors readily accessible material analogues, reducing reliance on space-grade materials.

The beam elements in the design are constructed using braided metallic cables, or wire rope, consisting of small-diameter metallic strands twisted together to form larger-diameter ropes or cables. Using braided metallic cables minimizes material stress compared to solid rods due to the helical structure and lower stiffness of individual cables.²⁷ A 1 × 19 strand 1 mm diameter braided cable composed of 312 stainless steel was selected for this study. This configuration has 19 individual strands twisted together and was selected due to its high stiffness and availability. The thickness, number of individual cable strands, and bundle configuration all influence the stiffness of the overall cable. 312 stainless steel was selected due to its corrosion resistance, high temperature strength, high availability, and large range of different types. There is a large variety of different materials and configurations, which, combined with the structural parameters, make these structures highly tunable, allowing use in a wide range of target environments.

The selection of stainless steel and metallic cables in general means some of the more challenging space conditions can be precluded from experimental validation. Stainless steels have a long space heritage, they are ubiquitous in space missions and applications and used to support nearly every aspect of spaceflight. Notably, stainless steel was used to construct flexible hoses for the Saturn-II²⁸ subject to extreme cryogenic temperatures. The mechanical behavior of steels in the presence of radiation, vacuum, and cryogenics is also well understood^29,30 giving us a stronger understanding of the material’s behavior in the large environment space. However, the flexibility and response to repeated large range deformations of systems constructed from these materials have not yet been characterized.

Cable structure modules were constructed using braided cables with 3D-printed or aluminum interfaces that hold each cable in its set helical configuration. Samples with eight cables each and angular offsets of 90°, 180°, 270°, and 360° were assembled for this study. Individual wire lengths are designed such that the total height of the sample at rest is 100 mm, independent of the angular offset, to enable comparisons between samples. Samples are labeled by the number of cables n, followed by the angular offset $θ$ , $n θ$ .

Mechanical testing

To characterize the mechanical behavior of the structure, we first examine its axial properties, as it allows the largest range of deformation. Samples were tested in compression and tension using an Instron 5545 Universal Testing Machine (UTM). Compression of the structure produces a profile shown in Figure 3. Where positive and negative displacement correspond to compression and extension from the neutral position, respectively.

FIG. 3.

Compression characteristics of a single cable structure measured using an Instron 5545 UTM. Indicative shapes of the structure at points along its compression profile are shown.

The profile in Figure 3 shows that the structure exhibits hysteresis with respect to its non-linear compression-displacement relationship. This is attributed to energy loss due to friction within the braided cable and the friction between cables in the structure. The presence of hysteresis and elastic behavior allows us to generally characterize this structure as exhibiting viscoelastic behavior.

The structure produces asymptotic behavior toward the end of the ranges, marking the transition between structural and material-dominated behavior. Under over-extension, the primary tension loading component comes from the steel cable, which is many times stiffer than the structure itself. Under compression, the main loading components are from interference between the cables and the compressive resistance of the rigid interface itself.

We note that a snap-through point exists partway through the compressive phase, identified by a local minimum within the compression range, shown in Figure 3. The local maximum force exerted by the structure is due to the mechanics of the lattice spring structure, labeled in Figure 3, compared to the global maximum, which occurs when fully compressed and represents the behavior of the interface material. We define the range between the rest state of the structure and the local minima as the lattice region, for the profile in Figure 3: $0 < δ < 72$ . In this region, the structure’s behavior is dominated by the spring mesh. Outside of this region, the structure’s properties are dominated by the material behavior and are of lesser interest to this study.

Cryogenic environment

To test in cryogenic conditions like those present in space environments, such as the Moon and Europa, as seen in Table 1, we opt for direct immersion of our samples in liquid nitrogen (LN2). This guarantees that our samples stay below the boiling point of nitrogen −196 °C during testing. We utilize a custom automated compression testing rig with a LN2 bath, enabling cyclic compression testing, shown in Figure 4(A).

FIG. 4.

(A) The single-axis test rig with LN2 Immersion Bath. (B) A render of a structure undergoing compression in the LN2 bath, where blue is at rest and red is compressed. The structure’s displacement is labeled as $δ$ .

To evaluate the compressive behavior of the cable structures, a single-axis test rig was used. Twin lead screws control the axial displacement, while a bar load cell in a S-cell arrangement measures forces generated between the moving test platen and the fixed base. Displacement and force were recorded using a microcontroller and sent over a serial interface to a logging computer. The sample and load cell temperatures were measured using T-type thermocouples attached to a logging system. The load cell was cross-calibrated using weights against calibrated precision scales at a range of temperature points and measured using an HX711 load cell amplifier. A custom LN2 immersion bath, shown in Figure 4(B), was fabricated using 3D printing and PLA material. The two-walled design aims to reduce heat transfer and feature an open top to allow refilling of LN2 and venting of nitrogen gas during testing.

Cable structures were placed into the LN2 bath and left until a temperature equilibrium was reached. Samples were then compressed between an upper and lower displacement point at a set rate for the desired number of cycles. LN2 was added throughout the tests such that the sample stayed entirely submerged for the duration of the testing. We limit the analysis to the lattice region of the compression profile, where the behavior is dominated by the lattice structure.

Key factors affecting calibration of the test rig included material stability, platen buoyancy in LN2, and temperature drift of the load cell. To account for buoyancy and material effects, the platen was cycled into a filled LN2 bath with no sample, this allowed these effects to be captured and compensation to be applied during data analysis. To account for load cell drift, the temperature was monitored during testing, and calibrations were conducted before and after each test.

Three cable structure configurations were selected to initially compare structure behavior in ambient and cryogenic environments. The number of cables for each sample was held constant, $n = 8$ and the angular offset was varied to 90°, 180°, 270°, and 360°. These samples are referred to using the $n θ$ convention 8090, 8180, 8270, and 8360, respectively, throughout the remainder of the text. For each configuration, four repeats were tested both at ambient, 22 °C, and cryogenic, −196 °C, for 10 cycles between $0 < δ < 70$ at a constant speed of 4 mm/s.

Endurance testing was conducted using four new samples of an 8360 configuration, that being eight cables with 360° angular offset. The samples were cycled between 0 and 70 mm at 4 mm/s for 100 cycles, totaling approximately 2 h of time submerged in LN2 each.

Cryogenic grasping demonstrations

To provide preliminary validation of the cable structures’ ability for integration into larger robotic systems and demonstrate their potential for space-relevant manipulation tasks, we developed a soft robotic limb prototype and performed grasping exercises in a simulated space environment. We note that although this system is constrained in 2D, previously we have presented systems that can operate in multi-plane consisting of longer chains of modules,²⁵ however, due to the desired experimental conditions, this system is constrained.

The limb is constructed from two cable structures connected in series with a simple tendon-actuated end effector constructed from the same steel cable. The limb, operating within a 300 mm × 300 mm 3D-printed vessel filled with LN2 [Fig. 2(A)], is actuated by three Dynamixel XL-430 actuators located outside the bath, driving stainless steel tendons. Tendon tension is measured using bar load cells connected between the actuators and the limb. This design isolates the actuators, sensing components, and electronics from the cryogenic environment, a key benefit of cable-tendon actuation that may make it well-suited for space applications.

Three 3D-printed chess pieces (pawn, bishop, and rook), objects with distinct grasping characteristics, representing objects of interest in space, were placed within the LN2 bath. An operator controlled the limb to perform grasping and manipulation tasks with each chess piece, see Supplementary Videos S1–S2.

Material characterization

Samples exposed to endurance testing in cryogenics were compared against control samples and examined using scanning electron microscopy (SEM) on a SU7000 Electron Microscope. Samples were examined for signs of physical damage and plastic deformation, including microfractures or cracking on the external surfaces of the cables. The lower third of the cable structure was examined as these were subject to the tightest bend radius during testing and, as such, were expected to be exposed to the highest stress.

Results

Comparisons between ambient and cryogenic performance

The overall trend of the profiles in ambient and cryogenic environments is similar, as shown in Figure 5. The peak force and axial stiffness of all samples increased when submerged in LN2. The significance of the snapthrough region on some configurations decreased, where the rate of stiffness change increased with positive displacement. The range of the lattice region decreased with $θ$ , as $θ$ increases, we see the snapthrough point moving from $δ = 70$ mm for $θ = 90$ to $δ = 55$ mm for $θ = 360$ . The hysteresis of the system can be quantified by calculating the area inside the hysteresis loop, integrating between the compressive and return profiles to give total energy loss. The peak force of the lattice region and the total energy loss of the structure were selected as the key characteristics for analysis.

FIG. 5.

Comparison between ambient and cryo behavior for 10 cycles and four different cable configurations, shown to the right of each graph.

Mean changes in the peak force of the lattice region over 10 cycles are shown in Figure 6(A) with generated 95% confidence intervals from the four samples shown as shaded regions. The average of all configurations at ambient temperature is also plotted for comparison. We note that in general, the samples experience less change in force when in a cryogenic environment compared to ambient. The mean change in load for all configurations in cryogenics was less than 3%. The configuration with the lowest change in force over 10 cycles was the 8-cable 360° configuration, 8360. This is supported by our design, the cable has the largest angular offset, causing the largest bend radius, resulting in a lower material stress.

FIG. 6.

(A) Average peak force changes for four different cable structure configurations compared to the average ambient for all samples. (B) Average energy loss for each configuration compared to ambient for all samples.

The change in energy loss for each configuration is shown in Figure 6(B). Initially, energy loss decreases sharply for most cycles, however, trends indicate that this may be increasing toward the end of the cycling.

This analysis was limited to only 10 cycles for a range of cable structure configurations. The long-term behavior of these structures at cryogenic temperatures is also of interest. The 8360 configuration was selected for further endurance testing as the samples exhibited the lowest change in force while maintaining lower energy loss compared to other samples.

Mechanical functions in cryogenic conditions

A compression profile from a sample cycled 100 times is shown in Figure 7. In general, as the number of cycles increases, the structure generates more force throughout the compression profile. A key observation is that the return phase of the compression profile is less affected by cycling than the active compression phase. Also to note is that the changes in force are not equal throughout the compression phase, instead, there are two significant regions, both with varying rates of change at $δ = 35$ and $δ = 60$ . The range of the lattice region stayed relatively consistent, lying between 0 and 50 mm through the cycles.

FIG. 7.

Compression profile for an 8360 structure undergoing 100 cycles while submerged in LN2 bath.

Figure 8(A) shows the peak force of the lattice region plotted against the cycle number with a 95% confidence interval generated from three samples, shown as shaded regions. We observed relatively small changes in force that level out, noting no significant long-term force changes after around 80 cycles. The peak force starts to decrease at the beginning of the test, supporting observations made from the 10-cycle tests, before increasing. This can be attributed to the structure bedding in over a few cycles. We note that the confidence interval overall increases in area, where the trend stays consistent.

FIG. 8.

(A) Changes in peak force of the lattice region while under cryogenic conditions across 100 cycles. (B) Changes in the total energy loss of the system under the cryogenic condition.

The results of the increasing force in the compressive phase but not in the return should mean that the hysteresis of the system is also increasing with the number of cycles. Figure 8(B) shows the energy loss of the system against cycle number with a 95% confidence interval generated from our samples. We observe changes in energy loss, up to an average of 20% after 100 cycles, and seemingly trending upwards.

Considering our previous observation that the return phase of the system stays relatively constant compared to the compressive phase and the minimal force change, we can conclude that this energy change is occurring mainly outside of our lattice region.

Cryogenic grasping demonstrations

Figure 2(B) shows the limb’s bending angle and tendon tensions during a representative grasping experiment. The graph illustrates the coordinated actuation of the three tendons, with the left and right actuating tendons combined into a differential value referred to as bending tension, and gripping tension shown separately, as the limb engages and recovers objects. Key frames highlight the successful grasping and release actions and are marked on the graph to show corresponding angle and tension measurements. Tendon tensions during these manipulations ranged from −108 to 70 N for the bending tension and between 5 and 68 N for the gripper.

Metal structure morphologies after cryogenic treatment

Two specimens are shown in Figure 9, comparing a control sample to a sample that underwent endurance testing in the cryogenic environment. Specimens were imaged up to 5000× magnification and examined along their length for any signs of physical degradation. We observed no significant differences in the macro-structures present in the individual cables and no indication of microfractures. There was a significant visual difference between samples, which can be accounted for by the LN2 bath cleaning of foreign material left over from the manufacturing process. If the sample was physically degrading, we would expect to see physical signs of this with matching performance decreases in the compression profiles.

FIG. 9.

SEM images of a sample wire before (A and C) and after (B and D) cryogenic cycling. (A and B) were imaged with 100× magnification and images (C and D) with 2500×. SEM, scanning electron microscopy.

Nano-scale material properties, such as grain structure or crystal composition, cannot be observed with this method. We note that only the outer surface was examined, and the internal grain structures could have been modified. One additional limitation of this method is that while testing was performed under cryogenic conditions, SEM was performed in ambient conditions. However, as our interest lies in macro-scale damage to the material, we can assume that the material’s condition would not improve once it was returned to ambient temperatures.

Discussion

These results are supported by works exploring the behavior of stainless steel in cryogenic environments. Li et al.²⁹ showed that the yield strength and ultimate tensile strength of 316L stainless steel improve under cryogenic conditions. Similarly, Hecker et al.³¹ found that both strain hardening and crystal structure transformation rates increased with decreasing temperature under tensile testing for a 304 stainless steel. Our results indicate an increase in the overall stiffness of the material compared to ambient, as shown in our comparisons to ambient tests in Figure 5. However, we also showed the increase in the structure’s stiffness with the number of cycles, indicating that the material structure is changing due to loading cycles [Fig. 8(A)].

This further supports observations of no material degradation from our SEM analysis. If the changes in material properties are caused by transformation between metallic microstructures, then we would not observe microfractures or other surface defects from loading.

The transition between the lattice region and the interface, $δ = 70$ , provides an interesting local lower energy state where the structure can be stored with less force required. We also demonstrated that this point can be tuned depending on the angular offset. Toward space applications, these structures can be stored in highly compacted states, meeting requirements toward minimizing volume for conditions such as launch. For operation in wide temperature ranges, where jointed systems must compensate for thermal expansion, causing backlash and interference, these structures instead change length and stiffness according to the temperature. This reduces the design complexity of the mechanical robotic systems; however, it increases the reliance on accurate modeling and control algorithms.

The performance of the structures under cryogenic conditions shows promise toward reliable structures for soft robotic systems in space environments. Removing the reliance on elastic materials and instead implementing a structure-based approach using space-validated components may reduce the complexity of soft systems in extreme environments. Work continues to further explore these structures’ robustness and to implement systems that utilize the significant benefits of soft robotics in space.

Authors’ Contributions

W.F.-H.: Writing—original draft, conceptualization, methodology, formal analysis, investigation, software, data curation, and visualization. D.J.H.: Supervision, writing—review and editing, conceptualization, and methodology. L.Y.: Supervision, writing—review and editing, and methodology. R.A.: Supervision, writing—review and editing, conceptualization, and methodology.

Footnotes

Acknowledgments

The authors acknowledge the assistance and technical expertise of Kevin Farries and Thomas Anthony and the support, facilities, and technical assistance of Adelaide Microscopy and the Andy Thomas Center for Space Resources (ATCSR) at the University of Adelaide.

Author Disclosure Statement

The authors have no conflicts of interest to disclose.

Funding Information

The authors would like to acknowledge the Australian government for the financial support provided through this research was supported by an Australian Government Research Training Program (RTP) Scholarship .

Supplemental Material

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Supplementary Material

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