Programmable Shape Morphing Metasponge

Smart materials respond to environmental signals by changing their microstructure and physical properties. Programming multiple behaviors and functions into a single material could increase its utility and adaptability to ever‐changing environmental conditions. A swellable and stretchable metamaterial hydrogel or “metasponge” engineered to morph into customized sizes and shapes that dynamically tune its physical properties and functions is reported. Multiple case studies that take advantage of the morphing properties of the metasponge, including robotic actuation, light guidance, optical and sonic invisibility (“cloaking”), adaptation of propulsion mode, sampling, and multiple biomedical applications, are illustrated. Developing multifunctional smart materials in which logic is programmed into the materials rather than electronic components could pave a new path to autonomy and dynamic responses in soft robots, sensors, and actuators.


Introduction
Smart materials can adapt and deform over time, achieving new functionalities. [1][2][3] A smart material response is programmed directly into its microstructure by selecting active materials with sensitivity to specific stimuli and environmental conditions. [4][5][6] In the past decade, there have been numerous reports of actuation stimuli that have been found to elicit responses in various systems. These stimuli include dielectric stimulation, which involves the application of an electric field to a material. [7][8][9] Environmental stimuli encompass a wide range of factors such as chemical, [10][11][12] humidity, [13][14][15][16] and heat [17,18] signals that can lead to swelling or contraction of active materials, which can in turn cause them to bend or flex. Moreover, externally applied physical stimuli such as magnetic [19][20][21][22][23] and light [24][25][26] fields have also been shown to induce actuation responses in materials. The robustness and tunability of smart materials have led to their adoption in diverse areas, ranging from biomedical [27][28][29] to industrial [30][31][32] applications. In general, smart materials are designed with specific functionality, such as generating mechanical, [33,34] shape, [35,36] or signal processing. [37,38] However, other functionalities require tethers to external hardware [39][40][41] or off-board electronics controls [42][43][44] increasing their complexity. The development of programmable materials could lead to the creation of constructs that can adapt to their surroundings and perform useful tasks.
Here, we report a simple, yet versatile "metasponge," a metamaterial (i.e., a synthetic composite material that exhibits properties not found in natural materials) [45] based swellable hydrogel, by integrating the properties of an absorbent hydrogel (sodium polyacrylate) within a highly stretchable and robust elastomer (Ecoflex [46] ). When the metasponge is submerged in an aqueous solution, the absorbent material swells within the elastomer matrix resulting in volumetric expansion. The elastomeric matrix enables the metasponge structure to withstand mechanical deformation while retaining hydrogel properties (e.g., water retention and diffusivity). Further, active regions composed of absorbent/elastomer can be spatially distributed within a passive support matrix (elastomer only, nonswelling), resulting in asymmetric swelling behaviors. The change in the mechanical properties of the metasponge (based on its swollen and unswollen state) was used for various applications, including dynamic camouflage (optical and sonic), light guiding, soft robotics, microfluidic valve, biomarker sampling, and drug delivery. With its tunable properties, simple structure, and compatibility with various fabrication methods, the metasponge holds potential as a material for soft robots and "smart" devices, providing responsiveness and multifunctionality.

Results and Discussion
A paste-like material results from mixing an equal volume of an absorbent material (sodium polyacrylate, dry powder) and an elastomer support matrix (ecoflex, uncured viscous liquid). The miscible paste can be molded or extruded into the desired shape ( Figure S1, Supporting Information). The metasponge grows in volume as the sodium polyacrylate polymeric chains uncoil by absorbing water molecules, within the passive elastomer matrix, creating a wetting network that propagates through the core of the metasponge (Figure 1a). The absorbent material is entangled within the mesh of the elastomer matrix; thus, there is nonsignificant leaking or release of the passive material from the metasponge. [47,48] In addition, the elastomer matrix provides mechanical robustness to the metasponge compared to a pure hydrogel structure ( Figure S2, Supporting Information) and enables it to expand compared to other stiff matrixes, such as polydimethylsiloxane (PDMS, Figure S3, Supporting Information).
The metasponge can be spatially distributed within a pure elastomer matrix to generate active (swelling) and passive (no swelling) zones within a single structure. To illustrate this capability, we designed a metasponge octopus-shaped construct that contained active material at the arms, chest, and head sections, while the rest of the body was composed of passive material. The dry state consists of a flat 2D structure, as the octopus is molded from a template mold. Upon suspension in an aqueous solution (containing a green, fluorescent dye), the metasponge's active arms and head swell up to curve. As a result, the octopus metasponge changes from a 2D to 3D structure ( Figure 1b). Multiple layers with distinct material compositions can be integrated into a single metasponge structure, as illustrated by a tree-like structure as in Figure 1c, which presents the selective growth of the green segment after overnight incubation in water.
The metasponge growth under different environmental conditions was studied in detail. A metasponge rectangular cuboid (3.5 mm Â 3.5 mm Â 1.5 mm) was used as a standard with equal absorbent and support matrix material ratio by volume unless otherwise stated. First, the swelling kinetics of the metasponge was evaluated by measuring the growth of individual structures (measuring ratio between final length L between L 0 initial length) during overnight incubation (Figure 2a). The metasponge presented a rapid growth profile during the first hour, almost doubling in length, followed by slower growth during its plateau for 12 h, resulting in a 3.5-fold increase in size. The slower growth as time passes is due to the more time it takes for the liquid to get deeper toward the center of the metasponge as the material reached its maximum swelling capacity. The metasponge swelling process is reversible when left drying overnight or in an oven to dehydrate the sample. After drying, the metasponge structure retained swelling capabilities for multiple cycles measured (Figure 2b and S4, Supporting Information). Figure 1. Metasponge actuation mechanism. a) Scheme and photograph of a dry and swollen metasponge fabricated by integrating an absorbent (sodium polyacrylate) within a stretchable elastomer matrix (Ecoflex). Scale bar: 3.5 mm. b) Photograph of an octopus design with spatially distributed metasponge region within an inert elastomer body (left). Photographs under ultraviolet light illustrate the spatially encoded growth of metasponge octopus before (middle) and after swelling (right) in a green, fluorescent solution. Scale bars: 30 mm. c) A tree-like structure produced with different material compositions, where only the top green metasponge structure (green) was designed to present swelling, while a base (brown) was designed to have minimal swelling. Scale bar: 3.5 mm.
The metasponge growth can be engineered by tuning the ratio between the absorbent and matrix materials. Using a higher ratio of absorbent results in a more brittle structure with less mechanical stability but higher growth. A higher ratio of elastomer material leads to reduced swelling as some absorbent chains are disconnected from the external network ( Figure 2c). We compared the swelling of the metasponge under different solvent polarity conditions, including water chloroform, toluene, and ethanol to evaluate the interaction of the solvent with the polyacrylic acid groups of the absorbent material ( Figure S5, Supporting Information). Highly polar solvents such as water presented a higher absorption rate when compared to nonpolar solvents. The nonpolar solvents showed less than 1.2-fold growth in the same incubation time. The metasponge grew differently under ionic environments (NaCl, CaCl 2 , FeCl 3 , NbCl 3 ) and molar concentrations (10 À3 , 10 À2 , 10 À1 , and 10 0 M). Ions present in the solution can compete with water molecules to interact with the negatively charged carboxylic groups from the absorbent material. The presence of positively charged ions with a higher charge (Fe 3þ and Nb 3þ ) hindered the growth of the material more significantly when compared to lower charge cations (Na þ and Ca 2þ ). Moreover, the molarity of the salt concentration played a significant role in the swelling of the metasponge structure. For instance, at low relative ionic strength (10 À3 M), the metasponge structure presented similar growth to water. In contrast, in high ionic strength (1 M), the metasponge structure did not present any significant growth compared to its original size ( Figure 2d). Incubation of a fully swollen metasponge in a high ionic concentration resulted in deswelling to half of its fully swollen size at the 90 min time point, and fully deswelled after overnight incubation (Figure 2e). We evaluated the swelling of the metasponge under different pH environments (Figure 2f ). Metasponges in solutions ranging from a pH of 4.5-9.5 presented similar swelling to that of a pH 7 solution. The swelling diminished in highly basic and acidic solutions. The higher concentration of Na þ from the NaOH solution (pH 12) is associated with reduced swelling (2-fold). On the other hand, the high concentration of H þ from the HCl (pH 2) leads likely to the protonation of the carboxylic acid groups from the sodium polyacrylate material, thus reducing interaction with the water molecules with the absorbent chains. These results indicate that the response of the metasponge can be engineered through response to solution properties (e.g., polarity, ionic strength, and pH).
The swelling of the metasponge can be accelerated by the function of temperature. The metasponge was incubated at different fluid temperatures (4, 23, 40 ºC) for 30 min. The metasponge grew at different rates based on the incubation temperate as follows: 2.7-fold at a high temperature (40 ºC), 2-fold at room temperature (23 ºC), and 1.4-fold at a cold temperature (4 ºC) ( Figure 2g). The combination of a higher temperature led to an increased diffusion rate of water molecules into the metasponge matrix driven by the faster movement of the solvent molecules and the change in mechanical proprieties of the polymer matrix resulting in enhanced growth. A highly porous metasponge was fabricated by including sucralose (sacrificial template) into the metasponge matrix, where the use of sacrificial materials in gels was also previously reported in the literature in order to increase surface area and improve the solution's access and penetration toward the center of the matrix. [49][50][51] The sacrificial layered metasponge was composed of a mixture with a volume ratio of 2 parts elastomer, 1 part sucralose, and 1 part absorbent. The sacrificial sucralose rapidly dissolved when placed under water, resulting in a porous metasponge structure that enabled it to grow faster ( Figure 2h). The sucralose asymmetric particles presented an average size of 5 AE 24 by 41 AE 13 μm, thus the initial pores are expected at this range. A hollow design can also be integrated to accelerate growth with increased surface area. A larger 5 mm sponge was perforated with 1 mm diameter holes through each face of the metasponge. These passages enabled the metasponge to grow faster than nonperforated metasponges (Figure 2i). These examples illustrate the tunability of the metasponge design, which can be engineered with different growth profiles.
The mechanical and physical properties of metasponge can change depending on its original and swollen state. The metasponge sustained constant mechanical strain and deformation, including stretching and torsion, in both original and swollen conditions, as shown in Figure 3a & Video S1, Supporting Information. Further, it is worth noting that the swollen state presents reduced stretchability as the swollen sodium polyacrylate particles inside the metasponge structure have already expanded the Ecoflex matrix.
A fully swollen metasponge (20 mg) withstood more than 100fold its weight, as demonstrated by adding a 20 g weight on top of it ( Figure 3b & Video S2, Supporting Information). The dry form of a metasponge fiber presented higher stretchability (%10-fold its original length before breaking) when compared to a swollen fiber (%3.2-fold its original length before breaking) (Figure 3c). The two states of the material have very distinct properties concerning optical and sonic characteristics. [52,53] In its original form, the metasponge is opaque and does not allow light to travel through. Its swollen state is translucent and allows light to travel from the input to the output port. Figure 3d illustrates a logo covered by a dry metasponge opaque film that blocks it. After swelling, the film became translucent, thus revealing the logo. When the metasponge was dry, the tight packing of the absorbent material reflected and diffused the incident light. This also applies to the elastomer matrix, which is opaque and a poor conduit for light. When the metasponge swelled, it obtained its optical properties from the water molecules and hydrogel material characteristics. Thus, the swollen metasponge can decrypt information or camouflage in its environment based on its state. Figure 3e shows a translucent metasponge film submerged in a solution camouflaged within rocks. The metasponge was used to guide light as an optical fiber, [54] as shown in the photographs in Figure 3f and S6, Supporting Information. Fibers made of only elastomer or dry metasponge did not present guidance, where the readout sensor only detected environmental noise. The swollen fiber was able to transmit light from a light source (Thorlabs), providing an output signal detected by a highresolution spectrometer (Ocean Optics) (Figure 3g & Video S3, Supporting Information).
The acoustic properties of the metasponge were evaluated using the experimental setup shown in Figure 3h,i. A thin film of the material was placed between a 1 MHz piezoelectric ultrasonic transducer (Olympus Corp) and a broadband needle hydrophone (Onda Corp.) in a tank filled with water. The acoustic transmission through the material was measured for three types of samples including: elastomer only (.005 V), metasponge (.05 V), and swollen metasponge (.15 V) (Figure 3h,i). The output voltage (which is proportional to the incident pressure field) measured across the hydrophone was negligible in the case of the elastomer, indicating poor acoustic transmission through the material. Due to the significant acoustic impedance mismatch between the elastomer and water, most of the incident acoustic energy from the transducer is either reflected or absorbed, causing the elastomer film to behave like a sound barrier. We observe a significantly higher hydrophone voltage and acoustic transmission in the case of the freshly swollen metasponge. This implies that the metasponge absorbed some water causing the acoustic impedance mismatch between the metasponge and surrounding liquid environment to reduce. [55] The greatest transmission is observed in the case of the swollen metasponge, as most of the material is swelled with water. This causes the sponge to have an impedance close to the surrounding liquid, which enables www.advancedsciencenews.com www.advintellsyst.com sound to pass through the film with minimal scatter. Thus, a thin film of metasponge can act as an acoustically reflecting or transmitting material by simply modulating its composition, thereby enabling the use of such materials in applications such as acoustic cloaking. The engineered growth of the metasponge can serve as an actuation mechanism for soft robots. A single structure can present distinct behaviors by fine-tuning the spatial distribution between active and passive material. For example, Figure 4a illustrates how different distributions of passive materials in a disk shape can serve as restriction to limit the growth of the metasponge. The active material in the pure metasponge grew at an even and regular rate in all directions. However, when the perimeter of the metasponge material was surrounded with passive material, the x-y growth was hindered, resulting only in z-direction growth. Thus, the passive material acts as a spring that applies a force upon the metasponge. A rigid skeleton of poly (methyl methacrylate) (PMMA) was used to constrain and direct the growth of the metasponge, as shown in Figure 4b, where a rigid plastic skeleton is introduced in three metasponge squares. When the metasponge squares swell, the rigid structure was lifted to create a bridge-like structure. The combination of swelling and non-swelling can enable directed growth. A star-shaped actuator was designed using a bottom layer of passive material with active top material. The metasponge materials swelled, while the passive support served as a resistor, generating a spring-like muscle that bent the arms of the star while they dynamically swelled, resulting in a fully closed sphere-like structure (Figure 4c). We conducted an experiment using star actuators with different thicknesses of the active layer (thin: 0.75 AE 0.04 mm, thick: 1.56 AE 0.09), each containing a 0.5 mm thick passive layer and presented that thinner active layer actuators resulted in faster actuation. Thicker materials presented slower actuation as they required more time for water to diffuse into the active metasponge matrix. Although water permeates the metasponge, the inner dry part of the metasponge behaves as a passive material that creates resistance to the material to flex ( Figure S7, Supporting Information). When the metasponge material was placed facing up, the swelling of the metasponge enables to "stand up" the whole star structure (Figure 4d and Video S4, Supporting Information). The metasponge was camouflaged, by adding texture to the passive layer, as illustrated by Figure 4e, where a sand was used to camouflage the star actuator in a sand-filled container. Upon growth, the star arms grew to capture a metallic nut (Video S5, Supporting Information). The star-shaped actuator provides both the volumetric expansion and the conformal trapping around the target as shown by tethered inflated actuators and the autonomy and deployability of untethered responsive actuators. The diffusion of liquid into the metasponge surface can also generate other actuation mechanisms that do not require swelling. We also designed a jellyfish containing platinum particles embedded in its matrix. The platinum catalyzed the peroxide solution into water and oxygen gas (Figure 4f ), the gas generated accumulated at the surface of the jellyfish resulting in buoyancy, and we note that the jellyfish was initially adhered to the surface and would not detach in the absence of peroxide fuel (Figure 4g & Video S6, Supporting Information).
The metasponge material can also be integrated into other devices or robots. To illustrate this principle, we engineered a small-scale millimeter-sized robotic car, consisting of a solid chassis, a magnetic core, and metasponge-based wheels (Figure 5a). The metasponge-based wheels are designed to have asymmetric growth, where the diameter of the wheel grows 1.6fold in diameter while the width of the wheel grows by threefold (Figure 5b). To actuate the robocar, a permanent magnet was rotated. The magnetic core responded by also turning; thus, each rotation dictates the cycle. [56] The displacement of the robocar is directly correlated to the diameter of the wheel. Figure 5c shows the experimental and theory displacement by a single rotation by the original and transform metasponge wheel. Figure 5d and Video S7, Supporting Information, illustrate an original and www.advancedsciencenews.com www.advintellsyst.com swollen robocar displacement under a single rotation. The displacement increased under multiple turns when the wheel had a large diameter. The robocar thus can change its locomotion behavior based on the size of the wheels. The swelling of the wheels can also help the robot navigate complex environments. For instance, when placing the original dry metasponge in a pool with rocks, the metasponge wheel was unable to climb through rocks larger than the diameter of the metasponge wheels. Additionally, the thin unswollen wheels provided little traction. However, when the metasponge wheels expanded, the robot climbed and navigated through the uneven environment (Figure 5e & Video S8, Supporting Information). The metasponge wheels can serve other functions apart from locomotion. A robocar was guided to a reservoir containing an acidic solution (pH 2) and was left sampling for 1 min. Next, the robocar was guided to a second reservoir containing a pH-responsive colorimetric solution (Hank's balanced salt solution). The absorbed acidic solution was transferred to the assay reservoirs changing its color from yellow to pink (Figure 5f & Video S9, Supporting Information). Similarly, the robocar can use a pH-sensing paper strip instead of the detection chamber ( Figure S8, Supporting  Information). These examples illustrate the multifunctionality of the metasponge-based wheels.
The metasponge's tunability and adaptability make it an excellent candidate for medical devices. For instance, metasponges can serve as autonomous valves in microfluidic devices. To show this application, a metasponge circle was placed at the center of a chamber in a microfluidic device and as a solution passed through the microchip pushed by a gravity-based pump, the metasponge swelled, blocking the channel, initially reducing and then stopping the flow leaving the microchannel at the outlet (Figure 6a & Video S10, Supporting Information). The flow stopped around the 2 min mark, enabling less than 3 mL of fluid to pass through the microfluidic chip, as shown in graphs in Figure 6b,c. Without the sponge, the full 10 mL passed through the channel. Metasponges can serve as autonomous volumetric flow control valves that close or increase flow resistance as a specific desired volume passes through the chamber.
Moreover, the metasponge can serve as the matrix of biologically active enzymes. Lactase enzyme powder was mixed during the fabrication in the sponge matrix to show this application. The lactase enzyme embedded in the metasponge can catalyze lactose into galactose and glucose (Figure 6d). The gel matrix is designed to trap active bionzymes and provide a porous matrix in which the sponge's interior can increase catalytic surface area. We evaluated the effect of the swelling on catalytic activity by incubating the metasponge containing lactase in milk solution for 1 h. As a control, we also measured the catalytic activity of an ecoflex þ enzyme material which did not have absorbent. The graph in Figure 6e illustrates the glucose generated as the by-product of lactose catalysis. The metasponge presented threefold degradation when compared to the control (Figure 6e). The enhanced surface area provided by the swelling of the metasponge enabled the lactase in the structure's interior to participate in the catalytic process. With the passive sponge, the catalytic enzyme in the interior was not accessible to the solution. The catalytic metasponge can be used over multiple cycles as the lactase is embedded in the matrix, as shown in Figure 6f. www.advancedsciencenews.com www.advintellsyst.com Another potential use of the metasponge relies on selective biomarker sampling in the body. In particular, the metasponge can potentially selectively sample in the gastrointestinal tract, as it has limited growth in an acidic environment found in the stomach but not at the relatively neutral pH found in the intestinal tract (Figure 6g). We used different biomarkers for testing applications. First, we incubated the metasponge mixed with nutrients in a solution containing fluorescent E. coli. After one day of incubation, we used a cross-section and imaged the metasponge using a confocal microscope. The micrograph in Figure 6h & Video S11, Supporting Information, shows that bacteria can migrate to the structure's interior. When the Figure 6. Demonstration of metasponge as a biomedical device application. a) Photographs illustrating a metasponge (orange) as an autonomous valve to close a flow in a microfluidic channel. Scale Bar: 5 mm. b) Output flow volume obtained from an initial 10 mL input, comparing no sponge (control) and metasponge (n = 5). c) Total output volume collected at the microfluidic channel outlet after 12 min (n = 5) d). Scheme and photograph illustrating metasponge as a matrix for biocatalytic enzymes. Lactase embedded in the metasponge decomposes lactose (milk) into galactose and glucose. e) Measured glucose by-product generated after incubation in different experimental conditions, including i) sponge w/o lactase, ii) elastomer w/lactase, and iii) sponge w/lactase in milk (lactose solution) (n = 4). f ) Reusability of metasponge w/lactase over multiple cycles. g) Schematic of metasponge used for selective sampling in the gut, inset graph illustrates the change in weight of the metasponge after incubation in different pH (n = 5). h) Use of metasponge for sampling bacteria, illustrated by a scheme and confocal image projections showing bacteria internalization towards the sponge's center. I) Qualitative photograph illustrating the hemoglobin capture efficiency after incubation of a metasponge in Guaiac solution. J) Navigation of a magnetically actuated metasponge for drug delivery through an ex vivo stomach model. Scale bar: 6 mm. k) Release of the metasponge cargo (blue dye) after one hour. Scale bar: 6 mm.
www.advancedsciencenews.com www.advintellsyst.com metasponge is not mixed with nutrients, the bacteria quantity in the metasponge diminishes, as there is not an incentive for them to migrate into the metasponge ( Figure S9, Supporting Information). As a second application, we investigated the ability of the metasponge to isolate hemoglobin from a liquid environment. Hemoglobin can be used as a marker for hidden blood in the gastrointestinal tract. [57] We incubated the metasponge in hemoglobin solution for one hour and used a colorimetric solution (detailed information in the experimental section) to detect the recovery of hemoglobin from the solution (Figure 6i). We also validated hemoglobin sampling via a commercial electronic sensor (AimStrip Hb Hemoglobin Meter) ( Figure S10, Supporting Information) and lateral flow immunoassay (Pinnacle Biolabs' Second-Generation FIT Tests) ( Figure S11, Supporting Information). The metasponge is not designed to be biodegradable inside the body, enabling its recovery, as the captured markers can then be isolated for further analytical downstream evaluation. Finally, we demonstrated the versatility of the metasponge as a sustained drug delivery platform. We created a smart pill, by integrating a rolling magnetically actuated small-scale robot with a metasponge loaded with a model drug (methylene blue). The smart pill navigates through the dry and wet surfaces of an ex vivo stomach model, demonstrating the ability to navigate through dry and wet surfaces (Figure 6j & Video S12, Supporting Information). After the smart pill reaches a liquid reservoir, it absorbs the liquid and releases the loaded drug (methylene blue) in a sustained release kinetic (1 h, Figure 6k). The slower release profile could be an alternative diffusion-based approach compared to the degradable polymeric material coatings. [47] These results suggest that the adaptability and robust behavior of the metasponge have broad potential applications in the biomedical field.

Conclusions
In this study, we demonstrated the simple fabrication and diverse uses of the metasponge. Integrating absorbent and elastomers into a single matrix enables robust functionality based on a reversible swelling mechanism. The metasponge is responsive to different environmental conditions that govern its swelling. The metasponge can be a mechanical actuator that deforms over time and can conformally grow and encompass its target object. The metasponge changes its physical parameters between dry and swollen states, including its mechanical, optical, and sonic properties, which can be used as encryption, camouflage, and optical fibers. Moreover, the metasponge material can be further adapted to other robotic designs. We used a metasponge car, where the swelling of wheels changes locomotion speed, behavior, and serves to sample liquid solution. We also demonstrated the feasibility of the metasponge as a medical device, including its use as an autonomous valve, a matrix for biocatalytic enzymes for biomarker sampling, and for drug delivery applications. The metasponge is limited to simple engineered functions, where rapid actuation and sophisticated communication are restricted by the slow growth kinetics (hours) and the requirement of being submerged in a liquid solution. The metasponge contributes to the field of smart materials through its potential to embed multiple functionalities and mechanical adaptability, unlike other systems that traditionally rely on tethered and electronic control. The actuation mechanism of the metasponge based on swelling can be accelerated using external fields and intrinsic design. Moreover, a thoughtful selection of active materials and their 3D design can lead to advanced functionalities. The simple fabrication, adaptability, and tunability of the metasponge hold exciting potential for reducing the gap between physical and computer intelligence, wherein the smart functional material could potentially serve as the logic element of autonomous machines.

Experimental Section
Metasponge Fabrication: The metasponge was fabricated by mixing two main components: 1) dry sodium polyacrylate powder (Carolina Scientific) and uncured liquid elastomer (Ecoflex 00-30, Smooth). The Ecoflex liquid solution was prepared by mixing a base and a curing agent in equal volumes. The powder was added to the mixed Ecoflex solution and mixed to generate a paste. The resulting paste was poured over a mold ( Figure S1, Supporting Information) and let in a vacuum pump for one minute, followed by incubation in an oven at 65°C to dry for 10 min to cure. This step can be done in a sequential manner to add an extra metasponge or elastomer layer. The molds were fabricated using a 3D printer (ANYCUBIC-Photon) or by engraving a clear cast acrylic sheet (McMaster-Carr) using a laser cuter (Versa Laser). In certain cases, color pigments were included in the metasponge or elastomer matrix for contrast purposes.
Metasponge Growth Kinetics: A sponge cube was used to analyze the growth of the metasponge. Each dry sponge was measured at its dry state first and then sequentially in different fluids for different time intervals. For revisability studies, sponges were left to dry overnight in a fume hood. The metasponge was incubated for 30 min to study the growth under different solvents. To evaluate the effect of different salt concentrations, the metasponges were left overnight in respective solutions for swelling. For deswelling studies, the growth was measure at different points over 90 min. The metasponge was incubated at different temperatures by placing water on a hot plate at 40°C, inside a fridge at 4°C, and at room temperature at 23°C. For the use of a sacrificial template, a commercially available sucralose (Splenda) was used to evaluate the effect, a 5 mm sponge was perforated with a hole puncher to make 1 mm holes through each face of a square metasponge.
Testing of Metasponge Physical Properties: A metasponge rectangle was used to evaluate the mechanical proprieties of the original and swollen states of the metasponge by manually stretching and rotating the metasponge. A 100 g weight was placed on top of a metasponge to evaluate resiliency over multiple compression cycles. The light-guiding capability of the metasponge optical fibers was quantified using a light source (OSL2 Fiber Illuminator, Thorlabs) and a power meter/spectrometer collection setup (high resolution spectrometer HR2000þ, Ocean Optics). The light was illuminated through an objective (100x). The illumination angle was controlled with an angular stage ( Figure S6, Supporting Information). The incident angle was changed to get a maximum light transfer through the fiber. Further, we have also measured the light transmission capability of the metasponge fiber under original and swollen conditions. The light was illuminated on one end of the metasponge fiber, and transmitted light was measured on the end of the metasponge fiber using a power meter (high-resolution spectrometer HR2000þ, Ocean Optics).
For acoustic measurements, a thin film of the material was placed between a 1 MHz piezoelectric immersion ultrasonic transducer (A302-AU 1.63" PTF, Olympus Corp.) and a broadband needle hydrophone (HNC-1000, Onda Corp.) in a tank filled with DI water. A 20-cycle sinusoidal tone burst at 1 MHz was generated by an arbitrary waveform generator (Agilent 33500B) and amplified using a 50 dB RF power amplifier (E&I 240L), before being fed to the piezoelectric transducer. The signal received by the hydrophone was amplified and recorded using a Picoscope 5242D oscilloscope. The recorded data were then processed using MATLAB.
www.advancedsciencenews.com www.advintellsyst.com Metasponge Actuators: A metasponge square was fabricated for bridgelike structures by placing a rigid PMMA rod and elastomer layer over the metasponge square top face to hold it in place. For the star experiments, a cast of metasponge material was cast first, followed by a secondary cast of elastomers only. Next, the metasponge was placed in a container and filled with water to evaluate growth. To camouflage the sponge, the elastomer layer was mixed with sand. Finally, the jellyfish metasponge was mixed with platinum black (Sigma Aldrich) and submerged in 3% peroxide solution.
Metasponge Wheels: A 1.5 mm poly (methyl methacrylate) sheet was cut using a laser cutter to generate a chassis. The interior of the chassis contained a chamber to place a 3 mm in diameter neodymium magnet (K&J Magnetics). Two metasponge wheels containing a passive material at its perimeter were added to the chassis. To enable the locomotion of the robocar, a 38 mm square neodymium magnet was placed under a flat table substrate and rotated by hand. The external magnet's magnetic force applies torque to the magnetic core embedded in the chassis. The robocar was incubated for two hours to perform comparative studies of dry and swollen wheels.
Similarly, for navigation in complex environment studies, the robocar was actuated through a chamber containing rocks and blue dyed water (for contrast) and left for 1 h to evaluate climbing ability. For liquid sampling assays, two 1.5 mm deep circular chambers were fabricated. The first chamber contained an acidic solution (pH 2), and the second chamber contained a sensitive colorimetric solution (Hank's balanced salt solution, Sigma Aldrich) that changes color from yellow at neutral pH (%7) to purple at an acidic environment.
Microfluidic Metasponge: A microfluidic chip was designed and constructed using laser cut acrylic sheets stacked together via adhesive sheets. A metasponge molded into the shape of a circle with a tab was place inside the chip. A gravity-based pump was placed at approximately 50 cm above the top surface of the microfluidic chip and 10 mL of water, colored with fluorescent paint, was added to the pump. The output of the liquid that passed through the microfluidic chip was collected, in containers, at 2-minute intervals and each container was measured for analysis.
Enzymatic Metasponge: The metasponge was fabricated as normal with the addition of 550 mg of Lactaid (commercially available lactase). The Lactaid and sodium polyacrylate were mixed first as a dry powder and then put into the elastomer. After mixing, the metasponge dried in a mold overnight. For experiments, each sponge was placed in a 35 mm Petri dish. About 4 mL of 2% skim milk was added to each petri dish and placed on a rotator mixer for a set period. Glucose measurements were taken with Keto-Mojo GK þ Sensors.
Metasponge Biomarker Sampling: Metasponge squares were incubated in an E. coli (ATTC, 25922GFP) solution and left incubated overnight. The metasponge was sliced into half and imaged using a confocal microscope to study the internalization of the bacteria into the gel. The metasponge containing nutrients (2:1:1, elastomer, sodium polyacrylate, LB broth powder) presented higher internalization when compared to bare metasponge. The metasponge was placed for one hour in 8-9 g dL À1 human hemoglobin (Sigma-Aldrich). Samples of hemoglobin-filled metasponge were then used with Pinnacle Biolabs' Second-Generation FIT Tests. For the guaiac procedure, samples were placed in a mixture of guaiac gum powder (Santa Cruz Biotechnology) with 200 proof ethanol and 3% hydrogen peroxide. Germaine Laboratories' AimStrip Hb Hemoglobin Meter was used to detect hemoglobin concentrations, and controls were measured against Germaine Laboratories' AimStrip Hb Control Set.
Metasponge for Drug Delivery: A PMMA (McMaster-Carr) 1.5 mm sheet was cut in the shape of a pill, containing two chambers for a 3 mm in diameter neodymium magnet and a central chamber to hold a metasponge. To absorb the molecules, the metasponge was incubated in a 1 mM methylene blue (model drug antiseptic) overnight. After fully swollen, it was left to dry overnight and placed in the interior of the pill chamber. A double-sided medical adhesive was used to hold in place the metasponge (ARcare@90 445, Adhesives Research). A porcine stomach purchased at the local supermarket was used as a model tissue to evaluate the locomotion of the metasponge pill. The actuation was performed as previously described in the robocar.

Supporting Information
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