Session G21: Soft Robotic Matter III: Biohybrid and Synthetic Meso/Microrobots

Fabrication and Modeling of Hydrogel-based Microrobots for Advanced Functionalities

Microrobots have attracted plenty of attention due to their potential biomedical applications such as drug delivery, cell-manipulation, and non-invasive surgery. In the past years, the research has been focused on the mobilities of different microrobot designs. However, this focus has been changed to the advanced functionalities of microrobots recently along with the development of fabricating smart polymers at the microscale. These smart polymers such as hydrogels and liquid crystalline polymers are soft and stimuli-responsive so that they can deform significantly under stimulus like pH, temperature, and light. After applying the smart polymers into microrobots, applications such as active adaptive locomotion and micromanipulation can be achieved. We report on the fabrication and modeling of hydrogel-based microrobots for adaptive locomotion and micromanipulation under various stimulations. These microrobots with advanced functionalities may be useful for future bio-applications for transporting drugs in blood vessels with different sizes and manipulating cells actively.   

Two-dimensional mechanics with atomically thin solids at the air-water interface

Manipulation of micro-objects at the air-water interface brought broad interest from the micro-robotics community because of the intricate interplay between the mass and shape of the moving object and the property of the water surface. For example, a floating object induces three-dimensional curvatures to the surrounding water surface depending on its mass and shape, which leads to an attractive force to neighboring floating objects and the “Cheerios effect”. Reducing the thickness and mass of a floating object would reduce such long-range interactions, ultimately enabling two-dimensional mechanical motions without inter-object interactions. Here, we realized such two-dimensional mechanical platform made with atomically thin (~ 0.6 nm) monolayer MoS₂ films, by combining materials synthesis, precise patterning, and surface actuation. For this, we use laser patterning directly applied to MoS₂ on a water surface to generate arbitrary shapes and photoactivated surfactants to produce lateral, spatially controlled external forces. Using this platform, we study the translocation and controlled shape change of various MoS₂ films, the latter of which is related to the out-of-plane deformation of the MoS₂. Our results provide essential components for realizing programmable 2D micro-mechanical systems at the ultimate thickness limit.   

Curvature Controlled Phase Separation in Self-Propelled Colloidal Particle Systems

In many biological systems, the curvature and topology of the surfaces, on which cells or other organisms live, influence their collective properties. We find that Gaussian curvature also affects active colloidal systems and can lead to new dynamic phases like cyclic phase separation and dissipation states or positionally targeted clustering. We explain these phenomena by studying computationally the phase behavior of active particles confined to the surfaces of positively curved spheres and negatively curved gyroids. We show that geometrical effects decrease or increase the critical density of motility induced phase separation (MIPS) for positive or negative curvature, respectively, while topological effects dominate at very high curvature. We also prove theoretically that the change in the onset of clustering can be explained by the nature of parallel lines in spherical and hyperbolic space. Lastly, we observe that the dense MIPS clusters fluidize upon introducing curvature due to the increase of defect patterns within the crystalline order. Our findings demonstrate a promising tool to design the emergent behavior of active colloids and indicate a mechanism to control their clustering and dynamics for robotic and other applications.   

Fabrication and Simulation of Functional Electro-Thermal Micro-Origami

Origami inspired assemblages can convert 2D surfaces into 3D structures, which is beneficial for creating small-scale robots. With origami principles, one can first fabricate the micro-scale robots on a flat surface using standard photolithography based methods and then fold these systems into desired 3D configurations. These micro-origami have broad applications as bio-medical grippers, micro-containers, transducers, meta-materials, and more. This presentation will demonstrate a novel eletro-thermal micro-origami system developed by our group. This electro-thermal micro-origami can achieve rapid elastic folding for swift robotic motion and permanent plastic folding for 3D reconfiguration. Moreover, by integrating both elastic folding and plastic folding within the same device, these robots can achieve complex shape-morphing that is beyond the simple fold and unfold motions possible with current micro-origami systems. We also present a simulation platform that can capture the behavior of these micro-origami, allowing for design and optimization of these soft robotic systems.   

Chemical design of motile Janus droplets

Solubilizing, self-propelling droplets have emerged as a rich chemical platform for exploration of active behaviors, but isotropic droplets rely on spontaneous symmetry breaking to sustain motion. The introduction of permanent asymmetry, e.g. in the form of a biphasic Janus droplet, has not been explored previously as a comprehensive design strategy for active droplets, despite the widespread use of Janus structures in motile solid particles. Here, we uncover the chemomechanical framework underlying the self-propulsion of active, biphasic Janus oil droplets solubilizing in aqueous surfactant. We elucidate how droplet propulsion is influenced by the degree of oil mixing, droplet shape, and oil solubilization rates for a range of oil combinations. A key finding is that for droplets containing both a mobile (solubilizing) and non-mobile oil, the degree of partitioning of the mobile oil across the Janus droplets’ oil-oil interface plays a pivotal role in determining the droplet speed and swimming direction. In addition, spatiotemporal control over droplet swimming speed and orientation is demonstrated through the application of local thermal gradients applied via induced via joule heading and laser spot illumination. We also explore the interactions between collections of Janus droplets including the spontaneous formation of multi-droplet spinning clusters that rotate predictably based on symmetry. Our findings provide insights as to how the chemistry and structure of multiphase fluids can be harnessed to design microswimmers with programmable active and collective behaviors.   

Kinetics of self-folding at the microscale

3D shells can be obtained from the self-folding of 2D templates of interconnected panels, called nets. To design self-folding, one first needs to identify what are the nets that fold into the desired structure. In principle, different nets can do it. However, recent experiments and numerical simulations show that the stochastic nature of folding at the microscale might lead to misfolding and so, the probability for a given net to fold into the desired structure (yield) depends strongly on the topology of the net and experimental conditions. Thus, the focus has been on identifying what are the optimal nets that maximize the yield [1]. But, what about the folding time? For practical applications, it is not only critical to reducing misfolding but also to guarantee that folding occurs in due time. Here, we consider as a prototype the spontaneous folding of a pyramid.  We find that the total folding time is a non-monotonic function of the number of faces, with a minimum for five faces. We show that it is the interplay between two different sets of events (first and subsequent edge closing) that explains the non-monotonic behavior. Implications in the self-folding of more complex structures are discussed.   

Locomotion of magnetoelastic membranes

Achieving locomotion through viscous fluids is critical for the development of multifunctional microscale robots. We establish a theoretical framework for the actuation of magnetoelastic membranes composed of superparamagnetic particles. We develop a phase diagram for the dynamic modes of circular magnetoelastic membranes in precessing magnetic fields. Above a critical magnetic precession frequency, circumferential and radial waves propagate within the membrane. When we introduce hydrodynamic effects, two aspects are critical for membrane locomotion: the amplitude of the circumferential wave and the asymmetry of the membrane. The wave amplitude is controlled by a magnetoviscous parameter, and the inversion symmetry of the membrane is broken via truncation. By programming a magnetic field with simple steps, membrane swimming is achieved. These results apply to diverse membrane shapes and lay the foundation for predicting the locomotion of magnetoelastic membranes in viscous fluids.   

Bioinspired chemical motors for self-propelled microrobots

Self-propelled chemical motors provide new tools to power machines and devices at the microscale. Inspired by the locomotion of aquatic insects, a set of motors powered by Marangoni propulsive forces generated by surface tension gradients have been developed over the years. However, Marangoni motor systems present limitations in their applications due to poor performance, short lifetime, low efficiency, poor control, and the need for hazardous fuel chemicals or hazardous environments. We have developed a self-propelled micromotor system from a cephalopod-derived protein and an anesthetic metabolite capable of operating in physiological conditions. This protein motor system surpasses previous Marangoni motors due to its dynamic nanostructure, enhancing performance by consuming less fuel (energy efficiency by controlling its release) and increasing its lifetime. These motors offer great versatility as they can be coated on a wide array of substrates and materials across length scales, with opportunities as modular power sources for microrobots and small-scale devices. These bioinspired chemical motors enable the wider design of self-powered microrobots without limitations in their swimming media, with potential applications in drug delivery and environmental remediation.   

Bioinspired Robotic Actuators by Electrochemical Oxidation of Liquid Metal Droplets

Progress in artificial muscles rely on new architectures that combine soft matter with mechanisms for converting stimuli into mechanical work. These architectures depend on the intrinsic compliance of soft materials and liquids that matches natural muscles. Liquid metal, in particular eutectic gallium-indium (EGaIn), is promising for creating an artificial muscle because of its ability to generate significant force and shape change by low voltage stimulation, which electrochemically modifies the surface tension. Thanks to their tendency to alloy with metals such as copper, surfaces of EGaIn droplets can be locally constrained such that their deformation is translated into motion of copper, which enables a wide range of motions for robotic actuation.

We present a generalized framework for designing liquid metal actuators (LMA), where the EGaIn droplets are configured with copper pads to generate a desired robotic actuation. We demonstrate by theory and experiments that LMA performs with higher work densities at smaller length scales, making them a favorable actuator at smaller scales with volume constraints. We also show that common actuation modalities, including linear contraction, rotation, bending, and twisting, are achievable using a variety of biologically-inspired designs.   

Emergent biohybrid active materials

Conventional robots are made from hard materials. Bio hybrid robots use living cells and extracellular matrix (ECM), in addition to engineered scaffolds. These living materials become active through a self-assembly process. Initially, they are dispensed as a liquid mixture of cells and ECM. After the ECM cures, the cells interact with one another as well as with the extra cellular matrix. These interactions result in a collective behavior leading to the emergence of the active material, i.e., the tissue. We will show that this emergence relies on a phase transition process as a function of cell-cell distance. If the cell-cell distance is high then the cells cannot interact with each other, and the collective behavior is suppressed. With increasing cell density, cell-cell distance decreases to a critical value when the cells begin to interact. Cells then approach each other, and the collective behavior proceeds in a feed forward way, eventually forming an active tissue with ordered structure, i.e., cells undergo a disorder to order transition. For example, muscle tissues for biohybrid robots can be formed only when the initial cell density is above a critical value. A simple mathematical model will be presented that captures this phase transition process.   

3D printed living robots with self-training capabilities

Biohybrid robotic systems are designed based on the combination of synthetic materials and biological entities aiming to acquire improved performance or properties that are difficult to mimic by their artificial counterparts. By integrating biological components (i.e. skeletal muscle cells) in robotic systems, it is possible to implement some of the most desirable capabilities from such living entities, including self-organization, self-healing or adaptability.

The exploitation of 3D printing as a versatile fabrication tool that has opened new possibilities for the creation of advanced bio-hybrid robots. In our study, we develop a skeletal muscle-based swimming biobot formed by a 3D-printed serpentine spring integrated into a 3D cell laden scaffold. The spring skeleton provides dynamic mechanical self-stimulation during the cell differentiation process, promoting a greater maturation and alignment of the muscle fibers that results in a higher force output. Upon electrical stimulation, the biobot exerts a directional swimming motion at the liquid-air interface, achieving a maximum velocity of 800 µm/s, 791 orders of magnitude higher than the fastest skeletal muscle-based swimming biobot up to date, providing a useful tool to create advanced robotic systems with programmable actuation.   

Biohybrid robotic jellyfish for potential applications in biology, soft robotics, and ocean exploration

Using robotic systems to control live animals as biohybrid robots offers advantages, such as natural self-healing for damage tolerance and leveraging animal metabolism to offset power costs. In this work, we present a biohybrid robot that uses a microelectronic system to induce swimming in live jellyfish. By driving jellyfish bell contractions at faster frequencies than observed in natural behavior, we demonstrate enhanced swimming speeds of nearly threefold in laboratory and in situ experiments, with only a twofold increase in cost of transport to the animal. This suggests the possibility for both faster and more efficient swimming capabilities in jellyfish, and compared to other swimming robots, the microelectronic system uses 10 to 1000 times less external power per mass. The experimental results are consistent with a hydrodynamic model that uses morphological and time-dependent input parameters. With future work to increase maneuverability and incorporate various sensors, we can potentially use biohybrid robotic jellyfish as a tool to study jellyfish biology, address challenges in soft robotics, and monitor the ocean alongside traditional underwater vehicles.   

Imaging Guided Microrobot for In Vivo Biomedical Applications

While synthetic micromotors have been evaluated extensively under in vitro conditions for over a decade, their in vivo function has rarely been explored. In particular, existing micro/nanomotor platforms face major challenges for deep tissue imaging and motion control in vivo. I will introduce our works on photoacoustic computed tomography (PACT)–guided micromotors for applications in intestines in vivo. The micromotors enveloped in microcapsules are stable in the stomach and exhibit efficient propulsion in various biofluids once released. The migration of micromotor capsules toward the targeted regions in the intestines has been visualized by PACT in real time in vivo. Near-infrared light irradiation induces disintegration of the capsules to release the cargo-loaded micromotors. The intensive propulsion of the micromotors effectively prolongs the retention in intestines. The integration of the newly developed microrobotic system and PACT enables deep imaging and precise control of the micromotors in vivo and opens the door to a number of in vivo and clinical applications of synthetic motors.