Osmosis-driven actuation offers a promising strategy for developing untethered, environmentally responsive soft and shape shifting mechanisms and robots. In this work, we explore the use of superabsorbent polymer (SAP) pellets as large-scale, shape-morphing actuators. Upon exposure to water, these approximately 2mm diameter spherical pellets undergo a dramatic volumetric expansion, up to 300 times their initial volume, generating actuation forces of approximately 10 N under constrained conditions. We further demonstrate reversible cyclic actuation via controlled swelling-deswelling using ethanol-water solutions. Finally, we integrate these systems into a shape-morphing wheel design to enable adaptive locomotion that passively transitions between terrestrial and aquatic environments. Our findings demonstrate SAP-based osmotic actuators as an environmentally-driven solution for soft robotics, and shape-shifting soft hybrid mechanisms.

To deploy robots outside of laboratory environments, they must be able to locomote on natural, unstructured terrain. While perception and control strategies for terrain navigation and obstacle avoidance have been developed for human-scale robots, microrobots are often too small to carry the sensors, computing power, and energy required to implement such techniques. Instead, this work presents passive foot designs for improving open-loop, quasi-static locomotion of a 1.6 g, 45 mm quadruped robot over rough terrains. The feet were evaluated by tracking the distance travelled on an uneven terrain of progressively increasing feature heights. Our insect tarsi inspired rigid foot designs improved performance somewhat, and by adding passive compliance via a viscoelastic hinge on the heel and toe, we increased the distance travelled by 168% over the original design. By exploring the design space of foot geometries and compliance, this work lays the foundation for understanding how passive foot design facilitates locomotion over uneven terrains.

Insects can locomote readily in challenging environments, such as over steep inclines and across obstacle-laden terrains, which still frustrate robots of similar size. In this work, inspired by the passive compliant properties of insect limbs, we use the insect-scale Harvard Ambulatory Microrobot and multilayer microfabrication techniques as platform to study the ability of passive mechanisms to improve open-loop running in rough terrains. We tested the performance of different limb designs in vivo, exploring how the magnitude, directionality, and distribution of compliance incorporated into the leg impacted robot performance. Limbs were evaluated on both a featureless substrate and an increasingly-adversarial 3D-printed terrain designed to mimic natural environments. We tested the limbs using a trotting gait in the quasi-static (2 Hz) and body dynamics (25 Hz) stride frequency regimes. Performance was reported as bodylengths traveled per gait cycle on the featureless substrate, and as the largest feature height the robot was able to overcome in the terrain. The work presented here provides design principles for a passive limb that expands the terrain accessible to small robot; we find a limb with a single asymmetrical joint is able to improve quasi-static terrain traversal by 203% relative to a rigid limb.

Compliant robotic grippers are more robust to uncertainties in grasping and manipulation tasks, especially when paired with tactile and proprioceptive feedback. Although considerable progress has been made towards achieving proprioceptive soft robotic grippers, current efforts require complex driving hardware or fabrication techniques. In this paper, we present a simple scalable soft robotic gripper integrated with high-deformation strain and pressure sensors. The gripper is composed of structurally-compliant handed shearing auxetic structures actuated by electric motors. Coupling deformable sensors with the compliant grippers enables gripper proprioception and object classification. With this sensorized system, we are able to identify objects' size to within 33% of actual radius and sort objects as hard/soft with 78% accuracy.

In this paper, we present sensor-controlled antagonistic pneumatic actuators (SCAPAs) that integrate proven soft robotic actuators and sensors into a simplified, controllable design. The antagonistic actuators together compose a bidirectional bending actuator with embedded capacitive strain sensors. By designing the SCAPAs from the ground-up for closed-loop control, we are able to minimize both the number of constituent components and the types of materials used, and further streamline the manufacturing processes. These improvements are embodied in the multipurpose use of a single conductive fabric sheet for both actuation and sensing, integrated into an otherwise all-silicone device. Such reduced material complexity allows us to use simple finite element analysis (FEA) models to predict the performance of a given design. We compare various designs to maximize sensor effectiveness using FEA and experimentally verify the suitability of select designs for state reconstruction. After converging on our final design, we demonstrate that this design evaluation process enables the use of simple control strategies to achieve closed-loop control.

In this paper, we describe the fabrication and testing of a stretchable fabric sleeve with embedded elastic strain sensors for state reconstruction of a soft robotic joint. The strain sensors are capacitive and composed of graphite-based conductive composite electrodes and a silicone elastomer dielectric. The sensors are screenprinted directly into the fabric sleeve, which contrasts the approach of pre-fabricating sensors and subsequently attaching them to a host. We demonstrate the capabilities of the sensor-embedded fabric sleeve by determining the joint angle and end effector position of a soft pneumatic joint with similar accuracy to a traditional IMU. Furthermore, we show that the sensory sleeve is capable of capturing more complex material states, such as fabric buckling and non-constant curvatures along linkages and joints.

Materials exhibiting variable stiffness properties have great potential for use in the growing field of soft robotics. Soft structural materials allow a robot to fit into enclosed spaces, resist shock and vibration, or even reconfigure its geometry and adapt to various environments. Rigid structural materials on the other hand allow environmental interactions through application of force and load-bearing capabilities. Materials that can be selectively switched between these two extremes could greatly expand the functionality of a robot that requires the properties of both. In this paper, we introduce a conductive epoxy composite that is self-softening through Joule heating via direct application of electrical current. The polymer can then become load-bearing and rigid again after being formed into a new shape. We demonstrate the capabilities of this material by attaching a pneumatic actuator and showing that the resulting variable stiffness device can be softened from its initial rigid state, reshape itself using the pneumatic actuator, then become rigid again and hold this new position without additional power being supplied to the actuator.

Future generations of wearable robots will include systems constructed from conformable materials that do not constrain the natural motions of the wearer. Fabrics represent a class of highly conformable materials that have the potential for embedded function and are highly integrated into our daily lives. In this work, we present a robotic fabric with embedded actuation and sensing. Attaching the same robotic fabric to a soft body in different ways leads to unique motions and sensor modalities with many different applications for robotics. In one mode, the robotic fabric acts around the circumference of the body, and compression of the body is achieved. Attaching the robotic fabric in another way, along one surface of a body for example, bending is achieved. We use thread-like actuators and sensors to functionalize fabric via a standard textile manufacturing process (sewing). The actuated fabric presented herein yields a contractile force of 9.6N and changes in length by approximately 60% when unconstrained. The integrated strain sensor is evaluated and found to have an RMS error of 14.6%, and qualitatively differentiates between the compressive and bending motions demonstrated.