Soft Robotics, Wearable Devices and Biomedical Devices
Soft robotics are expected to better incorporate human-machine-environment interactions and have the potential to revolutionize cooperative robots for healthcare and military applications. We have been working on the grand challenge of “soft grasping”. One approach is to use smart materials with tunable stiffness as ligaments for biomimetic fingers, which greatly simplifies the design. The following images first show the bending of one individual finger and then grasping and twisting of objects using improved version of three such fingers:
Another approach to soft grippers is to use reversible dry adhesion to achieve grasp-and-release of objects. This once again is enabled by smart materials with tunable stiffness. Using this approach, we have achieved soft grasping for objects with flat and smooth surfaces. We are working on ways to generalize to objects with arbitrary geometries. Below on the left is a schematic showing how the dry adhesion of such a soft gripper can be tuned, and on the right is an image of such a soft gripper picking up a glass piece:
We have also worked on biomedical devices such as drug eluting stents, as well as soft shear sensors that can be incorporated into wearable devices. Please refer to Publications [3,4,9,13,15] for more details of our work on these critical applications.
Materials Engineering and Processing
Smart materials with tunable functionalities have found ample applications in soft robotics, wearable devices and biomedical devices. We have been working on smart composite materials with tunable stiffness. One design for this is a multi-layered composite containing both a soft heater and a rigidity tunable component such as shape memory polymer or low melting point alloy. We have done some thermal analysis on such a design and concluded that its activation is limited by heat transfer across the many layers. A much improved design is achieved by combining the soft heater and rigidity tunable component into one functional layer. We did this by mixing carbon black particles into co-polymer matrix, and then embed the mixture into a PDMS elastomeric matrix to form the final rigidity tunable elastomer:
We are currently working with colleagues at UNR and ASU on design of multifunctional composite materials with desired mechanical and thermal functionalities for soft robotics applications. Please refer to Publications [10-12] for more details of our work in this area.
Mechanics of Solids and Structures
For this topic, we have been mostly working on mechanical instabilities of thin structures such as reinforced thin rod buckling, and failure of engineering materials and devices. These fundamental mechanics problems are encountered in design and control of soft robotics, biomedical devices, and other engineering structures. Some of these also play important roles in creatures and structures found in nature.
For example, microtubules in cytoplasm may play an important role in the mechanical integrity of single cells. Their presence, together with the viscoelastic cytoplasm, can support much higher force than either of the two could. The mechanics of how this works has not been well-understood yet. We have used a table top macroscopic experiment to investigate the mechanics of reinforced thin elastic rod buckling within elastic medium. Figure below on the left shows the experimental setup and theoretical modeling tools we utilized, while figure below on the right shows the force-displacement curves for different rod sizes and different interfacial bonding conditions between the rod and the surrounding media.
We have also investigated the large deformation behavior such as failure of such composite structures to pinpoint the mechanical role microtubules play within single cells. We are currently working on the buckling behavior of thin wires under more complex loading conditions and boundary conditions. The research findings can also be used in design of biomedical devices such as a robotic needle. Please refer to Publications [6,8,16,17] for more details.
Cost for failure, fracture or fatigue, of engineering materials and structures is huge. We have investigated fracture failure of biomedical devices, small crack growth problem in stainless steels and fatigue initiation of LIGA Ni MEMS thin films. The design of new composite materials for novel applications should also incorporate failure-resistance component from the very beginning.
For example, the third generation of coronary drug eluting stents are a composite structure containing hundreds of sub-milimeter scale reservoirs with suspended polymeric formulations on its metallic structs (figure below to the left). The mechanical strength of this hybrid structure and the safety of use of this biomedical devices is dictated by the interface or coupling between polymeric formulation and metallic structs. We have investigated the failure behavior of these biomedical devices both before implantation and after implantation under physiological conditions using a punch test (figure below to the right).
Please refer to Publications [2-5,7] for more details of our work in this area.
There are fundamental thermal science problems inherent in the design of smart composite materials involving Joule heating. We use theoretical and numerical methods to tackle these fundamental challenges. Examples include thermal actuation within a multi-layered rigidity tunable composites and inverse heat transfer problem within a homogeneous conductive elastomer for rigidity tuning. Please refer to Publications [11,14] for more details.
To meet the ever increasing energy demand from human society, new technologies need to be developed to enhance energy usage efficiency and harvest energy from nature or surrounding environments. We have worked on nanofluids that can potentially enhance heat transfer rates (Publication ). We are also interested in materials that can be used in energy harvesting devices such as solar panels.