UC Berkeley

Smart materials initiate drastic physical property changes in response to external stimuli such as chemical environment, light illumination, temperature change, electrochemical reaction and electrostatic field etc. The physical property changes normally include shapes, colors, and structural properties. Moreover, the responsive properties can be programmed in a controllable manner by controlling the external stimuli which are useful for various applications such as smart textiles and apparel, chemical sensors, and intelligent medical instruments. Smart materials can be categorized by the external stimuli they respond to such as electro active polymers, electrostrictive polymers, light responsive materials, shape memory alloys, and thermal responsive materials etc.. The most commonly seen smart materials are based on polymers and in a few cases they can be inorganic materials. Therefore, it can be well expected that the responsive mechanism are very diversified. In response to the external stimuli, the physical property change can be induced by a phase transformation, mass transport, microstructure change or as simple as physical stretch.

In particular, smart materials that can initiate large strain change or large deformation have very useful applications in artificial muscle, medical surgery, smart curtains and camouflage etc.. While the applications are diversified, there are no single criteria to describe the figure of merit. In general, to demonstrate useful applications, it requires smart materials to have fast responsive, large force, large strain and easy operation. My research centers on the development of smart materials that have fast response, large force, and a simple actuation mechanism. The general strategy for my research is to utilize the superior mechanical, thermal and optical properties of carbon nanotubes to make nanocomposites.

In the first part of my work, a simple approach is described to fabricate reversible, thermally- and optically responsive actuators utilizing composites of poly(N-isopropylacrylamide) (pNIPAM) loaded with single-walled carbon nanotubes. With nanotube loading at concentrations of 0.75 mg/mL, I demonstrated up to 5 times enhancement to the thermal response time of the nanotube-pNIPAM hydrogel actuators caused by the enhanced mass transport of water molecules. Additionally, I demonstrated the ability to obtain ultrafast near-infrared optical response in nanotube-pNIPAM hydrogels under laser excitation enabled by the strong absorption properties of nanotubes. The work opens the framework to design complex and programmable self-folding materials, such as cubes and flowers, with advanced built-in features, including tunable response time as determined by the nanotube loading.

In the second part of my work, I demonstrate a very easy and low-cost fabrication route for making single-walled carbon nanotube/polycarbonate based bilayer thin film materials that can be operated solely by solar light. The as-made nanocomposite materials possess superb light responsive properties such as fast response (less than 1 second), extreme solar light sensitivity (as low as 10 mW/cm2), very large deformation (90 degrees), and reversible actuation. The underlying mechanism is found to be the thermal expansion mismatch between the carbon nanotubes and polycarbonate, and the fast actuation response and large deflection can be attributed to the excellent light absorbing property, high thermal conductivity, very mechanically coherent interface, large Young's modulus, and small thermal expansion coefficient of the carbon nanotubes. I have also shown that by utilizing nanotubes with different chiralities, chromatic actuators that are selected responsive to specific wavelength band can be achieved. As a benchmark for fast and reversible light responsive material, a fast moving light-driven motor has been demonstrated. The easy-to-make and low-cost materials open the ways for many advanced photomechanical as well as biological applications.

In the third part of my work, theoretical and experimental results are presented for low voltage and large deformation electrostatic nanowire actuators. The design utilizes nanowire forest to enhance the electrostatic field. The goal of this work is to address the high operation voltage challenges of conventional dielectric elastomer actuators. Although there are still experimental challenges, my work sheds light on the validity of the design concept. Further progress can leads to huge breakthroughs in artificial muscles and microrobots.