Dr. Nanshu Lu
Dr. Nanshu Lu joined the Department of Aerospace Engineering and Engineering Mechanics at The University of Texas at Austin as an Assistant Professor in fall 2011. She received her bachelor’s degree in Solid Mechanics from the Department of Engineering Mechanics at Tsinghua University, Beijing in 2005. She obtained her Ph.D. in Mechanics of Materials from Harvard University working with Professors Zhigang Suo and Joost Vlassak in 2009. She then received Beckman Postdoctoral Fellowship and became a postdoctoral researcher working with Professor John Roger at the University of Illinois at Urbana-Champaign.
The goal of Dr. Lu’s research lab is to investigate the mechanics and materials of bio-integrated soft electronics. The ultimate goal is to help develop high-quality, multifunctional flexible electronics in forms that can conform with the soft, curvilinear and time-dynamic surfaces of the human body for sensing, stimulation and energy harvesting. A main focus of the lab is to train the next generation of scientists. They will form a diversified group that is conducive to learning and carrying out multidisciplinary research.
Dr. Lu’s group seeks to understand the fundamental mechanisms of the deformation and adhesion of bio-integrated stretchable electronics through both theoretical and experimental approaches. Solid Mechanics especially thin film mechanics and fracture mechanics are applied to understand the mechanical behaviors of the inorganic-organic hybrid systems, facilitated by numerical methods such as Finite Element Modeling (FEM) and experimental characterization such as tension, indentation and micro-imaging. Novel microfabrication technologies such as micro-transfer printing and polymer patterning will be applied to manufacture mechanically-optimized, bio-compatible stretchable electronics. They collaborate with electrical engineers for the integration of active electronic systems. To validate the biocompatibility and functionalities of the final devices They also collaborate with biomedical engineers and medical doctors to carry out in vivo tests.
Electronic tattoos with thickness, effective elastic modulus, bending stiffness, and areal mass density matched to the epidermis have been developed. Laminating such devices onto the skin leads to conformal contact and adequate adhesion based on van der Waals interactions alone, in a manner that is mechanically invisible to the user. Such devices have been demonstrated for noninvasive, in vivo monitor of EEG, EKG and EMG. It also represents a platform technology for potential incorporation of temperature sensors, strain gauges, LED, solar cells, antennas and so on.
Multifunctional Balloon Catheter
Materials that integrate directly with the thin elastic membranes of otherwise conventional balloon catheters can provide diverse, multimodal functionality suitable for clinical use. As examples, we present sensors for measuring temperature, flow, tactile, optical and electrophysiological data, together with radiofrequency electrodes for controlled, local ablation of tissue. Use of such ‘instrumented’ balloon catheters in live animal models illustrates their operation, as well as their specific utility in cardiac ablation therapy.
Unconventional Strain Gauges
Flexible Strain Gauges
We have fabricated flexible strain sensors that use thin ribbons of single-crystalline silicon on plastic substrates. The devices exhibit gauge factors of 43, measured by applying uniaxial tensile strain, with good repeatability and agreement with expectation based on finite-element modeling and literature values for the piezoresistivity of silicon. Using Wheatstone bridge configurations integrated with multiplexing diodes, these devices can be integrated into large-area arrays for strain mapping. High sensitivity and good stability suggest promise for the various sensing applications.
Thin Metal Films on Polymer Substrates
Effect of Adhesion
When a freestanding plastically deformable metal film is stretched, it ruptures by strain localization, and the elongation is less than a few percent. When the film is deposited on a polymer substrate, however, strain localization may be retarded by the substrate. We have stretched Cu films well adhered on Kapton substrates up to 50% and only few microcracks in Cu can be found. The in situ electrical resistance during elongation agrees with a theoretical prediction. Micrographs show that the strain localization and debonding coevolve.
Effect of Annealing
We observe much lower strains to failure (approximately 10%) for polymer-supported nanocrystalline metal films, the microstructure of which is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. Therefore, we propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth, and film debonding from the substrate.
Effect of Film Thickness
We perform uniaxial tensile tests on polyimide-supported copper films with a strong (111) fiber texture and with thicknesses varying from 50 nm to 1 mm. Films with thicknesses below 200 nm fail by intergranular fracture at elongations of only a few percent. Thicker films rupture by ductile transgranular fracture and local debonding from the substrate. The failure strain for transgranular fracture exhibits a maximum for film thicknesses around 500 nm. As the film thickness increases from 200 to 500 nm, a decrease in the yield stress of the film makes it more difficult for the film to debond from the substrate, thus increasing the failure strain. As the thickness increases beyond 500 nm, however, the fraction of (100) grains in the (111)-textured films increases. On deformation, necking and debonding initiate at the (100) grains, leading to a reduction in the failure strain of the films.
Thin Ceramic Islands on Polymer Substrates
For a flexible electronic device integrating inorganic thin-film islands on a polymer substrate, the polymer can deform substantially, but the inorganic islands usually fracture at small strains. When SiNx islands are directly fabricated on polyimide substrate, they rupture after polyimide is stretched by only 2%. When SiNx islands are fabricated on elastomer-coated polyimide substrate, they remain intact even when polyimide is stretched by 20%. Calculations confirm that the elastomer reduces the strain in the SiNx islands by orders of magnitude.
In one design of flexible electronics, thin-film islands of stiff functional materials are fabricated on a polymeric substrate. Experiments have shown that for a given amount of stretch, the islands exceeding a certain size may delaminate from the substrate. We calculate the energy release rate using a combination of finite element and complex variable methods. Our results show that the energy release rate diminishes as the island size or substrate stiffness decreases. We also obtain an analytical expression for the energy release rate of debonding islands from a very compliant substrate.
For more information, see Dr. Lu’s research web site.