Liu Wang

A graphene-based electrochemical device with thermoresponsive microneedles for diabetes monitoring and therapy

Owing to its high carrier mobility, conductivity, flexibility and optical transparency, graphene is a versatile material in micro- and macroelectronics. However, the low density of electrochemically active defects in graphene synthesized by chemical vapour deposition limits its application in biosensing. Here, we show that graphene doped with gold and combined with a gold mesh has improved electrochemical activity over bare graphene, sufficient to form a wearable patch for sweat-based diabetes monitoring and feedback therapy. The stretchable device features a serpentine bilayer of gold mesh and gold-doped graphene that forms an efficient electrochemical interface for the stable transfer of electrical signals. The patch consists of a heater, temperature, humidity, glucose and pH sensors and polymeric microneedles that can be thermally activated to deliver drugs transcutaneously. We show that the patch can be thermally actuated to deliver Metformin and reduce blood glucose levels in diabetic mice.

Conformability of a Thin Elastic Membrane Laminated on a Soft Substrate With Slightly Wavy Surface

When laminating a thin elastic membrane on a substrate with surface roughness, three scenarios can happen: fully conformed (FC), i.e., the membrane completely follows the surface morphology of the substrate without any interfacial gap, nonconformed (NC), i.e., the membrane remains flat if gravity is not concerned, and partially conformed (PC). Good conformability can enhance effective membrane-to-substrate adhesion strength and can facilitate signal/heat/mass transfer across the interface, which are of great importance to soft electronics laminated on rough bio-tissues. To reveal governing parameters in this problem and to predict conformability, energy minimization is implemented after successfully finding the substrate elastic energy under partially conformable contact. Four dimensionless governing parameters involving the substrate roughness, membrane thickness, membrane and substrate elastic moduli, and membrane-to-substrate intrinsic work of adhesion have been identified to analytically predict the conformability status and the area of contact. The analytical prediction has found excellent agreement with experimental observations. In summary, an experimentally validated quantitative guideline for the conformability of elastic membrane on soft corrugated substrate has been established in the four-parameter design space.

A Thin Elastic Membrane Conformed to a Soft and Rough Substrate Subjected to Stretching/Compression

Nanshu Lu Department of Aerospace Engineering and Engineering Mechanics, Center for Mechanics of Solids, Structures and Materials, The University of Texas at Austin, 210 E. 24th Street, Austin, TX 78712; Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712; Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78712; Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712 e-mail: nanshulu@utexas.edu A Thin Elastic Membrane Conformed to a Soft and Rough Substrate Subjected to Stretching/Compression

Conformability of a Thin Elastic Membrane Laminated on a Rigid Substrate With Corrugated Surface

When laminating a thin elastic membrane on a substrate with surface roughness, three scenarios can happen: 1) fully conformed, i.e., the membrane completely follows the surface morphology of the substrate without any interfacial gap; 2) partially conformed; and 3) nonconformed, i.e., the membrane remains flat if gravity is not concerned. Good conformability can enhance effective membrane-to-substrate adhesion and can facilitate heat/signal transfer across the interface, which are of great importance for micromembranes or nanomembranes transferred on target substrates and for flexible electronics laminated on rough biotissues. To reveal the governing parameters in this problem and to predict the conformability, energy minimization method is implemented with two different interfacial models, adhesion energy versus traction-separation relation. Depending on the complexity of the models, one to four dimensionless governing parameters have been identified to analytically predict the conformability status and the point of delamination if partial conformability is expected. In any case, partial conformability is achieved only when membrane energy is considered.

Suction effects in cratered surfaces

It has been shown experimentally that cratered surfaces may have better adhesion properties than flat ones. However, the suction effect produced by the craters, which may be chiefly responsible for the improved adhesion, has not been properly modelled. This paper combines experimental, numerical simulation and analytical approaches towards developing a framework for quantifying the suction effect produced by isolated craters and cratered surfaces. The modelling approach emphasizes the essential role of large elastic deformation, while the airflow dynamics, microscopic mechanisms, like surface tension and air permeation, and rate-dependence are neglected. This approach is validated using experimental data for isolated hemi-spherical craters. The modelling approach is further applied to spherical cap (not necessarily hemi-spherical) craters with the objective of identifying optimal geometric and material properties, as well as the minimum preload necessary for attaining the maximum suction force. It is determined that stiff polymers with deep craters are capable of producing large suction forces. For soft materials, central to biomedical applications, large suction forces can be attained by reinforcing deep craters with thin stiff layers. Parametric optimization studies of reinforced craters reveal that some of them perform beyond common expectations. However, those high-performance reinforced craters are prone to surface instabilities, and therefore the practical use of such craters may be problematic.

Stretchability, Conformability, and Low-Cost Manufacture of Epidermal Sensors

Epidermal sensors and electronics represent a class of artificial devices whose thickness, mass density, and mechanical stiffness are well-matched with human epidermis. They can be applied as temporary transfer tattoos on the surface of any part of human body for physiological measurements, electrical or thermal stimulation , as well as wireless communications. Except for comfort and wearability, epidermal sensors can offer unprecedented signal quality even under severe skin deformation. This chapter tries to address two fundamental mechanics challenges for epidermal sensors: first, how to predict and improve the stretchability and compliance when epidermal devices are made out of intrinsically brittle and rigid inorganic electronic materials; and second, when laminating on human skin , how to predict and improve the conformability between epidermal devices and the microscopically rough skin surfaces. Since the ideal use of epidermal devices would be one-time, disposable patches, a low cost, high throughput manufacture process called the “cut-and-paste” method is introduced at the end of this chapter.

Suction effects of craters under water

Octopus-inspired cratered surfaces have recently emerged as a new class of reusable physical adhesives. Preload-dependent adhesion and enhanced adhesion under water distinguish them from the well-studied gecko-inspired pillared surfaces. Despite growing experimental evidence, modeling frameworks and mechanistic understanding of cratered surfaces are still very limited. We recently developed a framework to evaluate suction forces produced by isolated craters in air. In this paper, we focus on underwater craters. The suction force-preload relation predicted by this framework has been validated by experiments carried out with an incompressible fluid under small and moderate preloads. Our model breaks down under a large preload due to multiple possible reasons including liquid vaporization. A direct comparison between liquid and air-filled craters has been carried out and the dependence on the depth of water has been revealed. We find that the suction forces generated by underwater craters scale with the specimen modulus but exhibit non-monotonic dependence on the aspect ratio of the craters.