Ginel C. Hill

Orientation angle and the adhesion of single gecko setae

We investigated the effects of orientation angle on the adhesion of single gecko setae using dual-axis microelectromechanical systems force sensors to simultaneously detect normal and shear force components. Adhesion was highly sensitive to the pitch angle between the substrate and the setas stalk. Maximum lateral adhesive force was observed with the stalk parallel to the substrate, and adhesion decreased smoothly with increasing pitch. The roll orientation angle only needed to be roughly correct with the spatular tuft of the seta oriented grossly towards the substrate for high adhesion. Also, detailed measurements were made to control for the effect of normal preload forces. Higher normal preload forces caused modest enhancement of the observed lateral adhesive force, provided that adequate contact was made between the seta and the substrate. These results should be useful in the design and manufacture of gecko-inspired synthetic adhesives with anisotropic properties, an area of substantial recent research efforts.

Effect of fibril shape on adhesive properties

Research into the gecko’s adhesive system revealed a unique architecture for adhesives using tiny hairs. By using a stiff material (β-keratin) to create a highly structured adhesive, the gecko’s system demonstrates properties not seen in traditional pressure-sensitive adhesives which use a soft, unstructured planar layer. In contrast to pressure sensitive adhesives, the gecko adhesive displays frictional adhesion, in which increased shear force allows it to withstand higher normal loads. Synthetic fibrillar adhesives have been fabricated but not all demonstrate this frictional adhesion property. Here we report the dual-axis force testing of single silicone rubber pillars from synthetic adhesive arrays. We find that the shape of the adhesive pillar dictates whether frictional adhesion or pressure-sensitive behavior is observed. This work suggests that both types of behavior can be achieved with structures much larger than gecko terminal structures. It also indicates that subtle differences in the shape of ...

Development of an SU-8 Fabry-Perot blood pressure sensor

This paper presents the fabrication method and testing of an interferometric pressure sensor designed for intravascular blood pressure measurements. A cap containing a pressure-sensing diaphragm was mounted onto the end of a fiber optic cable. The Fabry-Perot interferometer, formed between the reflective diaphragm and the fibers end, measured the diaphragm deflection. Microfabricated from the biocompatible polymer, SU-8, the device is fast, simple, and inexpensive to manufacture. Its small dimensions (<300/spl mu/m outer diameter) reduce the risk of infection and thrombosis and allow for its insertion into small vessels. The sensor showed a linear response to pressure from 0 to 125 mmHg with approximately 1-2 mmHg resolution. The use of an optical displacement transducer allowed a series of careful measurements of drift and hysteresis of SU-8 materials in different environments. This data may be valuable to other researchers working with SU-8.

Patterned cracks improve yield in the release of compliant microdevices from silicon-on-insulator wafers

The safe release of compliant structures is an ongoing challenge in microfabrication. The buried oxide (BOX) layer of silicon-on-insulator wafers is useful as an etch stop or sacrificial layer. However, when freed during processing, the BOX layer can buckle and crack from compressive stress, and these cracks can threaten the survival of delicate devices above the BOX layer. This work reports on the use of cracks patterned lithographically into the BOX layer prior to device release in two separate microcantilever fabrication processes. In both processes, the patterned cracks were found to inhibit spontaneous cracking in critical regions near or under devices and improve device yield. In the first process, the average yield of ultrasoft silicon cantilevers for magnetic resonance force microscopy improved by more than 60%. In the second process, the yield of piezoresistive silicon cantilevers for high-frequency force detection improved by more than 95% with the use of patterned cracks.

Patterned cracks in the buried oxide layer improve yield in device release from SOI wafers

Cracks patterned lithographically into the buried oxide (BOX) layer of silicon-on-insulator wafers were found to substantially improve yield in the release of ultrasoft silicon cantilevers. The BOX layer is useful as an etch stop and sacrificial layer in microfabrication. However, compressive stress in the BOX membrane can cause it to buckle and crack when released. These cracks can damage delicate structures in the device layer and reduce yield. Cracks were patterned into the BOX layer prior to avoid critical regions near or under devices; these patterned cracks reduced damage to devices from spontaneous cracks in the BOX layer and improved yield.

Force Sensing Optimization and Applications

Piezoresistance is commonly used in micro-electro-mechanical systems for transducing force, pressure and acceleration. Silicon piezoresistors can be fabricated using ion implantation, diffusion or epitaxy and are widely used for their low cost and electronic readout. However, the design of piezoresistive cantilevers is not a straightforward problem due to coupling between the design parameters, constraints, process conditions and performance. Here we discuss the equations and design principles for piezoresistive cantilevers, and present results from cantilevers and systems that we have developed for probing, mechanics studies and sensing, especially for low stiffness or large bandwidth applications.

Piezoresistive Cantilever Optimization and Applications

Piezoresistors are commonly used in microsystems for transducing force, displacement, pressure and acceleration. Silicon piezoresistors can be fabricated using ion implantation, diffusion or epitaxy and are widely used for their low cost and electronic readout. However, the design of piezoresistive cantilevers is complicated by coupling between design parameters as well as fabrication and application constraints. Here we discuss analytical models and design optimization for piezoresistive cantilevers, and describe several applications ranging from studying electron movement using scanning gate microscopy to measuring the biomechanics of whole organisms.