Microelectrode arrays now allow the simultaneous electrical recording and stimulation of large numbers of neurons in the central nervous system, providing a basis for major advances in neuroscience and in prosthetic devices for disorders such as epilepsy, deafness, and Parkinson's disease. However, the neuronal system is chemical as well as electrical, and for many of these applications, the development of miniature drug-delivery systems for in-vivo use is essential. This research has developed microvalves that can be integrated into these arrays to provide control over the chemical environment at the cellular level.

Microchannels for drug delivery are formed in the body of a multi-electrode probe by undercutting a selectively-etched boron-doped silicon and/or dielectric masking structure. The mask openings are then sealed using deposited dielectrics. In order to control fluid flow in these channels, integrated microvalves have been developed that require only two extra masks beyond the normal probe process. The normally-open valve structure uses pressure to deflect a corrugated circular diaphragm against a seat, blocking the flow path as needed.

Using stress-compensated silicon dioxide and silicon nitride corrugated diaphragms 400μm in diameter and 1-2μm thick, two different on-probe pneumatically-actuated valve structures were demonstrated. Two sacrificial 5μm-thick polysilicon layers sandwiched between dielectric layers were used to form the two chambers of the microvalves. Both valve types provide an open flow rate greater than 500pL/sec at an applied input pressure of 10kPa and meet the design target of a leak rate less than 25pL/sec at an actuation pressure of 35kPa.

Prototype thermopneumatically-actuated microvalves were also designed and fabricated. Simulations show that using the phase-change of a low-boiling-point liquid (42°C, cyclopentane), less than 10mW is required to generate a drive pressure of 35kPa, closing the valve in less than 20msec. The temperature rise in the surrounding tissue should be less than 2°C, making the valve safe for use in-vivo. This is the first reported microvalve suitable for on-probe use in a cellular drug-delivery system, and as a low-voltage low-power structure capable of high actuation pressure and throw, it should have many other applications as well.