MEMS fabrication process
Original designs for a variety of MEMS forceps were manufactured at the University of California Berkeley Microfabrication Laboratory. Forceps were constructed from two dimensional designs using boron doped silicon wafers (orientation (100), resistivity 0.02 ohm-cm). Photoresist was spun onto wafers and patterned using a photomask, then hard baked at 125°C. Deep etching was performed using the Bosch process in an STS plasma etcher. After cleaning wafers with acetone, piranha (H2SO4/H2O2), and deionised (DI) water rinse, a 1 μm thermal oxide coat was grown (1100°C, O2, steam). Oxide was removed from the backside of the wafer using 5:1 buffered oxide etch (aqueous hydrofluoric acid (HF) with ammonium fluoride). A ring of oxide was preserved at the outer diameter of the wafer, to preserve the full wafer thickness around the edge to prevent it from becoming too fragile to handle. Wafers were placed in 25% tetramethyl ammonium hydroxide at 60°C until the etch front reached the bottom of the plasma etched pattern (where only oxide windows remain). Wafers were rinsed in DI water and all oxide was removed with 49% HF. A 1 μm thick layer of wet thermal oxide was grown (1100°C, O2, steam) to remove sharp corners and stress concentrations, then all oxide was removed with 49% HF. Finally, silicon parts were obtained, and for some designs assembled with epoxy to a forceps actuator shaft.
Scanning electron microscopy
Scanning electron microscopy (EM) was done with a Jeol 6400. Silicon specimens were mounted on standard aluminium scanning EM stubs using colloidal carbon paint. Gold coating was not necessary for visualisation as the silicon alone is sufficiently conductive. Micrographs were obtained and stored digitally.
Forceps design and construction
Two different actuator designs were developed to control forceps operation. The initial design was an electrically heated thermal expansion actuator, which incorporated tweezer tips and heat sink fins as a single piece (fig 1A). A manual potentiometer was used to apply current to rapidly heat and cool the semiconductive silicon of the thermal actuator, causing expansion and contraction in length, to open and close the tips via a lever linkage.
Figure 1 Prototype MEMS intraocular forceps. (A) Scanning electron micrograph of an early version of micro-tweezers with thermal expansion actuator. The device incorporates heat sink fins into the body of the tweezers; electric (more ...)
The second generation of forceps used a more conventional mechanical actuation (figs 1B and 2). MEMS silicon forceps tips were joined to a 20 gauge stainless steel instrument shaft enclosing a spring loaded opening mechanism, which included a microcalibration system for fine adjustments of the forceps tip excursion. To maximise stability, the mechanical actuator itself is electrically activated, via wire connections to a control switch, much like the automated MPC scissor familiar to retinal surgeons.7
Figure 2 Scanning electron micrographs at different magnifications (A, B) of mechanically actuated MEMS forceps with serrated jaws. In the background is a commercial stainless steel subretinal forceps. The shaft of the mechanical actuator (more ...)
Several tip configurations were designed to be suitable for intraocular surgery. The stiffness of tips made for these forceps ranged from 1 nanonewton/μm to 100 micronewtons/μm.
For surgical testing in human cadaver eyes, an eye cup was formed by excising the cornea, iris, and lens, then filling the vitreous cavity with balanced salt solution after vitrectomy. Surgery was done in an “open sky” fashion under the operating microscope (Zeiss Op-Mi6, Carl Zeiss, Germany). For in vivo surgical testing, standard three port 20 gauge vitrectomy (Storz Millenium, Rochester, NY, USA) was performed on adult New Zealand White rabbits (Charles River Laboratories Inc, Wilmington, MA, USA), anaesthetised with 3–5% isoflourane mask inhalation (Baxter, Deerfield, IL, USA). Both lensectomy and lens sparing vitrectomies were done. The MEMS instruments were introduced into the eye through standard sclerotomies made with a 20 gauge microvitreoretinal (MVR) blade. At the completion of surgery animals were euthenised with intramuscular ketamine (30–50 mg/kg, Fort Dodge Animal Health, Ft Dodge, IA, USA) and xylazine (5–10 mg/kg, Phoenix Pharmaceutical Inc, St Joseph, MO, USA), followed by intramuscular sodium pentobarbital (>150 mg/kg, Schering-Plough, Kenilworth, NJ, USA) and bilateral thoracotomy. All rabbit experiments were done in accordance with UCSF committee on animal research guidelines.