Photoresists are light-sensitive resins used in a variety of technological applications. In most applications, however, photoresists are generally used as sacrificial layers or a structural layer that remains on the fabrication substrate. Researchers in the Allbritton Group, as described in the Journal of Micromechanics and Microengineering, have fabricated thin layers of patterned 1002F photoresist released to form a freestanding film. Films of thickness in the range of 4.5–250 µm were patterned with through-holes to a resolution of 5 µm and an aspect ratio of up to 6:1. Photoresist films could be reliably released from the substrate after a 12 h immersion in water. The Young's modulus of a 50 µm-thick film was 1.43 ± 0.20 GPa.
The investigators demonstrated the use of the films as stencils for patterning sputtered metal onto a surface, and these 1002F stencils were used multiple times without deterioration in feature quality. Furthermore, the films provided biocompatible, transparent surfaces of low autofluorescence on which cells could be grown. Culture of cells on a film with an isolated small pore enabled a single cell to be accessed through the underlying channel and loaded with exogenous molecules independently of nearby cells. Thus 1002F photoresist was patterned into thin, flexible, free-standing films that will have numerous applications in the biological and MEMS fields.
A cell separation strategy capable of the systematic isolation and collection of moderate to large numbers, 25–400, of single cells into a targeted microwell was reported in Analyst in a collaborative work between the Allbritton and Ramsey groups. An array of microfabricated, releasable, transparent micron-scale pedestals termed pallets and an array of microwells in poly(dimethylsiloxane) (PDMS) were mated to enable selective release and retrieval of individual cells.
Cells cultured on a pallet array mounted on a custom designed stage permitted the array to be positioned independently of the microwell locations. Individual pallets containing cells were detached in a targeted fashion using a pulsed Nd:YAG laser. The location of the laser focal point was optimized to transfer individual pallets to designated microwells. In a large-scale sort (n = 401), the accuracy, defined as placing a pallet in the intended well, was 94% and the collection efficiency was 100%. Multiple pallets were observed in only 4% of the targeted wells. In cell sorting experiments, the technique provided a yield and purity of target cells identified by their fluorescence signature of 91% and 93%, respectively. Cell viability based on single-cell cloning efficiency at 72 h post collection was 77%.