Design

Materials

Process

Laser Micromachining of Polymers for Biomedical Applications

C.A. Brayfield¹, J.P. Leonard² June 2009
(1)Department of Bioengineering, (2)Department of Materials Science and Engineering University of Pittsburgh, Pittsburgh PA 15261

This project involves micromachining of complex shapes for fluid delivery in biomedical applications. The unique ability of the excimer laser to deliver intense beams of deep UV light is used to selectively vaporize virtually any material in selected regions. In laser micromachining, the beam is focused to a small spot on the surface of the material to be removed. The fluence is typically 1-5 J/cm², delivered in a nanosecond timescale pulse. Because photon energies (5.0 eV in the KrF system) are well above the bandgap of most materials, most energy is absorbed within a few nanometers the surface. The result is a rapid heating and vaporization of up to a few hundred nm of material with a single shot. Repeated pulses in the same spot will erode ever deeper to create holes or channels of controlled depth. Using micromachining, a wide array of holes, wells, and channels can be rapidly fabricated in polymers and ceramics commonly used in biological systems engineering.

Figure 1. SEM micrograph of holes micromachined in a polyethersulfone (PES) tube. Holes are placed at 100um spacing along the axis of the tube. Hole diameter is approximately 10um at the outer surface of the tube.

Figure 2. SEM micrograph of inside surface of PES tube showing single hole. Diameter of hole is under 5um at this surface.

In the current project it is necessary to open ports through the walls of a microporous polyethersulfone (PES) tube. It is especially challenging because of the curved shape and flexibility of the tube, which is easily crushed. Excimer micromachining is an ideal non-contact technique that can produce a series of identical holes running down the length of the tube. Micromachining such a porous structure is especially challenging due to the nonuniform way that material is removed.

The process is not without complications, such as heating of regions to the sides and below the micromachined area. This can cause degradation, stresses or viscoelastic flow, especially in polymers. As well we find that the plume of material ejected can be redeposited on the areas surrounding the hole, and surface chemistry can be altered due to the heating or other photochemical effects. We are currently pursuing several experimental challenges relating to sidewall slope control and hydrophilic/phobic surface modification.

Publications relating to this project

C.A. Brayfield, K.G. Marra, J.P. Leonard, X.T. Cui and J.C. Gerlach, "Excimer laser channel creation in polyethersulfone hollow fibers for compartmentalized in vitro neuronal cell culture scaffolds", Acta Biomaterialia 4, 244 (2008).