Student Project Abstracts 2005 - Pluto - University of Washington

Hydrogel Materials for Two-Photon MicrofabricationMayen UdoetukNorfolk State UniversityDr. Joe Perry and Vincent ChenGeorgia Institute of TechnologyOBJECTIVEThe purpose of the research is to develop a hydrogel materialsystem for two-photon microfabrication. Problems that were addressedin this research project are:1) For what composition can sufficient crosslinking beachieved by ultra-violet (UV) exposure such that the patternedstructure will not wash away upon development?2) Can hydrophilic monomers be crosslinked using currenttwo-photon absorbing dyes?3) What are the requirements of the material and the structureto ensure that pores and other desired features within thestructure are maintained after swelling?Potential practical applications of these materials are, for example,microstructures to be used for cell adhesion, which canfind applications in tissue engineering, and structures that respondto the solvent environment for sensors or microfluidics. Mostresearch in microfabrication to date has been carried out with acrylateand epoxy type polymers. 1 However, in applications thatinvolve biological molecules or require the adhesion of cells ontopolymeric structures, biocompatible materials need to be used.Hydrogel systems have hydrophilic surfaces and are of particularinterest due to their ability to swell and de-swell, depending ontheir environment.RESEARCH METHODSThe research problems were approached by testing resins basedon hydrophilic monomers and two-photon absorbing dyes or UV initiators.Photolithography is a process of transferring spatial patternsonto a surface of a material by exposure to light with a mask. Stepsusually involved in this process include substrate cleaning, adhesionlayer formation, photoresist application, soft baking, mask alignment,exposure, development, and hard-baking.The hydrophilic monomers and crosslinker that were usedinclude N-vinylpyrrolidone (VP), hydroxyethylmethacrylate(HEMA), and ethyleneglycol-bis-methacrylate (EGMA). Resinscontaining various amounts of these compounds were prepared.The initiator used for UV exposure was 1-[4-(methylthio)phenyl]-2-methyl-2-morpholinopropan-1-one, (UV initiator #7).The amounts of initiator used were 0.1, 0.2, and 0.5 wt%, seetable 1.1). To ensure that the structures remained attached to thesubstrate, the surface of the substrate was modified with an adhesionpromoter (95% methanol and 5% 3-(trimethoxysilyl)propylmethacrylate). The adhesion promoter solution was spin-coatedonto microscope slides and cover slips, and baked on a hotplateat 90°C for 2 minutes.Five drops of the hydrophilic resin were placed on surfacetreated microscope slides and cover slips, and were exposed toUV light from a mercury UV lamp (254nm) for various times.Solubility tests were done in order to find a solvent that wouldwash away any resin left after exposure, while leaving the polymerizedstructure intact. All of the following were found toperform well as development solvents: dichloromethane, toluene,ethanol, dioxane, cyclohexane, dimethyl formamide, and 4-methyl-2-pentanone. 4-Methyl-2-pentanone was chosen for usein the experiments.Several sample geometries were tested but some gave an inadequateamount of crosslinking. The initial setup included theresin on top of a microscope slide modified with adhesion promoter,with the UV light shining down on it. Due to the thicknessof the resin, the UV light did not penetrate to the resin-substrateinterface in order to yield sufficient crosslinking. Two geometriesthat were found to provide sufficient exposure and crosslinking atthe interface are shown in diagram 1.1.Experiments were conducted to determine the swelling propertiesof the crosslinked materials. The dimensions of the lineswere measured before and after swelling with water at room temperature.The feature dimensions were measured using opticalmicroscopy.Bulk materials of the hydrogels were prepared using setup 1in order to better visualize the swelling. Fluorescence and transmissionimaging were used to characterize and document thefabricated structures. A laser was used for 3D microfabricationof structures 3 . A test structure consisting of a vertical stack ofcrossing lines was designed and fabricated using two-photon microfabrication.1-[4-(methylthiophenyl]-2-methyl-2-morpholinopropane-1-one (UV #7)was varied in the compositions. 0.1, 0.2, and 0.5 wt% were used.CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 123