Pulsed Laser Deposition of Collagen Thin Films
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and the residual acid and buffer impurities left behind in the film after evaporation. To deposit thin films of collagen, we have used pulsed laser deposition (PLD). Collagen has been ablated by low wavelength UV radiation in clinical studies investigating the utility of lasers in corneal surgery. However, these studies were not concerned with film formation, 2 but rather with the clean etching of the remaining collagen target. Both ArF (193 nm) and KrF (248 nm) UV lasers have been successfully used to etch cleanly the surface of collagen without thermal or photo degradation of the remaining material. At UV wavelengths the clean etching of collagen has been described as a photochemical unzipping of the molecule. 3 Reformation of the polymer as a coherent thin film on a substrate via this same photoablative unzipping mechanism has been demonstrated with other organic polymers such as polyimide4 and polytetrafluoroethylene. 5 Thus, the reformation of collagen on a substrate to form thin coherent films would also be likely. In this paper, the pulsed laser deposition of collagen films with preserved chemical functionalities and surface morphologies that are a function of the PLD growth parameters is reported. EXPERIMENTAL Collagen (Type I) pellets for deposition targets were pressed from commercially available bovine Achilles tendon (Sigma) without further purification. Thin films of collagen were prepared in a PLD vacuum apparatus, as described in detail elsewhere. 6 The stainless steel deposition chamber had a base pressure of 10-6 Torr. Depositions were conducted at 300 mTorr background gas pressure. The ability to deposit films by PLD in a reactive background gas has been shown to be an advantage to the technique for a variety of different materials. For example, for PLD of hydroxyapatite (HA), water vapor has been shown to be necessary in the deposition gas 65 Mat. Res. Soc. Symp. Proc. Vol. 414 ©1996 Materials Research Society
environment to maintain the hydroxyl groups on the molecule. 6 Anticipating possible integration of PLD of collagen with PLD of HA, a water vapor enriched inert gas environment (produced by bubbling Ar through water prior to introduction to the chamber) was selected as the reactive gas environment for this study. PLD of collagen in this reactive gas environment was compared to PLD of collagen in inert Ar. Si (100) was selected as the substrate because it is transparent in the IR and allowed characterization of the films by FT-IR. The substrate was positioned normal to the target pellet at a distance of approximately 4 cm. The beam from a KrF (248 nm) excimer laser was focused on the pellet at a 450 angle of incidence. The energy density of the beam ranged from 0.2 - 1.5 J/cm 2 , with an energy of 250 mJ/pulse, -30 ns pulse width, and repetition rate of 10 Hz. Each film was deposited at room temperature with 10,000 shots of the laser. Monitoring the temperature of the substrate during deposition by means of a thermocouple attached to the substrate surface demonstrated that the temperature
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