We report photoinitiated chemical vapor deposition (piCVD), a gentle synthetic method for the preparation of ultrathin films (approximately 100 nm) of the hydrogel poly(hydroxyethyl methacrylate) (pHEMA). piCVD occurs near room temperature and requires only mild vacuum conditions. The deposited films swell rapidly and reversibly in buffer solution, and the swelling properties can be controlled via the deposition conditions. Analysis of the swelling data indicates that the mesh size of the hydrogel creates a selectively permeable coating. The mesh is large enough to allow small molecule analytes to permeate the film but small enough to prevent the transport of large biomolecules such as proteins. X-ray photoelectron spectroscopy (XPS) shows that the films decrease nonspecific adhesion of the protein albumin by nearly 8-fold over bare silicon. A dry process, piCVD is suitable for coating particles with diameters as small as 5 microm. The absence of solvents and plasmas in piCVD allows films to be directly synthesized on optode sensors without degradation of sensitivity or response time.
Several property requirements have led to the modification of hydrogels by incorporating functional groups. The current work seeks to achieve graded functional group incorporation into hydrogel thin films using the one-step technique of photoinitiated CVD (piCVD). The functional group pentafluorophenylmethacrylate (PFM) is copolymerized with hydroxyethyl methacrylate (HEMA). Because PFM reacts easily with amine groups, the incorporation of PFM results in a platform for subsequent functionalization. The graded copolymer confines the PFM to the near surface region (∼20 nm) allowing the control of the hydrogel film properties independently of the surface reactivity. Although homogeneous incorporation of PFM in the hydrogel matrix inhibits swelling, the swollen water content of pure pHEMA is nearly preserved in the graded copolymer. FTIR spectroscopy shows that the absorption peaks corresponding to the fluorinated phenyl ring in the graded copolymer disappear after functionalization with O,O-bis (2aminoethyl) polyethylene (PEG-diamine), suggesting a nearly complete conversion of the PFM bonds.
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