UC Berkeley

Polymers are widely used in bioengineering for a wide range of applications, including substrates for in vitro cell culture and scaffolds for in vivo tissue engineering. Because polymer surfaces are usually non-polar and exhibit low biocompatibility, surface chemical modification must be used to enhance biocompatibility. In this study, biopolymer surfaces were modified by various plasma treatments and the resulting surface properties were characterized in detail by various microanalysis techniques. Although surface chemistry modification of biopolymers is important, modification of the near-surface structure of biopolymers is also critical because it affects cell attachment, proliferation, and infiltration, which is of paramount importance in the fabrication of scaffolds for tissue engineering.

Plasma polymerized fluorocarbon (FC) films grafted onto Ar plasma-treated low-density polyethylene surfaces were shown to increase the surface shear strength while maintaining low friction. These surface characteristics illustrate the potential of FC films as coating materials of bioinstruments, such as catheters used for the treatment of diseased arteries where blood flow is restricted by plaque deposits onto the inner wall of the vessel. In addition to FC film grafting, plasma polymerization with diethylene glycol dimethyl ether monomer was used to graft non-fouling polyethylene glycol (PEG)-like films on various substrates to prevent both protein adsorption and cell attachment, which is of great importance to the fabrication of non-clotting artificial grafts for bypass surgery.

Non-fouling PEG-like films were used to chemically pattern substrate surfaces for single-cell culture. Polystyrene culture dishes coated with a PEG-like film were chemically patterned using a silicon shadow mask or a poly(dimethyl siloxane) (PDMS) membrane mask, fabricated by standard lithography methods, to locally remove the PEG film by Ar plasma etching through the mask windows. Another surface chemical patterning method for long-term single-cell culture was accomplished with polystyrene and parylene C surfaces by taking advantage of the change in surface hydrophilicity induced by plasma treatment. These surface chemical patterning methods were used to regulate the shape and size of smooth muscle cells (SMCs). A strong effect of the shape and size of SMCs on proliferation rate was observed, which was correlated to changes in nuclei shape and volume of the SMCs.

In contrast to solid polymers, plasma surface treatment of fibrous polymer materials to improve biocompatibility has received relatively less attention. Thus, another objective of this dissertation was to explore how plasma surface modification with inert (e.g., Ar) and reactive (e.g., NH3) gas plasmas can be used to enhance cell attachment, growth and infiltration into fibrous polymer scaffolds. Poly(L-lactide) (PLLA) microfibrous scaffolds synthesized by electrospinning were plasma treated with Ar and NH3 gases to improve cell affinity and incorporate functional groups for biomolecule immobilization. Both Ar and NH3 plasma treatments were shown to improve the cell attachment and growth onto the fabricated microfibrous scaffolds, while surface functional groups produced by NH3 plasma treatment were also effective in immobilizing biomolecules.

In addition to the surface chemistry, the structure of biopolymer materials also impacts the effectiveness of tissue engineering scaffolds. Using microfabrication technology to produce a patterned PDMS template for electrospinning, patterned PLLA microfibrous scaffolds with different structures were fabricated and their potential for tissue engineering was demonstrated by in vitro and in vivo cell culture experiments.

The results of this thesis indicate that surface chemistry and structure modification of biopolymers by combining plasma treatment with microfabrication/micropatterning techniques is an effective method of engineering surfaces for single-cell culture and scaffold materials with tailored two- and three-dimensional structures that enhance cell growth and infiltration. The findings of this work have direct application in the development of patterned surfaces for controlled single-cell attachment, which is of particular value to studies of individual cell behavior, and scaffolds for tissue engineering and repair.