Photolithographic patterning of polyethylene glycol hydrogels

Abstract

A simple, inexpensive photolithographic method for surface patterning deformable, solvated substrates is demonstrated using photoactive poly(ethylene glycol) (PEG)-diacrylate hydrogels as model substrates. Photolithographic masks were prepared by printing the desired patterns onto transparencies using a laser jet printer. Precursor solutions containing monoacryloyl-PEG-peptide and photoinitiator were layered onto hydrogel surfaces. The acrylated moieties in the precursor solution were then conjugated in monolayers to specific hydrogel regions by exposure to UV light through the transparency mask. The effects of UV irradiation time and precursor solution concentration on the levels of immobilized peptide were characterized, demonstrating that bound peptide concentration can be controlled by tuning these parameters. Multiple peptides can be immobilized to a single hydrogel surface in distinct patterns by sequential application of this technique, opening up its potential use in co-cultures. In addition, 3D structures can be generated by incorporating PEG-diacrylate into the precursor solution. To evaluate the feasibility of using these patterned surfaces for guiding cell behavior, human dermal fibroblast adhesion on hydrogel surfaces patterned with acryloyl–PEG–RGDS was investigated. This patterning method may find use in tissue engineering, the elucidation of fundamental structure–function relationships, and the formation of immobilized cell and protein arrays for biotechnology.

Introduction

Controlling cell–microenvironment interactions is important in generating tissue engineered constructs that mimic native tissue architecture and for guiding cellular differentiation and organization. Recent advancements in patterning technologies have significantly enhanced our ability to spatially control surface chemistry and topography at the micrometer level and thus our ability to tailor cell microenvironment. Common patterning methods include including photolithography [1], [2] and soft lithographic approaches such as microcontact printing [3], microfluidic patterning [4], [5], and micromolding [6], [7]. These techniques have been widely used for the high fidelity patterning of rigid substrates, such as modified silicon or glass [8], [9]. However, the surface patterning of deformable, solvated, biocompatible platforms relevant to tissue engineering, such as hydrogels, has not received a similar degree of attention.

Given the importance of deformable, hydrated substrates as scaffolds for soft tissue engineering applications, development of methods for direct, high fidelity patterning of these platforms is desirable. This work develops transparency-based photolithography as a simple, versatile, and inexpensive technique for surface patterning bioactive peptides and 3D structures onto hydrated, photoactive poly(ethylene glycol) (PEG)-based hydrogel substrates. PEG-diacrylate (PEGDA) hydrogels are biocompatible and intrinsically resistant to protein adsorption and cell adhesion. In addition, acrylate-terminated PEG macromers undergo rapid polymerization upon exposure to UV light when in the presence of appropriate photoinitiators [10], [11]. Thus, the material properties and photoactivity of PEGDA hydrogels can be exploited to tailor in desired bioactivity via light-based patterning [12]. In transparency-based photolithography, masks are prepared by printing the desired patterns onto transparencies using a standard laser jet printer, obviating the need for expensive equipment and clean room use for mask fabrication. The range of feature sizes and shapes that can be generated using this methodology are investigated, and the effects of UV irradiation time and precursor solution concentration on the patterning outcome are characterized. Furthermore, the feasibility of using these patterned hydrogels to control cell behavior is evaluated by examining human dermal fibroblast (HDF) adhesion onto ACRL–PEG–RGDS patterned hydrogels.