Sequential formation of covalently bonded hydrogel multilayers through surface initiated photopolymerization

Abstract

A novel method for the sequential formation of hydrogel multilayers is described. Formation of the first layer is based on surface initiated photopolymerization of hydrogel precursors on eosin derivatized surfaces. In order to attach subsequent layers it is necessary to be able to functionalize intermediate hydrogel layers with eosin. In the present work, this is accomplished by introducing poly(ethylene glycol) amino acrylate (NH₂-PEG-Acr) along with other hydrogel precursors such as poly(ethylene glycol) diacrylate (PEG-DA) on the intermediate layers. The pendant amine groups allow functionalization of the intermediate layers with eosin for subsequent photopolymerization of new hydrogel layers. The process can be repeated sequentially to construct multilayered hydrogel membranes. The NH₂-PEG-Acr monomer can be formed by coupling cysteamine to PEG-DA by a conjugate addition reaction. The approach to multilayer formation could allow the incorporation of specific functionalities or compositions within each hydrogel layer so that multifunctional membranes can be formed. It could also be implemented, through proper photopatterning procedures, for the formation of 3-D hydrogel structures. The mild photopolymerization conditions employed using visible (514 nm), rather than ultraviolet light would make this technique especially attractive for tissue engineering, drug delivery, biomaterials, biosensor development and other situations where the elements incorporated are sensitive to UV light exposure.

Introduction

Hydrogels are becoming increasingly popular since they were first discovered due to their high water content and tissue-like physical and mechanical properties. These important characteristics lend to their use in systems where pH or component sensitivity, water absorption, and biocompatibility are essential. Hydrogels can be formed by a variety of methods including crosslinking by physical, ionic, and covalent interactions [1]. Photopolymerization can allow covalent crosslinking of liquid prepolymer solutions in the presence of a photoinitiator and light [2], [3]. Due to their biocompatibility, hydrophilicity, and permeability, crosslinked poly(ethylene glycol) (PEG)-based hydrogels [4] have been used as immunoprotective barriers in tissue engineering for therapeutic cell transplantation to prevent the rejection of transplanted cells by the host's immune system [5]. The semipermeable PEG hydrogel membrane allows sufficient immunoprotection while allowing the diffusion of small molecules such as nutrients (e.g., oxygen, glucose and amino acids) and metabolism products through the membrane. For applications involving treatment of type I diabetes, it has been demonstrated that encapsulated islets are able to secrete insulin in response to varying levels of glucose in the body and that insulin permeates through the membrane. As a result, islet cells encapsulated in hydrogels do not lose their viability, glucose sensing, or insulin secretion capabilities [5], [6], [7], [8].

Another application of PEG hydrogels is the formation of degradable thin hydrogel barriers formed on the inner surface of injured arteries by interfacial photopolymerization [9]. It has been shown that these barriers dramatically reduced thrombosis and intimal thickening in rat and rabbit models of vascular injury, as the barrier blocked contact between blood and the damaged vessel. PEG hydrogel barriers have also been used for the sustained release of macromolecular drugs to the arterial media in order to prevent constriction of blood vessels following catheter-based interventions to open obstructed coronary arteries [10].

The effectiveness of PEG hydrogels to encapsulate mesenchymal stem cells (MSC) has also been investigated recently by Williams et al. [11] who demonstrated that PEG-based photopolymerizing hydrogels can be used as scaffolds for chondrogenesis induction using bone marrow-derived MSC. The findings of their study indicated that PEG-DA hydrogels supported MSC survival, phenotypic differentiation, and accumulation of chondrogenic extracellular matrix. Further evidence indicated that chondrogenesis could be enhanced by the addition of chondrogenic growth factor. Hydrogels formed by this technique will help the development of stem cell-based therapies for cartilage repair or augmentation therapies.

Hydrogels have also been used for the development of cell-based biosensing devices for applications such as high-throughput drug screening or pathogen detection. Koh et al. immobilized different cell phenotypes in arrays of PEG hydrogel microstructures based upon UV initiated free radical polymerization demonstrating the potential to create multiphenotype cell-based biosensors [12].

The above-mentioned applications of hydrogel membranes usually involve single layers that are generally formed in a single procedural step. However, it is difficult to obtain single membranes that possess all the properties required for a specific application [13]. Some applications may require the membrane to possess a diverse range of properties such as pH sensitivity, high water absorption, and size exclusion capabilities. Although sometimes these properties can be attained with a single material, it is difficult to obtain multiple properties in a single layer that can satisfy the intended application of the material or device. Thus, the capability to construct multiple layers of varied composition in a single membrane or device would increase the range of applications of these systems. Hydrogel multilayers, as will be presented in this study, may provide more precise properties when a single material cannot meet the requirements for the application. This may be achieved by varying the composition and/or concentration of the precursor solution in each individual layer. For example, some layers may possess specific responsive properties such that they can change their physicochemical properties with external stimuli, such as pH [14], temperature [15], and solvent composition [16] while other layers may have specific permeation or sensing functions.

Here we present a method that allows sequential formation of covalently attached hydrogel multilayers. The method extends our previous work on interfacial photopolymerization of PEG hydrogels on eosin-functionalized surfaces [17]. The technique presented in this study allows the sequential formation of crosslinked PEG hydrogel multilayers and 3-D patterns that can have a variety of applications when multiple functionality/composition is desirable.