Protein-based Biointerfaces to Control Stem Cell Differentiation

JORGE ALFREDO UQUILLAS PAREDES, ALESSANDRO POLINI AND WOJCIECH CHRZANOWSKI

Faculty of Pharmacy, University of Sydney, Sydney, Australia

1.1 Modification of Biomaterial Surfaces with Proteins

It is already well-established that cellular responses to foreign materials when implanted in vivo are guided mainly by the surface characteristics. The modulation of characteristics such as roughness, porosity, chemical and biological composition allows the regulation of material integration within the body as well as the guidance of specific responses, e.g., cell adhesion, cell detachment, cell proliferation, differentiation, or metabolic activity. Material integration within the body is critically important for orthopedic implantable devices. Desired integration results in successful single surgery, thus reduces costs and complications related to adverse reactions and implant revisions. In this chapter, we described the methods of surface modification and functionalization, which enable the effective regulation of stem cells behavior for orthopedic applications. In particular, the chapter focuses on immobilization of different signaling molecules such as proteins, peptides, growth factors on the surface and, in three dimensions, to modulate cell adhesion and to trigger and regulate osteogenesis, osteointegration, and osteoregeneration. Furthermore, examination methods for the immobilization of biomolecules on surfaces are described in detail to explain the advantages and disadvantages of each method.

1.1.1 Introduction

Implantation of exogenous materials into the body triggers common body responses, and in some cases adverse inflammatory reactions. The body tries to ward-off the foreign object and protect from its 'perceived' negative influence on healthy cells and tissues. As a consequence the exogenous materials are encapsulated in a vascular, fibrous sacks which often limit the functionality of the implanted material. In the last decades research has been focused on the possibility of addressing this problem and creating surfaces that mimic the body's environment, thus enabling the host cells to accept and interact with the foreign object. Such interactions are facilitated by different cues incorporated into the surface, which includes topographical, mechanical, structural, chemical and biochemical signals. These are capable of stimulating specific cellular responses which include cell adhesion, proliferation, differentiation and, ultimately, cell death. The design of specific cues, single or combined, is guided by the application and tailored for specific needs.

Historically, one of the most successful approaches, which can be called biomimetic, was a deposition of hydroxyapatite coatings on the surface of implants that were placed within the bone environment. Hydroxyapatite, a natural component of bone tissues, when present on surfaces of metallic or polymeric devices allows for significant improvement of implant integration within the body. The most significant limitation is the adhesion of the coating to the substrate. It has to be noted that some of the implants are heavily bent (pre-operatively) and 'hammered' into the host tissue. This creates significant sheer and bending stresses that can lead to failure of the coating. Nevertheless, hydroxyapatite coatings have been clinically used and verified.

The cross-talk between a bioactive layer and the tissue can similarly be achieved using different types of mineral, ceramic, glass–ceramic materials that include tri-calcium phosphate, bioglasses and phosphate glasses. These materials are coated onto the surface or are used as a base component to produce implants. The primary advantage of these materials is that they can be degradable and compounded with different ions (Zn, Ag, Co, Sr), which provides additional biochemical cues for cells at the interface. Zn, Co and Ag have been reported to have antimicrobial activity, while Sr and Zn have been demonstrated to support bone formation and are used clinically in the treatment of osteoporosis. Furthermore, each of these materials interacts with body fluids differently and can guide the adsorption of proteins and growth factors and other components from the body fluids and blood. Subsequently, these biomolecules modulate interactions with cells and tissues. Hence, by a careful selection of the material it is possible to stimulate cellular responses through the structural and chemical composition, and also to guide the adsorption of desired biomolecules that stimulates biological responses, e.g., adsorption of cell adhesive proteins to hydroxyapatite.

1.1.2 Influence of Modified Surfaces on Cell Adhesion

The communication of cells with surfaces is mediated by pre-adsorbed protein layers. The composition...