Biofunctional Hydrogels - Biomaterials - page 178

8.2 Synthetic Hydrogels

Synthetic polymers serve as foundation elements for hydrogel scaffolds, offering precise control over
many of the chemical and physical characteristics of these matrices. This control, or “tuning,” is a strong
advantage over naturally derived materials, which can suffer from purity issues and batch-to-batch vari-
ation. From a bioactivity standpoint, the hydrophilic nature of synthetic hydrogels discourages attach-
ment of proteins and therefore prevents cell interaction with these materials, which are generally not
recognized through traditional biological adhesion mechanisms. As such, the polymeric materials act as
bioinert platform into which specific bioactivity can be designed. It should be noted, however, that some
biofunctionalities are actually based on the inherent inertness of these matrices. Consider, for example,
the application of a thin hydrogel to the luminal surface of an injured blood vessel to act as a barrier to
thrombosis (Hill-West et al. 1994), or a similar method to prevent the formation of postoperative tissue
adhesions in peritoneal sites (Yaacobi et al. 1993). As the field of biomaterials grows, however, strate-
gies to impart specific and complex bioactivities to hydrogel matrices are becoming more common and
eagerly pursued.

8.2.1 Common Polymeric Hydrogel Materials

Hydrophilic polymers such as poly(hydroxyethyl methacrylate) (PHEMA), poly(vinyl alcohol) (PVA),
poly( N -vinyl-2-pyrrolidone) (PNVP), and poly(ethylene glycol) (PEG) are readily formed into highly
hydrated materials whose physical and chemical properties are amenable to interaction with cells and
tissues. Table 8.1 shows some recent applications of these frequently used hydrogel materials. It is also
common for hydrogels to appear in heterogeneous formulations made up of two or more hydrophilic
monomers. This diversity allows for greater control over the properties of hydrogel matrices. For example,
PHEMA materials can be made to take in greater amounts of water with the inclusion of methacrylic
acid as a comonomer or designed to swell less by incorporating the more hydrophobic methyl methac-
rylate (Ratner et al. 2004).
PEG is an especially popular biomaterial choice for applications including cell substrates, tissue
engineering scaffolds, and drug delivery devices. While the options for formulating PEG hydrogels
are extensive, in one specific strategy photoreactive PEG diacrylate mixed with a chemical initiator
(2,2-dimethoxy-2-phenyl-acetophenone) is polymerized upon exposure to long-wavelength UV light
(10 mW/cm 2 , 365 nm). This rapid polymerization process is amenable to cell encapsulation (Bryant
et al. 2000). The low viscosity of PEG precursor solutions means that they are readily formed into vari-
ous geometries (e.g., tubular structures representative of blood vessels) and that the hydrogels can be

TABLE 8.1 Recent Applications of Common Synthetic, Biofunctional Hydrogel Materials

Material Abbreviation Application References

Poly(ethylene glycol) PEG Proangiogenic matrix Leslie-Barbick et al. (2011);

Moon et al. (2010)

Drug/protein delivery Zustiak and Leach (2011)

Poly(hydroxyethyl
methacrylate)

PHEMA Stem-cell differentiation

Cardiac tissue scafold a

Guvendiren and Burdick (2010)
Madden et al. (2010)

Drug-eluting contact lens Anderson et al. (2009); Xu et al.

(2010)

Poly( N -isopropyl acrylamide) PNIPAAm Injectable glucose sensor Shibata et al. (2010)

Poly( N -vinyl-2-pyrrolidone) PNVP Spinal tissue scafold a Boelen et al. (2007)

Poly(vinyl alcohol) P VA Cartilage scaffold Bichara et al. (2011)

Drug-eluting sensor coating Vaddiraju et al. (2009)

a Used as a copolymer.