4.4. BASIC SCAFFOLD FABRICATION FORMS 47

salt concentrations or pH they form a stable hydrogel made of flexible nanofibers. Such peptide nanofibers have been shown to create a favorable cell microenvironment upon injection into the infarct, and as systems for growth factor delivery [3] (details in Chapter 10).

Poly (N -isopropylacrylamide) (PIPAAm) has been mainly applied for the creation of intact cell sheets. When PIPAAm is grafted onto tissue culture plates by electron beam irradiation, the grafted surfaces become slightly hydrophobic under cell culture condition, at 37◦C, but readily become hydrated and hydrophilic below the polymer LCST (low critical solution temperature of PIPAAm), 32◦C.Thus, the attachment and detachment of cells on the culture surface can be controlled by simple temperature change. With the temperature-responsive culture surfaces, cells can be non-invasively harvested as intact cell sheets along with their deposited ECM, and subsequently be attached to host tissue. Using this technique, several cell sheets could be layered on top of each other to create multilayered three-dimensional cell constructs [14, 15].

Other types of more traditional synthetic polymers that have been used in tissue engineering, but not as much in cardiac tissue engineering, especially in their injectable forms include: the biodegradable poly(α-hydroxyacid) [poly ε-caprolactone (PCL), polylactic acid (PLA), polyglycolic acid] and their copolymers [10, 16, 17, 18]. These polymers were adapted from the field of drug delivery systems and the main initial motivation for their use in tissue engineering stems from their biodegradability, biocompatibility, and FDA approval. These polymers have been mainly used as solid macroporous scaffolding for cardiac patch reconstruction.

4.4BASIC SCAFFOLD FABRICATION FORMS

Scaffolds can be fabricated in different shapes, sizes, and internal structure (porosity, pore size, and architecture of the pore structure (isotropic or anisotropic). In tissue engineering of patches, either with seeded cells or without cells, the two main scaffolds in use are hydrogels and macro-porous solid scaffolds.

4.4.1HYDROGELS

Hydrogel is a network of polymer chains that are water-insoluble, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are superabsorbent (they can contain over 99% water) natural or synthetic polymers. Hydrogels also possess a degree of flexibility very similar to natural ECM, due to their significant water content.The hydrogels can be prepared from natural and synthetic polymers by physical/ionic interactions (alginate) or via chemical cross-linking (collagen, HA, and others). Cells are incorporated/encapsulated in the hydrogel during fabrication. Due to their resemblance to ECM texture, hydrogels are extensively being investigated as ECM replacements for damaged ECM after MI.They are delivered either by intramyocardial injection or by catheter-based techniques via the intracoronary route [19, 20, 21, 22, 23, 24].

484. BIOMATERIALS – POLYMERS, SCAFFOLDS, AND BASIC DESIGN CRITERIA

4.4.2MACROPOROUS SCAFFOLDS

Macroporous scaffolds are characterized by large pore size (50-200 μm in diameter) and matrix porosity (70-90%). The pore size in scaffolds should be at least 50 μm in diameter to enable vascularization (blood vessel penetration) after their implantation. The pore size and architecture as well as the extent of pore interconnectivity are major effectors on cell seeding, cell penetration from the host, and cell organization into a tissue. The most common techniques for preparing macroporous scaffolds are: solvent casting particulate (porogen) leaching, non-solvent induced phase separation, thermally induced phase separation, foaming process, microsphere sintering, and electrospinning.

Recently proposed fabrication techniques are: rapid prototyping, solid free-form, shape deposition manufacturing, fused deposition modeling, 3D printing, selective laser sintering, stereolitographic technique, and molecular self-assembly [25].

Macroporous alginate scaffold, commercially available from Life Technologies Incorporation as AlgiMatrixTM, has been developed by our group, by a controlled freeze-dry technique of calcium cross-linked alginate solution [26, 27]. The scaffold porous structure was dependent on the freezing regime (rate and direction) (Fig. 4.3A, B). When the calcium crosslinked alginate solutions were slowly frozen at −20◦C, in a nearly homogenous cold atmosphere, the resultant scaffold had an isotropic pore structure; the pores were spherical and interconnected. In contrast, when the cooling process was performed under the unidirectional temperature gradient along the freezing solution, an anisotropic pore structure was attained. This pore architecture influenced the shape of the tissue organized in the different scaffolds (Fig. 4.3C-E).