Tissue Engineering: Focus on the Cardiovascular System - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 173

will be introduced, and their cell-entrapping qualities will be detailed. The
high water and natural proteins and/or synthetic polymer content of hydro-
gels provide an extracellular matrix (ECM)-like environment supportive of
prolonged cell survival. Scaffold-“free” tissue engineering approaches will be
introduced, including the cell sheets and cell aggregation techniques. In the
cell sheet approach, confluent and intact cell layers are harvested and over-
laid to create 3D tissue constructs. The cell aggregation technique, also void
of biomaterials, encourages cellular self-assembly within rotating shakers,
eventually leading to 3D tissue patch construction. Use of natural, acellular
ECM components derived from native tissues will also be described as a
common scaffolding method. Lastly, protein-engineered biomaterials, com-
posed entirely of recombinant proteins, will be described, as well as their
advantageous design flexibility.
Polymeric Porous Scaffolds
Porous matrices are commonly employed as scaffolds for tissue-engineer-
ing purposes. They allow for direct cell seeding where cells can fill the
micropores, proliferate, adhere to scaffold walls, and assume their shape.
Particulate leaching methods using microspheres or salt grains offer regu-
lation over scaffold microstructure, porosity, and interpore connectivity
[94, 95]. Synthetic polymers, such as poly-lactic acid (PLA), poly-glycolic
acid (PGA), and the PLA-PGA co-polymer [96, 97], serve as typical scaffold
materials in such protocols. Polyglycerol sebacate (PGS) [98] and polycapro-
lactone (PCL) [94] represent an additional two commonly used scaffold
reagents. Synthetic polymers allow for simple tailoring of the scaffold's
mechanical, morphological, and degradative properties. In many cases,
scaffolds are supplemented with natural ECM matrix components (matri-
gel, collagen, or fibronectin) to foster cell adhesion. Porous scaffolds can
also be designed using naturally occurring proteins, such as collagen,
gelatin, fibrin, and alginate [99], all of which support cell adhesion and
proliferation.
In a collaborative work with Gepstein et al. [11], we have demonstrated that
hES-CMs, endothelial, and embryonic fibroblast cells seeded within a PLLA-
PLGA (50/50) scaffold successfully form 3D human, vascularized cardiac
muscle constructs (Figure 8.1). The patch was occupied with differentiated
CMs arranged in a sarcomeric pattern and exhibited synchronic beating, as
well as contractions responsive to both positive and negative chronotropic
agents [11]. The endothelial and embryonic fibroblast cells were shown to be
responsible for formation of the intense inter-CM vascular networks observed
in vitro (Figure 8.1a). Transplantation of these vascularized human cardiac
constructs into rat myocardium led to intense graft vascularization and for-
mation of functional human blood vessels [12] (Figure 8.2c, d). Implanted
hES-CMs continued to thrive and mature and underwent elongation and
directed alignment [12] (Figure 8.2a, b).