Engineered Muscle Actuators: Cells and Tissues - Biomimetics: Biologically Inspired Technologies - page 256

interfaces. The cellular components of muscle can be chemically removed while retaining the
detailed architecture of the muscle ECM. Preliminary results indicate the success of the reintro-
duction of myogenic cells into these natural ECM scaffolds. This approach to engineering muscles
as actuators has several advantages, among these are that heterogenic cells can be introduced into
the preexisting matrix. For example, skeletal-cardiac chimeric muscles could be employed or
myogenic precursors from an entirely different species. The main advantage of the use of
natural ECM scaffolds is that the fine architecture of the entire muscle organ is retained by the
acellularized ECM scaffold. It is possible to perfuse the scaffold using the remnant vascular bed
ECM to reintroduce cells and later to provide perfusion to the reengineered muscle organ. The
acellularized muscle ECM also has matrix architecture specific to the MTJ and tendon, which may
be advantageous in the development of this very critical tissue interface. The principal disadvantage
of this approach is that the ECM scaffold architecture is limited to those architectures that are
available in nature.

9.9 TISSUE INTERFACES: TENDON, NERVE, AND VASCULAR

For any type of muscle actuator, it will be essential to provide appropriate tissue interfaces. In some
cases, the tissue interfaces are already in place and specific measures must be taken to maintain
them properly. In other cases, their formation must be guided and facilitated. Based upon our in vivo
work, we have demonstrated that muscle phenotype can be controlled and maintained in the
absence of innervation via electrical stimulation. A considerable volume of published research
has been directed toward the promotion of adult phenotype in muscle tissue in culture directly
by electrical stimulation, in the absence of nerve-derived trophic factors or depolarization via
the neuromuscular junction and related synaptic structures. It remains to be demonstrated,
however, that muscle can be guided through the necessary developmental stages in the absence
of innervation to achieve adult phenotype. Adequate and functional vascular and tendon interfaces
to muscle engineered in vitro are also yet to be demonstrated, although they are the topic of
intensive research.

9.9.1 Vascular Tissue Interface

Nutrition and oxygen delivery in static culture conditions always limit the cross-sectional area,
particularly for tissues with high metabolic demand, such as muscle. Therefore, a 3-D organ culture
system with perfusion of a vascular bed within the muscle tissue is a core objective of current
research. Cell types associated with angiogenesis, such as endothelial cells, are also crucial players
in organ development (Bahary and Zon, 2001). Endothelial progenitor cells from peripheral blood
are readily isolated, and have been shown to incorporate into neovessels (Asahara et al. , 1997) and
also have potential to expand to more than 10 19 -fold in vitro (Lin et al., 2000). Furthermore,
functional small-diameter neovessels can be created in culture by using endothelial progenitor cells
(Kaushal et al., 2001).

9.9.2 Strategies for Engineering Functional Vascularized Muscle Tissue

There are three strategies for generating vascularized muscle constructs:

(1) Recellularization of an acellular muscle construct.

(2) Coculture of myoblasts with endothelial cells and growth factor stimulation for induction of the

endothelial cells to form capillary like structures.

(3) Induction of sprouting of microvessels into temporarily implanted tissues or from vascularized and

perfused tissue explants (such as adipose) cultured adjacent to the engineered muscle.