Page 268 - Biomimetics : Biologically Inspired Technologies

Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c009 Final Proof page 254 21.9.2005 3:10am

254 Biomimetics: Biologically Inspired Technologies

2004). This failure mode is most prevalent in engineered muscle maintained in culture. There are
two approaches to dealing with this in engineered muscle: (1) genetic enhancement and (2)
development of electromechanical tissue maintenance protocols. In the case of genetic enhance-
ment, the approach is to forcibly express desired genes in an attempt to promote the desired tissue
phenotype. The effectiveness of this approach is the core issue in gene therapy for diseases of
muscle, but this approach has not yet been demonstrated to be effective for engineered muscle ex
vivo. Optimal tissue maintenance protocols are a much more natural and subtle approach, based
upon the fact that all viable muscle cells contain the necessary genetic machinery to develop any
desired muscle phenotype, if the correct signals and growth conditions prevail. In addition to
genetic engineering of myocytes to enhance performance of tissue-based actuators, other potential
countermeasures include: (1) development of appropriate tissue interfaces to permit signal trans-
duction to the cellular machinery, (2) development of tissue and organ culture bioreactors to allow
the experimental determination of optimal control and maintenance protocols for ex vivo muscle
tissue, (3) use of these protocols to guide tissue development (cell phenotype and tissue architec-
ture), and (4) implementation of this technology into the hybrid actuator system. This topic is
currently an area of very active research. Success in terms of counteracting this failure mode in
engineered muscle will constitute an extraordinarily significant scientific contribution, as well as
providing the key enabling technology to the further development of practical living actuators.

9.7 SELF-ORGANIZING MUSCLE TISSUES

Self-organization within developing animals gives rise to an enormous array of muscle actuator
architectures. Each myogenic precursor cell contains the genetic potential to self-organize into
muscle tissue with the desired phenotype and tissue interface. The ability to guide the development
of self-organizing muscle tissues in culture will provide the systems engineer with the greatest
level of design flexibility, since it will in principle be possible to start with a small population of
muscle progenitor cells and guide them to self-organize into a muscle actuator of any imaginable
geometry. It will also be possible to construct hybrid actuators not found in nature, containing
regionally organized tissue structures, perhaps even consisting of fundamentally different types
of muscle tissue (skeletal, cardiac, or smooth), depending upon the functional requirements of
the actuator system. It is implicit in most muscle tissue engineering research programs that
skeletal muscle self-organization and development can be guided by the application of the correct
external cues. The general method of guided tissue self-organization in culture (Figure 9.1)
briefly is:

. Isolate and coculture the desired cells. The cells may be primary or from cell lines.
. Engineer a cell culture substrate with controlled adhesion properties for the cells.
. Provide permanent anchor points and surfaces to guide tissue architecture formation.
. Culture the cells to permit the formation of a cohesive monolayer.
. Induce monolayer delamination from the substrate at the appropriate point in cell differentiation
(the monolayer remains attached to the anchor points).
. Promote tissue self-organization and further development by applying external signals: chemical,
electrical, mechanical.

Self-organization of tissues in culture is one effective way to produce small functional tissue
constructs from a range of tissues. Examples include:

. Cardiac myocytes cocultured at confluence with fibroblasts will self-organize into long cylinders
and tapered cones in culture in 340 to 400 h. These constructs are electrically excitable and also
spontaneously contract as a syncytium to continuously generate significant mechanical work
cycles. Such constructs could be engineered to power cell-scaled implantable pumps, pumps for