Biomedical Engineering Reference
In-Depth Information
growth factors and morphogenes. Therefore, application of small chemical molecules that
target signaling pathways such as Shh, Wnt, FGF, or notch can be useful in neural-tissue
engineering.
Biosignaling Molecules (ECM, Growth Factors, and SMs) Presentation
Biosignaling molecules can be physically adsorbed and/or deposited onto the biomaterial
matrices through weak forces such as van der Waals forces, hydrogen bonding, or electrostatic
interaction. For example, laminin, polylysine, and collagen have been used to modify
poly(lactide-coglycolide) (PLGA) films [64], plasma-treated PLGA films and chitosan films
[65], and poly(
l
-lactic acid) (PLA) nanofibers [66]. Despite its technical simplicity, the poor
stability of biomolecule layer limits its usage in neural-tissue engineering. Blending ECM
reagents with biomaterials is a simple alternative approach to direct their deposition that
results in more stable and uniform distribution. For example, neural adhesion has been sig-
nificantly improved by the use of a blend of chitosan and 3% weight polylysine [67].
Electrostatic attachment is based on electrostatic interactions between ECM molecules and
biomaterials. It is similar to physical deposition and blending, and includes layer-by-layer
(LbL) assembly and electrochemical polymerization. For instance, LbL films comprised of
hyaluronic acid (HA)/collagen have been shown to promote cortical neuron adhesion on a
nonpermissive substrate, such as a glass coverslip [68]. The advantages of these films include
versatility and applicability to any charged substrate. The introduction of multiple linkages,
chemical or photocoupling can improve the films' stability. However, these methods are
relatively complicated compared to former techniques and factors such as pH, polyelectrolyte
loading, and ionic strength of the polyelectrolytes can affect composite stability.
Two main strategies for growth-factor presentation in tissue engineering exist to improve
its integration to biomaterials and prevent rapid diffusion, chemical immobilization, and
physical encapsulation.
Chemical immobilization
to the biomaterials includes chemical
cross-linking or affinity-based interaction of the growth factor to the polymer substrate
and/or cells chosen for tethering proteins to solid substrates.
Physical encapsulation
includes
the encapsulation, diffusion and pre-programmed sustained release of growth factor from a
degrading substrate or dissolving delivery systems into the surrounding tissue microenviron-
ment (Figure 15.1) [31]. For example, encapsulated FGF2 slowly diffuse from biodegradable
acidic gelatin hydrogels
in vivo
, whereas it does not release from the gelatin
in vitro
[69].
Incorporation of biodegradable PLGA microspheres within a porous scaffold can assist
with slowing the release of FGF-2 [70].
The immobilizations of a wide range of growth factors and other biomolecules to different
natural and synthetic solid substrates can be performed by means of
covalent
or
noncovalent
chemical approaches. Noncovalent attachment includes direct hydrophobic interactions with
excipient molecules through secondary interactions between growth factors and biomaterials
or indirectly via intermediate proteins/biomolecules [71]. In this way, chemically or physically
coated biopolymeric gels have been used as ECM-mimicking materials (e.g., heparin, fibro-
nectin, laminin, collagen, and gelatin) [72-74]. Small oligopeptides mimicking key fragments
such as RGD [71, 75] and a variety of synthetic hydrogels can provide specific biological
binding sites that can tether growth factors, morphogens or their small molecule agonists into
the area of interest.
Several other techniques have been developed to overcome loose binding of ECM
biochemical cues and/or growth factors to scaffolds. In this regard, covalent attachments
that permit molecular escape via diffusion and provide more stable delivery systems are of
great interest. These attachments include the use of heterofunctional cross-linkers such as
thiol, amine, carboxylate, and hydroxyl to conjugate ECM components or growth factors to
