Page 497 - Handbook of Biomechatronics

Bioinspired and Biomimetic Micro-Robotics for Therapeutic Applications 491

phosphorylcholine (Durrani et al., 1986). The intention is to simply hide the
surface, with an “engineered natural look” as the authors put it, using synthetic
polymers.
Another solution is to use hydrogels, which have a cross-linked matrix in
between long polymer chains, as biocompatible coatings (Peppas et al.,
2000). Hydrogels are soft materials having porous polymer matrices con-
taining more aqueous content in volume than the polymer molecule itself.
There are several areas hydrogels are used and they can be engineered in a
way that they can imitate the elasticity of living tissue. A different coating
approach is immobilization of polymers such as poly(ethylene glycol),
poly(vinyl chloride), and poly(e-caprolactone) in order to make surfaces
more invisible to blood cells and blood proteins (Merrill and Salzman,
1983; Rusu et al., 2006). Also, Ti coating can be used as a biocompatible
material concealing other toxic elements in some cases, since it has a rather
inert nature (Servant et al., 2015).
Furthermore, there are special chemicals to that effect known as “surface
additives.” They are used to engineer biocompatible surfaces while preserv-
ing the physical properties of the material beneath the surface, such as hep-
arin (Larm et al., 1983), and “thrombin inhibitor compounds”(Salvagnini et al.,
2005). Heparin can also be used for surface functionalization during
manufacturing process (Dreyfus et al., 2005) thus serving two purposes at
once. Surface functionalization is depicted in Fig. 9.

Fig. 9 Surface functionalization of a cargo: nonimmunogenic molecules of the organic
or inorganic source could be used to mask the surface as to keep the specific immune-
response proteins away. Similarly, molecules (proteins) of various sorts could be used to
functionalize the surface, for example, bacteria could use these proteins as an umbilical
to the surface and they can tow the cargo. Either way, the expected end result is a clan-
destine micro-robotic agent.