Conclusion and future
perspectives
The primary aim of surface engineering of biomaterials is improving
their biological performance by controlling over the interaction between
the surface and living system. It is suggested that physicochemical cues
of the surface are intrinsically linked revealing that the alteration of
the surface topography will lead to localized changes in surface
chemistry. Surface engineering methods usually are combined to obtain
optimal results. For example, chemically non-reactive surface requires
pre-activation via surface oxidation, functional groups introduction or
ionizing irradiation for further surface grafting or biomolecules
immobilization.
Non-specific protein deposition underlies all undesirable biological
reactions and triggers other biomolecules and cell adhesion accounting
for “biofouling”. PEG is considered as the “gold standard” in
reducing bioadhesion and widely applied in anti-biofouling applications
not only in biomedical applications but marine applications. However,
PEG can suffer oxidative damage that limits its non-fouling feature for
long-term applications[66]. Development of
non-PEGylated hydrophilic surface with comparable protein-resistant
property to PEGylated surface but better thermal and oxidative stability
is of great interest. For example, dextran as a natural
phosphorylcholines is studied as a PEG alternative for antifouling
surface coating of biomaterials in long-term
applications[1]. Zwitterionic polymers are another
alternatives for antifouling surface modifications that exhibit even
stronger hydration effect than PEG[66]. A curcumin
loaded zwitterionic polymersome was incorporated in PDMS contact lenses
to improve the antibiofouling and antimicrobial
properties[9]. Bacteria acidify the local
environment like tumor cells. Creation of stimuli-responsive
antibacterial surface such as pH-sensitive can offer an on-demand
strategy to address the resistant bacteria. Silver-releasing coatings
are widely adopted due to their bactericidal ability. However, there are
concerns arisen from their potential side effects to proteins. Such
effects seem to be limited in applications with easy excretion of silver
such as urinary catheters, or where the benefits outweigh the risk, such
as skin wound dressings[3].
Thrombosis and intimal hyperplasia account to the major causes of
blood-contacting device failure due to the unsatisfactory
hemocompatibility. Achieving fully endothelialization on the luminal
surface is termed as the ultimate solution for anti-coagulation. When
designing proper surface of blood-contacting devices, the competitive
growth of ECs and SMCs should be considered. Heparinization is a common
accepted technique to enhance the antithrombogenicity of biomaterial.
The mechanisms by which heparinized devices modulated those cellular
behavior remains unclear. Possible reasons could be the interchanges
between heparin and thrombospondin that impairs migration and
proliferation of SMCs; and binding between heparin and angiogenesis
growth factors that accelerates
endothelialization[67]. Metallic implants are
currently used in many hard tissue applications especially in load
bearing conditions. Improving the tribology performance of biomaterials
through surface coating can mitigate the abrasive debris and enhance the
corrosion resistance. Super-lubricous coating offers a new perspective
in surface engineered implants for articulating joint with relatively
low wear generation.
Retaining the pluripotency in cell culture stage and maintaining the
differentiated phenotype of stem cells are both critical. Stem cells can
respond to the mechanical cues generated by the surface engineered
substrate and convert them into biochemical cues. Biomolecules
immobilization such as growth factors and peptides can provide direct
biological cues to stem cells. Full-length proteins are prone to undergo
conformational change and proteolytic degradation induced by surface
properties. On the contrary, peptides are preferred owing to higher
stability and easier control of surface
density[68]. Intermediate crosslinker is favored
to conjugate biomolecules to the surface due to the avoided direct
contact between biomolecules and biomaterials. However, the mechanisms
of interactions between cell and biomaterials surface are not fully
defined yet since there are a few cell-ligand interactions identified as
present. Moreover, the behavior of engineered surface can vary acrossin vitro and in vivo studies since living body presents a
dynamic and more complex environment. For example, platelet adhesion to
micropatterned surface can be mitigated in in vivo due to the
hemodynamics. Which suggests that long-term in vivo effects of
surface engineering are necessary to fully understand the performance of
biomaterials.