Optimizing protein adsorption and antimicrobial
properties
The biological response varies considerably across the in vivoapplications resulting in different requirements of biomaterials design.
However, all undesirable biological reactions are associated with a
series events know an “biofouling” that started with protein
adsorption following by other biomolecules and cell adhesion. Therefore,
creating a non-biofouling surface with minimal non-specific protein
adsorption is of great importance to avoid undesirable biological
response. There are two main conditions in which form non-adhesive
surfaces as suggested: strongly hydrophilic or strongly
hydrophobic[4]. Construction of hydrophilic
surfaces of biomaterials is more favored in biomedical applications.
Various polymers have been studied to generate a hydrophilic and
protein-resistance surfaces. Among them, PEG has been the most widely
used polymers for antifouling applications. PEG is a water-soluble
amphiphilic polyether and termed as the “gold standard” of antifouling
polymers[5].
PEG was coupled to a titanium oxide (TiO2) surface by a
3,4-dihydroxyphenylalanine (DOPA) derivative as
cross-linker[6]. The DOPA was used to construct a
hydroxylated surface to graft PEG via an amination reaction. The
antiadhesive property of PEG functionalized TiO2 surface
was assessed through the protein adsorption of BSA. Results revealed a
reduction of BSA adsorption by a factor of 4 on PEG-surface compared to
bare surface. Similarly, PEG was grafted to PVDF porous membranes via an
amination reaction with reactive graphene oxide (GO)
additives[7]. The antifouling ability and
hydrophilicity of PEGylated PVDF/GO surface were significantly enhanced.
The flux recovery rate (FRR) of PEGylated surface was 90.2% with a
total fouling rate
(Rt) of 20.7%,
whereas the FRR of original PVDF/GO surface was 86% with a
Rt of 26.7%. In one demonstration, a click reaction was
conducted on silicon surface to create amine terminated layer for
coating PEG with improved grafting density and
uniformity[8]. The PEGylated silicon surface
showed no fouling of human serum albumin and relatively lower adsorption
of lysozyme. Silicon based biomaterials have been widely used for the
development of ophthalmic devices such as contact lenses and intraocular
lenses[9]. However, silicon-based contact lenses
are always associated with limited wettability and excessive protein
adsorption leading to ocular discomfort[10]. PEG
coating was applied to intraocular lenses for improving hydrophilicity
and antifouling property[11]. There is a
commercial PEG based contact lens coating technology, Tangible™
Hydra-PEG, to improve the lubricity and antifouling ability of contact
lens.
In addition to macroscopic surfaces, PEG can be incorporated to
nanoparticle system to confer protein-repellent properties. NPs are
widely used in nanomedicine and drug delivery applications. Whereas the
protein corona formed on the surface of nanoparticle can induce fast
uptake by macrophages, the reduced targeting efficiency and lower cancer
cell uptake specifically for anticancer drug
delivery[12]. PEG can be grafted on a wide range
of NPs such as inorganic NPs, magnetic metallic NPs, polymeric NPs and
nanoscale metal-organic frameworks. Mesoporous silica NPs was first
surface modified with PEI-coated carbon dots for effective
transepithelial transport and then coated with PEG for better mucus
permeability and oral bioavailability[13]. PEG was
coated on biodegradable PLGA NPs to improve the mucus permeability and
retention of NPs as well[14]. The in vivoanimal studies indicated a considerably improved colorectal retention of
PEG-modified PLGA NPs compared to pristine PLGA NPs. The retention of
PEG-modified PLGA NPs reached 2-hours post-administration in contrast to
15 min of bare PLGA NPs. A sequential antifouling surface can be
constructed on porous silica NPs by grafting PEG via a photo-triggered
system. PEG was conjugated to PEI surface with biotin conjugates as
targeting molecules via a photo-cleavable ortho-nitrobenzyl linker; and
PEI was conjugated to the surface of silica NPs. PEGylation afforded the
antifouling property of NPs and avoided the clearance by macrophages.
Upon light irradiation, the outer PEG layer would be detached from PEI
surface leaving the negatively charged carboxylic acids. Together with
positive charged amine groups on the surface, a zwitterionic surface was
generated that preserves targeting efficiency of biotin and offers
further antifouling property.
Zeolitic imidazolate framework (ZIF-8) NPs with encapsulated
doxorubincin (DOX) was modified by PEG in
one-pot[15]. PEGylation endowed improved colloidal
stability of ZIF-8 NPs in both water and cell culture medium. There was
a pH-sensitive drug releasing behavior of DOX@ZIF-8/PEG NPs and higher
cytotoxicity to hepatocellular carcinoma cells than free DOX suggesting
an enhanced cancer cell targeting ability. Drugs can be conjugated to
PEG directly prior to decorate the surface of drug carrier. Curcumin was
coupled to the hydroxyl groups of PEG to form drug conjugates; then the
PEGylated curcumin was physically attached to magnetic
Fe3O4 NPs. Such PEG modified drug
delivery system possessed higher drug loading efficiency and a
pH-sensitive drug releasing profile.
Construction of an anti-fouling surface using aforementioned techniques
can benefit the reduction in bacterial adhesion. However, unlike the
“passive” strategy relying on the production of low adhesive surface,
an “active” approach involves an antibacterial surface based on the
incorporation of antibacterial agents[1].
Metal NPs or ions can be incorporated in antibacterial surface. Silver
related antibacterial activity has been widely studied. There are
numbers of silver incorporated commercial products in healthcare such as
Silverlon® surgical dressing and Palindrome™ Precision
SI-silver ion antimicrobial dialysis catheter. Silver has been coated on
single-walled carbon nanotubes (SWCNT) to achieve antibacterial
activities[16]. Results showed a stronger
bactericidal activity against foodborne pathogens of PEGylated silver
coating than non-PEGylated silver coating. The in ovoadministration of PEG/Ag-SWCNT indicated undetectable toxic effects on
development of chicken embryo. Silver doped HA coating was deposited on
NiTi alloys through electrodeposition to obtain an antibacterial and
bioactive surface for orthopedic applications[17].
A composite coating composed of nano-HA and silver NPs on Ti6Al4V dental
implants was developed for enhanced biocompatibility and additional
antimicrobial property[18]. Since the HA coating
was a porous layer, the antibacterial ability of silver layer would not
be masked. There was some initial release of silver ions in the first 24
h immersion in cell culture media followed by a slow release. The
initial release could be clinically beneficial for an early infection
control.
Quaternary ammonium compounds (QACs) are antimicrobial materials and
effectively against various bacteria. The most accepted mechanism of the
antibacterial ability of QACs is the disruption of cell membrane due to
the sufficiently long cationic polymer chains[20].
Another explanation is the disruption of divalent cations on cell
membranes by the highly charged surface[21].
However, both explanations lead to concern about the potential
cytotoxicity to human cells by QACs, which quite limited their progress
in clinic[3,22].
Antimicrobial peptides (AMPs) are widely used in antibacterial coatings
due to their broad-spectrum antimicrobial activity.
Magainin I (Mag) has been bonded to
TiO2 surfaces via PEG
crosslinker[6]. There was a significant reduction
in Listeria ivanovii adhesion to PEG-Mag modified surface
compared to bare surface by a factor close to 2. Even though some
bacteria were adhered to modified surface, such bacteria exhibited
abnormal morphology revealing a detrimental effect from the
antimicrobial surface. Another AMP, ε-poly-L-lysine (EPL), was
dip-coated to 3D PCL/HA scaffolds to confer antibacterial
property[23]. The EPL modification endowed a
notable improvement in hydrophilicity and broad-spectrum antibacterial
activities of PCL/HA scaffolds. Such antimicrobial activities againstS. aureus , E. coli and S. mutans . can retain for 3
days.
Various antibiotics can be immobilized on the surface of biomaterials to
directly achieve bactericidal effects. Triclosan has been encapsulated
into multilayer films composed of PEG, PCL, chitosan and
PAA[24]. Such films were deposited on PDMS
substrates by layer-by-layer self-assembly. Zone of inhibition (ZOI) and
bacterial LIVE/DEAD staining assays verified the high efficiency of this
antibiotics delivery system. The ZOI against E. coli was 15 mm
while ZOI against S. aureus was 3 mm. The multilayer film allowed
a sustained release of triclosan up to 7 days enabling long-term
antibacterial function. Importantly, the sustained antibiotics release
was stimuli-responsive that can be triggered by pH and bacteria stimuli,
which may address the problems of resistant bacteria. As implanted the
triclosan loaded multilayer coated substrates in rabbit models, the
implants related infection was considerably eliminated with an infection
rate of 16.7% in comparison with infection rate of 83.3% in multilayer
alone modified group.
Polymers with bactericide effects can also be employed as antibacterial
coating such as chitosan and PHMB. Acid treated carbon nanotubes were
incorporated in PCL fibers to enhance mechanical strength as well as to
create negatively charged surface[25]. Chitosan as
a positively charged polysaccharide can be strongly immobilized to the
surface of PCL fibers through electrostatic attraction. ZOI assay
confirmed the acquired antibacterial function of PCL fibrous mats
attributed to chitosan with ZOI against E. coli of 11.15 ± 0.21
mm and ZOI against S. aureus of 8.38 ± 0.19 mm. A hemostatic and
antibacterial sodium alginate/gelatin sponge was fabricated by surface
engineering with PHMB and hyaluronic acid by alternately spraying them
on the sponge layer-by-layer. Hyaluronic acid was deposited on top of
PHMB to endow the surface with better biocompatibility. When encountered
with Gram-positive bacteria such as S. aureus , the bacteria
secreted hyaluronidase would degrade the hyaluronic acid layer leading
to subsequent exposure of PHMB to perform the bactericide function. This
bacteria-stimulated antibacterial sponge showed an on-demand strategy
and exhibited excellent in vivo anti-infection performance.
However, its antibacterial property against Gram-negative bacteria such
as E. coli was quite limited due to the masking effect of
hyaluronic acid layer on PHMB.