264
hexamethyldisiloxane (HMDS) and PDMS. Siloxanes have been generally recognized as safe (GRAS) in consumer products. According to Lassen et al. (2005),
siloxanes have low general toxicity and no reported genotoxic potential, apart from
some data showing inflammatory effects of silicones found mainly in cosmetics.
The International Agency for Research on Cancer (IARC) has assessed that there is
evidence suggesting lack of carcinogenicity in humans of breast implants made of
silicone, for female breast carcinoma (IARC 1999). Certain studies stated that siloxane surfactants used as adjuvants in agriculture showed a negative impact on honey
bees when ingested in high concentrations (Chen et al. 2018). Nonetheless, biocompatibility is a complex problem which depends on a number of factors, and must be
evaluated for each particular case (Mojsiewicz-Pieńkowska et al. 2016).
In biological research, PDMS is used as a support in microfluidic devices. Kuo
et al. (2016) observed a long-range signal delivery on 10- and 20-μm PDMS barriers
in microfluidically tetracycline inducible expression systems, thus suggesting that
this communication depends largely on the PDMS barrier for the delivery signal,
since it is a ‘wired’ cell communication instead of wireless. In another example, the
siloxane coating was able to significantly reduce bacterial adherence, and this effect
was assumed to be mainly a result of the high hydrophobicity of the coating substrate (Pfeiffer et al. 2016). On the other hand, for many biomedical applications the
hydrophobic behavior is a drawback. The low surface energy of PDMS elastomers
used in prostheses can cause abrasion, irritation and ulceration of tissues, as well as
bacterial colonization. It is known that an appropriate hydrophobic/hydrophilic balance of a drug molecule is essential for cell uptake. Biofouling (adsorption of contaminating matter, e.g. nonspecific proteins or organisms) is another serious problem
in relation to biomedical implants and devices. Polymers with antifouling properties
must be hydrophilic and electrically neutral, and must have H-bond acceptors but no
H-bond donors (Chapman et al. 2001). Here comes the role of the special functionalization of siloxanes, in order to modify the polarity and make the final material
more hydrophilic. The strategy to improve clinical performance by modifying the
surface of silicone elastomers is based on chemical, physical and combinations of
both methods (Abbasi et al. 2001; Hron 2003). In general, physical methods produce Si-OH groups, and therefore temporary hydrophilic surfaces, which can be
chemically modified with a permanent hydrophilic effect. The antifouling coatings
for PDMS were already reviewed by Zhang and Chiao (2015). Usually, these are
attached either by physical adsorption or by covalent bonding of appropriately
designed PEG/poly(ethylene oxide) (PEO) derivatives, polyzwitterions or other
compounds on the PDMS surface with or without preliminary UV/plasma treatment.
Goncalves et al. (2013) superficially polymerized PDMS using poly(ethylene
glycol)methacrylate by surface-initiated ATRP, preceded by UV/ozone exposure
and covalent bonding of the initiator (1-trichlorosilyl-2-(chloromethylphenyl)ethane) onto the hydroxylated PDMS via chemical vapor deposition. These authors
succeeded to obtain hydrophilic surfaces (with water contact angles around 60°),
thus resulting in the inhibition of bacterial adhesion and the non-cytotoxic effect on
human skin fibroblasts. In this same line, the surface modification of PDMS substrates by using ABA-type block copolymers which comprise poly(2- methacryloyl
C. Racles et al.
hexamethyldisiloxane (HMDS) and PDMS. Siloxanes have been generally recognized as safe (GRAS) in consumer products. According to Lassen et al. (2005),
siloxanes have low general toxicity and no reported genotoxic potential, apart from
some data showing inflammatory effects of silicones found mainly in cosmetics.
The International Agency for Research on Cancer (IARC) has assessed that there is
evidence suggesting lack of carcinogenicity in humans of breast implants made of
silicone, for female breast carcinoma (IARC 1999). Certain studies stated that siloxane surfactants used as adjuvants in agriculture showed a negative impact on honey
bees when ingested in high concentrations (Chen et al. 2018). Nonetheless, biocompatibility is a complex problem which depends on a number of factors, and must be
evaluated for each particular case (Mojsiewicz-Pieńkowska et al. 2016).
In biological research, PDMS is used as a support in microfluidic devices. Kuo
et al. (2016) observed a long-range signal delivery on 10- and 20-μm PDMS barriers
in microfluidically tetracycline inducible expression systems, thus suggesting that
this communication depends largely on the PDMS barrier for the delivery signal,
since it is a ‘wired’ cell communication instead of wireless. In another example, the
siloxane coating was able to significantly reduce bacterial adherence, and this effect
was assumed to be mainly a result of the high hydrophobicity of the coating substrate (Pfeiffer et al. 2016). On the other hand, for many biomedical applications the
hydrophobic behavior is a drawback. The low surface energy of PDMS elastomers
used in prostheses can cause abrasion, irritation and ulceration of tissues, as well as
bacterial colonization. It is known that an appropriate hydrophobic/hydrophilic balance of a drug molecule is essential for cell uptake. Biofouling (adsorption of contaminating matter, e.g. nonspecific proteins or organisms) is another serious problem
in relation to biomedical implants and devices. Polymers with antifouling properties
must be hydrophilic and electrically neutral, and must have H-bond acceptors but no
H-bond donors (Chapman et al. 2001). Here comes the role of the special functionalization of siloxanes, in order to modify the polarity and make the final material
more hydrophilic. The strategy to improve clinical performance by modifying the
surface of silicone elastomers is based on chemical, physical and combinations of
both methods (Abbasi et al. 2001; Hron 2003). In general, physical methods produce Si-OH groups, and therefore temporary hydrophilic surfaces, which can be
chemically modified with a permanent hydrophilic effect. The antifouling coatings
for PDMS were already reviewed by Zhang and Chiao (2015). Usually, these are
attached either by physical adsorption or by covalent bonding of appropriately
designed PEG/poly(ethylene oxide) (PEO) derivatives, polyzwitterions or other
compounds on the PDMS surface with or without preliminary UV/plasma treatment.
Goncalves et al. (2013) superficially polymerized PDMS using poly(ethylene
glycol)methacrylate by surface-initiated ATRP, preceded by UV/ozone exposure
and covalent bonding of the initiator (1-trichlorosilyl-2-(chloromethylphenyl)ethane) onto the hydroxylated PDMS via chemical vapor deposition. These authors
succeeded to obtain hydrophilic surfaces (with water contact angles around 60°),
thus resulting in the inhibition of bacterial adhesion and the non-cytotoxic effect on
human skin fibroblasts. In this same line, the surface modification of PDMS substrates by using ABA-type block copolymers which comprise poly(2- methacryloyl
C. Racles et al.