Additive Manufacturing and Surface Modification of Biomaterials using Selfassembled Monolayers - Biointerfaces: Where Material Meets Biology - page 42

including blood purification (algae removal from the bloodstream using
SAM-coated nanoparticles), drug delivery, surface modification of implants
and to study the cellular properties such as interfacial reactions between
cells and organelles and/or proteins and implants. 62

d n 3 r 4 n g | 0

2.3.2.1 Types of Self-assembled Monolayers
Some of the current monolayer self-assembly chemistries include thiols,
disulfides and thiols on gold; silanes on silicon dioxide and hydroxy func-
tionalised surfaces; fatty acids and phosph(on)ates on metal oxides and
isonitriles on platinum. Alkanethiol SAMs remain the most popular system
despite being limited to noble metals. 63 Thiols on gold surfaces have been
well studied and used for a variety of applications including corrosion pro-
tection, semi-conductors, biomaterial and biosensing applications. 64 Alka-
nethiol monolayers have also been formed on 316L stainless steel; however,
alkanethiol SAMs formed on 316L SS are not as stable as they are on gold
surfaces due to the complex surface chemistry of 316L SS. 58
Similarly, the use of silane monolayers on silica surfaces is popular owing
to their excellent grafting properties to silica surfaces. 63 The use of silane
monolayers on metal oxide surfaces such as titanium and cobalt-chromium
were also studied in recent years for drug delivery. 65,66 Mani et al. reported
that the silane monolayers formed on a cobalt-chromium alloy surface was
due to the covalent bonding of the SAMs with the alloy (Si-O-Cr and Si-O-
W). However, their study reported that these monolayers remained ordered
and bound to the alloy surface for only 7 days under in-vitro conditions. 66
Ajami et al. studied the functionalisation of silane monolayers with an anti-
bacterial drug on an electropolished titanium surface. 65 This study con-
cluded that the drug was released in a sustained manner over a 4-week
period under in-vitro conditions. Although the release of a drug over 4 weeks
is desirable, drug delivery from the implant surface for a longer period may
be necessary in clinical application. In such cases, this system cannot be
used. Apart from thiols and silane monolayers, phosphonic acids have
gained considerable interest in recent years due to their ability to bond to a
range of metal oxide surfaces and their relatively good hydrolytic stability
under physiological conditions. 67,68

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2.3.2.2 Overview of Phosphonate Monolayers
Phosphonates have three oxygens and a carbon directly attached to a
phosphorus molecule. The binding mechanism of these phosphonic acid
monolayers to metal oxide surfaces can be a monodentate, bidentate or
tridentate as represented in Figure 2.7. 61,69 This binding mechanism de-
pends on both the surface and the nature of the organophosphorus com-
pounds. Similar to thiol on gold, phosphonic acid monolayers adsorb on a
metal surface with a tail-up orientation with a tilt angle of the hydrocarbon
chains of about 301 with respect to the normal. 70 Phosphonic acid SAMs