Gradient surfaces have become invaluable tools for the high-throughput characterisation of biomolecule- and cellmaterial
surface interactions as they allow for the screening and optimisation of surface parameters such as surface
chemistry, topography and ligand density in a single experiment. Here, we have generated surface chemistry gradients on
oxidised porous silicon (pSi) substrates using silane functionalisation. In these studies, pSi films with a pore size of 15-
30 nm and a layer thickness of around 1.7 ìm were utilised. The manufacture of gradient surface chemistries of silanes
was performed using a simple dip coating method, whereby an increasing incubation time of the substrate in a solution
of the silane led to increasing surface coverage of the silane. In this work, the hydrophobic n-octadecyldimethyl
chlorosilane (ODCS) and pentafluorophenyldimethyl chlorosilane (PFPS) were used since they were expected to
produce significant changes in wettability upon attachment. Chemical gradients were characterised using infrared (IR)
spectroscopy, X-ray photoelectron spectroscopy (XPS) and sessile drop water contact angle measurements. In addition,
the surface chemistry of the gradient was mapped using synchrotron IR microscopy. The ODCS gradient displayed
sessile drop water contact angles ranging from 12° to 71°, confirming the successful formation of a gradient. IR
microscopy and an XPS line scan confirmed the formation of a chemical gradient on the porous substrate. Furthermore,
the chemical gradients produced can be used for the high-throughput in vitro screening of protein and cell-surface
interactions, leading to the definition of surface chemistry on nanostructured silicon which will afford improved control
of biointerfacial interactions.
The ability to evaluate and control the cellular response to substrate materials is the key to a wide range of biomedical
applications ranging from diagnostic tools to regenerative medicine. Gradient surfaces provide a simple and fast method
for investigating optimal surface conditions for cellular responses such as attachment and growth. By using two
orthogonal gradients on the same substrate, a large space of possible combinations can be screened simultaneously. Here,
we have investigated the combination of a porous silicon (pSi) based topography gradient with a plasma polymer based
thickness gradient. pSi was laterally anodised on a 1.5 × 2.5cm2 silicon surface using hydrofluoric acid to form a pore
size gradient along a single direction. The resulting pSi was characterised by SEM and AFM and pore sizes ranging from
macro to mesoporous were found along the surface. Plasma polymerisation was used to form a thickness gradient
orthogonal to the porous silicon gradient. Here, allylamine was chosen as the monomer and a mask placed over the
substrate was used to achieve the thickness gradient. The analysis of this chemistry based gradient was carried out using
profilometry and XPS. It is expected that orthogonal gradient substrates will be used increasingly for the in vitro
screening of materials used in biomedical applications.
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