Ali E. Özçam

Formation of surface-grafted polymeric amphiphilic coatings comprising ethylene glycol and fluorinated groups and their response to protein adsorption

Amphiphilic polymer coatings were prepared by first generating surface-anchored polymer layers of poly(2-hydroxyethyl methacrylate) (PHEMA) on top of flat solid substrates followed by postpolymerization reaction on the hydroxyl terminus of HEMA’s pendent group using three classes of fluorinating agents, including organosilanes, acylchlorides, and trifluoroacetic anhydride (TFAA). The distribution of the fluorinated groups inside the polymer brushes was assessed by means of a suite of analytical probes, including contact angle, ellipsometry, infrared spectroscopy, atomic force microscopy, and near-edge x-ray absorption fine structure spectroscopy. While organosilane modifiers were found to reside primarily close to the tip of the brush, acylchlorides penetrated deep inside PHEMA thus forming random copolymers P(HEMA-co-fHEMA). The reaction of TFAA with the PHEMA brush led to the formation of amphiphilic diblocks, PHEMA-b-P(HEMA-co-fHEMA), whose bottom block comprised unmodified PHEMA and the top block was made of P(HEMA-co-fHEMA) rich in the fluorinated segments. This distribution of the fluorinated groups endowed PHEMA-b-P(HEMA-co-fHEMA) with responsive properties; while in hydrophobic environment P(HEMA-co-fHEMA) segregated to the surface, when in contact with a hydrophilic medium, PHEMA partitioned at the brush surface. The surface activity of the amphiphilic coatings was tested by studying the adsorption of fibrinogen (FIB). While some FIB adsorption occurred on most coatings, the ones made by TFAA modification of PHEMA remained relatively free of FIB.

Poly(vinylmethylsiloxane) elastomer networks as functional materials for cell adhesion and migration studies.

Cell migration is central to physiological responses to injury and infection and in the design of biomaterial implants. The ability to tune the properties of adhesive materials and relate those properties in a quantitative way to the dynamics of intracellular processes remains a definite challenge in the manipulation of cell migration. Here, we propose the use of poly(vinylmethylsiloxane) (PVMS) networks as novel substrata for cell adhesion and migration. These materials offer the ability to tune independently chemical functionality and elastic modulus. Importantly, PVMS networks are compatible with total internal reflection fluorescence (TIRF) microscopy, which is ideal for interrogating the cell-substratum interface; this latter characteristic presents a distinct advantage over polyacrylamide gels and other materials that swell with water. To demonstrate these capabilities, adhesive peptides containing the arginyl-glycyl-aspartic acid (RGD) tripeptide motif were successfully grafted to the surface of PVMS network using a carboxyl-terminated thiol as a linker. Peptide-specific adhesion, spreading, and random migration of NIH 3T3 mouse fibroblasts were characterized. These experiments show that a peptide containing the synergy sequence of fibronectin (PHSRN) in addition to RGD promotes more productive cell migration without markedly enhancing cell adhesion strength. Using TIRF microscopy, the dynamics of signal transduction through the phosphoinositide 3-kinase pathway were monitored in cells as they migrated on peptide-grafted PVMS surfaces. This approach offers a promising avenue for studies of directed migration and mechanotransduction at the level of intracellular processes.

Self-assembly fronts in collision: impinging ordering organosilane layers

Colliding autocatalytic wave-fronts of self-assembling organosilane (OS) layers are generated through the controlled positioning of sources of the volatile OS material at the edges of a silica wafer and through adjustment of the container dimensions in which the wafer sources are placed. The concentration profiles and molecular orientation of the OS colliding wave-fronts are assessed by means of combinatorial near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. For systems involving self-assembly wave-fronts developing from the same OS precursor molecule, the shapes of interfacial region arising from front collision are centro-symmetrical and slowly ‘heal’ to form a uniform OS layer. In contrast, heterogeneous systems, involving OS molecules having different chemistries exhibit different rates of advance and highly non-symmetrical concentration profiles after front collision. We discuss the general nature of our OS colliding front data in terms of a mean field model of colliding reaction–diffusion fronts that generalizes a model introduced before for describing single OS front propagation.