Papers by Nicholas Spencer
Soft Matter
Adhesive and nanotribological properties of end-grafted poly(acrylamide) (PAAm) films with variou... more Adhesive and nanotribological properties of end-grafted poly(acrylamide) (PAAm) films with various degrees of crosslinking, and in the presence of solvents over a broad spectrum of quality, were investigated by means of colloidal-probe atomic force microscopy. The solvent consisted of a mixture of water (good solvent for PAAm) and methanol (bad solvent for PAAm). Adhesion measurements carried out on brush (uncrosslinked) structures revealed significant pull-off forces in solvent mixtures that placed the polymer at its glass transition. These pull-off forces, which were orders of magnitude higher than those measured in either pure solvent, were significantly reduced in the presence of crosslinking. The nanostructures of PAAm films with different crosslinking degrees were elucidated in their maximum collapsed state by means of atomic force microscopy, and the parameters influencing adhesive properties of films with brush structures at their glass transition were investigated. Complex ...
ACS Applied Materials & Interfaces
We report the interaction of surface-tethered weak polyacid brushes, poly(methacrylic acid), with... more We report the interaction of surface-tethered weak polyacid brushes, poly(methacrylic acid), with a weak polybase poly(l-lysine)-graft-poly(ethylene glycol), in solution. The grafted polyacid brushes, grown directly from the silicon substrate by UVLED surface-initiated polymerization, acts as a nanotemplate for the solution-phase polybase, which penetrates into the brushes, forming a polyelectrolyte complex (PEC), whose mechanical and nanotribological properties are markedly influenced by the electrostatic assembly conditions. The mechanical effects are amplified due to the architecture of the specific polybase used, which contributes approximately 2k Da per unit charge to the overall system, resulting in an efficient filling of the polyacid brushes, which thus acts as a scaffold. The distribution of the adsorbed copolymers in the PEC films has been investigated by means of confocal microscopy. The unique structure of the PEC films provides a system whose mechanical and nanotribolog...
The ocular surface, Jan 16, 2015
The mechanical forces between the lid wiper and the ocular surface, and between a contact lens an... more The mechanical forces between the lid wiper and the ocular surface, and between a contact lens and the lid wiper, are reported to be related to dry eye symptoms. Furthermore, the mechanical forces between these sliding partners are assumed to be related to the ocular signs of lid-wiper epitheliopathy (LWE) and lid-parallel conjunctival folds (LIPCOF). Recent literature provides some evidence that a contact lens with a low coefficient of friction (CoF) improves wearing comfort by reducing the mechanical forces between the contact lens surface and the lid wiper. This review discusses the mechanical forces during spontaneous blinks from a tribological perspective, at both low and high sliding velocities, in a healthy subject. It concludes that the coefficient of friction of the ocular surfaces appears to be strongly comparable to that of hydrophilic polymer brushes at low sliding velocity, and that, with increased sliding velocity, there is no wear at the sliding partners` surfaces tha...
Chemical & Engineering News, 1967
Methods in Cell Biology, 2014
Surface-chemical and -morphological gradients can be extremely useful in cell-biological research... more Surface-chemical and -morphological gradients can be extremely useful in cell-biological research as high-throughput screening tools-for example, exposing a given set of cells to many different surface conditions at once, under identical ambient conditions, in order to monitor cell behavior such as proliferation or specific gene expression. They can also be used to investigate the effects of gradients themselves on cell behavior, such as migration. A number of simple, reliable techniques for both chemical- and morphological-gradient fabrication have been developed in our laboratories and are described in detail in the following.
Advanced Materials Interfaces, 2014
Physical review letters, Jan 27, 2013
Grazing-incidence Rutherford backscattering and angle-resolved x-ray photoelectron spectrometry a... more Grazing-incidence Rutherford backscattering and angle-resolved x-ray photoelectron spectrometry are used to determine the ion-concentration profiles near the surface of a solution consisting of a salt (TEABr) in a weakly polar organic liquid (polyethylene glycol) with atomic-layer depth resolution. The predictions of a model, in which ions in solution are repelled from the surface due to a screened Coulomb interaction with their image charge, are in good agreement with measured ion profiles. This contrasts with the behavior of salts in aqueous and highly polar organic solutions.
Microelectronic Engineering, 2015
ABSTRACT Arrays with well-defined particle registration are of high importance in device fabricat... more ABSTRACT Arrays with well-defined particle registration are of high importance in device fabrication for biosensors, electronics and optics. Also, materials exhibiting gradient variations of properties (e.g. wettability) in one, two or three dimensions have proven their capability for high-throughput screening of various interactions (e.g. cell–surface interactions). Here, we present the fabrication of cross-gradient particle arrays (CGPA) featuring a gradual cross-over from one particle type to another, while keeping the overall particle density constant. CGPAs were prepared by means of a capillary assembly setup assisted by a microfluidic chip. This setup offers a high level of control over the capillary assembly process with respect to the composition and location of the assembled arrays. A CGPA can be a versatile template or precursor for fabricating surface-bound gradient materials, e.g. by selectively transferring one population of assembled particles or by functionalizing particles before or after assembly. The resulting two complementary gradients may be used in combinatorial studies of biological and chemical interactions. We demonstrate the application of a CGPA as a two-level security feature with unclonable finger-print-like patterns. Moreover, we show the possibility of obtaining a 2D CGPA by capitalizing on diffusive transport within the capillary bridge perpendicular to the assembly direction.
International Journal of Environmental Research and Public Health, 2014
The aim of this study was to provide an overview of studies in which the impact of the 2008 econo... more The aim of this study was to provide an overview of studies in which the impact of the 2008 economic crisis on child health was reported. Structured searches of PubMed, and ISI Web of Knowledge, were conducted. Quantitative and qualitative studies reporting health outcomes on children, published since 2007 and related to the 2008 economic crisis were included. Two reviewers independently assessed studies for inclusion. Data were synthesised as a narrative review. Five hundred and six titles and abstracts were reviewed, from which 22 studies were included. The risk of bias for quantitative studies was mixed while qualitative studies showed low risk of bias. An excess of 28,000-50,000 infant deaths in 2009 was estimated in sub-Saharan African countries, and increased infant mortality in Greece was reported. Increased price of foods was related to worsening nutrition habits in disadvantaged families worldwide. An increase in violence against children was reported in the U.S., and inequalities in health-related quality of life appeared in some countries. Most studies suggest that the economic crisis has harmed children's health, and disproportionately affected the most vulnerable groups. There is an urgent need for further studies to monitor the child health effects of the global recession and to inform appropriate public policy responses.
Biomaterials, 2014
Tissue engineering using scaffold-cell constructs holds the potential to develop functional strat... more Tissue engineering using scaffold-cell constructs holds the potential to develop functional strategies to regenerate bone. The interface of orthopedic implants with the host tissues is of great importance for its later performance. Thus, the optimization of the implant surface in a way that could stimulate osteogenic differentiation of mesenchymal stem cells (MSCs) is of significant therapeutic interest. The effect of surface roughness of polycaprolactone (PCL) on the osteogenic differentiation of human bone-marrow MSCs was investigated. We prepared surface roughness gradients of average roughness (Ra) varying from the sub-micron to the micrometer range (∼0.5-4.7 μm), and mean distance between peaks (RSm) gradually varying from ∼214 μm to 33 μm. We analyzed the degree of cytoskeleton spreading, expression of alkaline phosphatase, collagen type 1 and mineralization. The response of cells to roughness divided the gradient into three groups of elicited stem cell behavior: 1) faster osteogenic commitment and strongest osteogenic expression; 2) slower osteogenic commitment but strong osteogenic expression, and 3) similar or inferior osteogenic potential in comparison to the control material. The stem-cell modulation by specific PCL roughness surfaces highlights the potential for creating effective solutions for orthopedic applications featuring a clinically relevant biodegradable material.
Tribology Letters, 2010
We have investigated the tribological properties of surfaces with adsorbed poly(L-lysine)-graft-p... more We have investigated the tribological properties of surfaces with adsorbed poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) sliding in aqueous glycerol solutions under different lubrication regimes. Glycerol is a polar, biocompatible liquid with a significantly higher viscosity than that of water. Macrotribological performance was investigated by means of pin-on-disk and mini-traction-machine measurements in glycerol-PLL-g-PEG-aqueous buffer mixtures of varying compositions. Adsorption studies of PLL-g-PEG from
Tribology Letters, 2013
ABSTRACT This paper reports on the frictional properties of smooth rubber substrates sliding agai... more ABSTRACT This paper reports on the frictional properties of smooth rubber substrates sliding against rigid surfaces covered with various densities of colloidal nano-particles (average diameter 77 nm). Friction experiments were carried out using a transparent poly(dimethyl siloxane) (PDMS) rubber contacting a silica lens with silica nano-particles sintered onto its surface. Using a previously described methodology (Nguyen et al., J Adhesion 87:235–250, 2011), surface shear stress and contact-pressure distribution within the contact were determined from a measurement of the displacement field at the surface of the PDMS elastomer. Addition of silica nano-particles results in a strong, pressure-independent enhancement of the frictional shear stress as compared to the smooth lens. The contribution of viscoelastic losses to these increased frictional properties is analyzed in the light of a numerical model that solves the contact problem between the rubber and the rough surface. An order-of-magnitude agreement is obtained between experimental and theoretical results, the latter showing that the calculation of viscoelastic dissipation within the contact is very sensitive to the details of the topography of the rigid asperities.
Tribology Letters, 2013
We report a highly effective macroscopic, nonaqueous lubrication strategy, employing polymer-brus... more We report a highly effective macroscopic, nonaqueous lubrication strategy, employing polymer-brushbased surface modification. Poly(alkyl methacrylates) with hexyl, dodecyl and octadecyl side chains have been prepared on both silicon and iron substrates by means of surface-initiated atom-transfer radical polymerisation. Frictional properties of the three polymers have been evaluated in hexadecane, ethanol and toluene. For the case of the poly(dodecyl methacrylate), the study additionally involved nine base lubricating fluids of different chemical natures, with viscosities ranging from 4 to 2,200 cSt at 20°C. By constructing a set of Stribeck curves at low values of speed 9 viscosity (at constant load), it was possible to identify an extended hydrodynamic lubrication regime and, for P12MA-based coatings, a low-friction, brush-lubrication regime, reinforced by time-dependant effects, which appeared to replace the mixed and boundary regimes observed for conventional lubricants. Self-mated poly(dodecyl methacrylate) brushes maintained low coefficient-of-friction values (e.g. 0.012 in hexadecane) over thousands of reciprocating cycles ([100 m total distance) under low speed 9 viscosity conditions (0.1 cm/s and 4 cSt).
Tribology International, 2005
We have investigated the influence of surface modification of an elastomer, poly(dimethylsiloxane... more We have investigated the influence of surface modification of an elastomer, poly(dimethylsiloxane) (PDMS), on its aqueous lubrication properties. A dramatic reduction in frictional forces has been observed upon hydrophilization by oxygen-plasma treatment or by surface coating with amphiphilic co-polymers, when PDMS was slid against PDMS in an aqueous environment. This effect is attributed to the removal of the strong hydrophobic interaction between PDMS surfaces in water, thereby enabling the isoviscous-elastic lubrication (or soft EHL) mechanism to predominate. This study demonstrates the significance of surface modification in allowing effective soft EHL of an elastomer. q
The Journal of Physical Chemistry B, 2005
PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol... more PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.
Soft Matter, 2008
Surface gradients of chemistry or morphology represent powerful tools for the high-throughput inv... more Surface gradients of chemistry or morphology represent powerful tools for the high-throughput investigation of interfacial phenomena in the areas of physics, chemistry, materials science and biology. A wide variety of methods for the fabrication of such gradients has been developed in recent years, relying on principles ranging from diffusion to time-dependent irradiation in order to achieve a gradual change of a particular parameter across a surface. In this review we have endeavoured to cover the principal fabrication approaches for surface-chemical and surface-morphological gradients that have been described in the literature, and to provide examples of their applications in a variety of different fields.
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Papers by Nicholas Spencer