The very low sliding friction at natural synovial joints, with friction coefficients µ < 0.002 at pressures up to 5 MPa or more, has not to date been attained in any man-made joints or between model surfaces in aqueous environments. We find that surfaces bearing grafted-from polyzwitterionic brushes in water can have µ values down to 0.0004 at pressures up to 7.5 MPa.This extreme lubrication is attributed primarily to the strong hydration of the phosphorylcholine-like monomers comprising the robustly-attached brushes, and may have relevance to a wide range of man-made aqueous lubrication situations. 3Rubbing of opposing bones during the articulation of mammalian joints is mediated by layers of articular cartilage coating their surfaces, which provide uniquely efficient lubrication as they slide past each other (1, 2). The associated very low friction at the high pressures of human joints such as hips or knees, with friction coefficients µ < 0.002, has not to date been emulated in man-made systems. Model studies (3-7) between smooth sliding surfaces bearing neutral or charged polymer brushes demonstrated sliding friction coefficients as low as µ < 0.001, values lower than with any other boundary lubricant system. As noted (8,9), in earlier studies with polymer brushes(3-6) the friction increases sharply at mean pressures P > ca. 0.3 MPa, which is far below the pressures, of 5 MPa or more, where low friction persists in nature. In the present work we sought to overcome the limitations of these earlier studies(3-6). We use polymer brushes, which make good boundary lubricants as they do not bridge the intersurface gap, that are strongly attached to each surface to resist being sheared off; and that are highly hydrated, to utilize the very efficient lubrication by hydration sheaths observed earlier(10, 11). 4 Figure 2 shows typical normal-force vs. surface separation profiles between pMPC brush-bearing mica surfaces in the standard crossed cylinder SFB configuration, in pure water and at salt concentrations ca. 0.01M and 0.1M NaNO 3 (the pMPC brush and monomer structure is inset to fig. 2A). Some contraction of the pMPC brushes is seen in the salt solutions relative to pure water. The highest normal loads F n applied are some 2 or more orders of magnitude higher than in earlier brush studies(4, 5) using the SFB. This leads to substantial flattening at the contact region, as indicated in the photo of the interference fringes, fig. 2B (and schematically in fig. 2C), from which the contact area A between the surfaces is measured directly (the mean pressures P across the flattened contact area are given by P = F n /A). Comparison with the control profiles from fig. 1A in the absence of polymer (dotted and broken curves in fig. 2A) reveals the extension of the unperturbed brushes from the macroinitiator layer, while fits to the force profiles provide more detailed information on the brush characteristics(12-section C, 13).We note the similarity of the profiles both on approach and separation of the surfaces (c...

Bacterial antibiotic resistance is becoming more widespread due to excessive use of antibiotics in healthcare and agriculture. At the same time the development of new antibiotics has effectively ground to a hold. Chemical modifications of material surfaces have poor long-term performance in preventing bacterial build-up and hence approaches for realising bactericidal action through physical surface topography have become increasingly important in recent years. The complex nature of the bacteria cell wall interactions with nanostructured surfaces represents many challenges while the design of nanostructured bactericidal surfaces is considered. Here we present a brief overview of the bactericidal behaviour of naturally occurring and bio-inspired nanostructured surfaces against different bacteria through the physico-mechanical rupture of the cell wall. Many parameters affect this process including the size, shape, density, rigidity/flexibility and surface chemistry of the surface nanotextures as well as factors such as bacteria specificity (e.g. gram positive and gram negative) and motility. Different fabrication methods for such bactericidal nanostructured surfaces are summarised.

Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century. Such monolayers generally consist of amphiphilic surfactants anchored by their polar headgroups; sliding occurs at the interface between the layers, greatly reducing friction and especially wear of the underlying substrates. This process, widespread in engineering applications, is also predicted to occur in biological lubrication via phospholipid films, though few systematic studies on friction between surfactant layers in aqueous environments have been carried out. Here we show that the frictional stress between two sliding surfaces bearing surfactant monolayers may decrease, when immersed in water, to as little as one per cent or less of its value in air (or oil). We attribute this to the shift of the slip plane from between the surfactant layers, to the surfactant/substrate interface. The low friction would then be due to the fluid hydration layers surrounding the polar head groups attached to the substrate. These results may have implications for future technological and biomedical applications.

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We have measured normal and lateral interactions across a range of different liquids between mica surfaces using a surface force balance (SFB). The mica surfaces were prepared either by melt cutting using Pt wire and standard procedures in our laboratories or by tearing sheets (that had not been exposed to Pt) off from a freshly cleaved sheet of mica. AFM micrographs revealed the substantial absence of Pt nanoparticles on the melt cut and torn-off mica surfaces. Normal-force versus surface-separation (D) profiles and shear force versus D measurements for purified water (no added salt), for concentrated aqueous NaCl solutions, and for cyclohexane revealed that in all cases the behavior of the highly confined liquids between melt-cut and between torn-off mica sheets was identical within experimental scatter. These results demonstrate directly that interactions measured between melt-cut mica surfaces as routinely prepared using established procedures in our laboratories and in other laboratories are free of the effect of any Pt contamination.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. 2018 PAMAM dendrimers have been conjectured for a wide range of biomedical applications due to their tuneable physicochemical properties. However, their application has been hindered by uncertainties in their cytotoxicity, which is influenced by dendrimer generation (i.e. size and surface group density), surface chemistry, and dosage, as well as cell specificity. In this review, biomedical applications of polyamidoamine (PAMAM) dendrimers and some related cytotoxicity studies are first outlined. Alongside these in vitro experiments, lipid membranes such as supported lipid bilayers (SLBs), liposomes, and Langmuir monolayers have been used as cell membrane models to study PAMAM dendrimer-membrane interactions. Related experimental and theoretical studies are summarized, and the physical insights from these studies are discussed to shed light on the fundamental understanding of PAMAM dendrimer-cell membrane interactions. We conclude with a summary of some questions that call for further investigations.a b s t r a c t a r t i c l e i n f o Available online 27 June

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