An aqueous-based surface modification of poly(dimethylsiloxane) with poly(ethylene glycol) to prevent biofouling

A survey of state-of-the-art surface chemistries to minimize fouling from human and animal biofluids

Fouling of artificial surfaces by biofluids is a plague Biotechnology deeply suffers from. Herein, we inventory the state-of-the-art surface chemistries developed to minimize this effect from both human and animal biosamples.

Preparation of anti-fouling silicone elastomers by covalent immobilization of carboxybetaine

The surface-grafted carboxybetaine (CB) layer could enhance the biocompatibility of polydimethylsiloxane (PDMS) and reduce the adsorption of protein and adhesion of bacteria efficiently.

PDMS substrate stiffness affects the morphology and growth profiles of cancerous prostate and melanoma cells

A deep understanding of the interaction between cancerous cells and surfaces is particularly important for the design of lab-on-chip devices involving the use of polydimethylsiloxane (PDMS). In our studies, the effect of PDMS substrate stiffness on mechanical properties of cancerous cells was investigated in conditions where the PDMS substrate is not covered with any of extracellular matrix proteins. Two human prostate cancer (Du145 and PC-3) and two melanoma (WM115 and WM266-4) cell lines were cultured on two groups of PDMS substrates that were characterized by distinct stiffness, i.e. 0.75 ± 0.06 MPa and 2.92 ± 0.12 MPa. The results showed the strong effect on cellular behavior and morphology. The detailed analysis of chemical and physical properties of substrates revealed that cellular behavior occurs only due to substrate elasticity.

Modification of silicone elastomer with zwitterionic silane for durable antifouling properties

Biofouling on medical devices generally causes adverse complications, such as thrombosis, infection, and pathogenic calcification. Silicone is a widely used material for medical applications. Its surface modification typically encounters undesirable "hydrophobic recovery", leading to deterioration of surface engineering. In this study, we developed a stable superhydrophilic zwitterionic interface on polydimethylsiloxane (PDMS) elastomer by covalent silanization of sulfobetaine silane (SBSi) to resist nonspecific adsorption of bacteria, proteins, and lipids. SBSi is a zwitterionic organosilane assembly, enabling resisting surface reconstruction by forming a cross-linked network and polar segregation. Surface elemental composition was confirmed by X-ray photoelectron spectroscopy (XPS), and the long-term stability of modification was accessed using a contact angle goniometer. The biofouling tests were carried out by exposing substrates to bacterial, protein, and lipid solutions, revealing the excellent bioinertness of SBSi-tailored PDMS, even after 30 day storage in ambient. For the real-world application, we modified commercially available silicone hydrogel contact lenses with developed zwitterionic silane, presenting its antibacterial adhesion property. Moreover, the cytotoxicity of SBSi was accessed with NIH-3T3 fibroblast by the MTT assay, showing negligible cytotoxicity up to a concentration of 5 mM. Consequently, the strategy of surface engineering in this work can effectively retard the "hydrophobic recovery" occurrence and can be applied to other silicone-based medical devices in a facile way.

Engineering biomaterials surfaces to modulate the host response

Undesirable host response is responsible for the surface induced thrombus generation, activation of the complement system and the inflammatory reactions by the blood-contacting biomaterials. The surface interaction of biomaterials with different blood components is thought to be the critical factor that dictates the host response to biomaterials. Surface engineering can be utilized as a method to enhance the biocompatibility and tailor the biological response to biomaterials. This review provides a brief account of various polymer brush based approaches used for biomaterials surface modification, both passive and bioactive, to make the material surfaces biocompatible and antibacterial. Initially we discuss the utilization of polymer brushes with different structure and chemistry as a novel strategy to design the surface non-fouling that passively prevent the subsequent biological responses. Further we explore the utility of different bioactive agents including peptides, carbohydrates and proteins which can be conjugated the polymer brush to make the surface actively interact with the body and modulate the host response. A number of such avenues have also been explored in this review.

Fabrication of a photocontrolled surface with switchable wettability based on host-guest inclusion complexation and protein resistance

A novel surface-modification strategy has been developed for the construction of a photocontrolled silicon wafer surface with switchable wettability based on host-guest inclusion complexation. The silicon wafer was first modified by guest molecule azobenzene (Azo) via a silanization reaction. Subsequently, a series of polymers with different polarities were attached to host molecule β-cyclodextrin (β-CD) to prepare β-CD-containing hemitelechelic polymers via click chemistry. Finally, a photocontrolled silicon wafer surface modified with polymers was fabricated by inclusion complexation between β-CD and Azo, and the surface properties of the substrate are dependent on the polymers we used. The elemental composition, surface morphology, and hydrophilic/hydrophobic property of the modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope, and contact angle measurements, respectively. The antifouling property of the PEG-functionalized surface was evaluated by a protein adsorption assay using bovine serum albumin, which was also characterized by XPS. The results demonstrate that the surface modified with PEG possesses good protein-resistant properties.

Modification of the adhesive properties of silicone-based coatings by block copolymers

The improvement of the (bio)adhesive properties of elastomeric polydimethylsiloxane (PDMS) coatings is reported. This is achieved by a surface modification consisting of the incorporation of block copolymers containing a PDMS block and a poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) block in a PDMS matrix, followed by matrix cross-linking and immersion of the obtained materials in water. Contact angle measurements (CA), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) showed the presence of the PDMAEMA block at the surface, drastic morphology changes, and improved adhesion properties after immersion in water. Finally, underwater bioadhesion tests show that mussels adhere only to block copolymer-filled coatings and after immersion in water, i.e., when the PDMAEMA blocks have been brought to the coating surface. These observations highlight the significant role of hydrophilic groups in the surface modification of silicone coatings.

Physical manipulation of the Escherichia coli chromosome reveals its soft nature

Replicating bacterial chromosomes continuously demix from each other and segregate within a compact volume inside the cell called the nucleoid. Although many proteins involved in this process have been identified, the nature of the global forces that shape and segregate the chromosomes has remained unclear because of limited knowledge of the micromechanical properties of the chromosome. In this work, we demonstrate experimentally the fundamentally soft nature of the bacterial chromosome and the entropic forces that can compact it in a crowded intracellular environment. We developed a unique "micropiston" and measured the force-compression behavior of single Escherichia coli chromosomes in confinement. Our data show that forces on the order of 100 pN and free energies on the order of 10(5) k(B)T are sufficient to compress the chromosome to its in vivo size. For comparison, the pressure required to hold the chromosome at this size is a thousand-fold smaller than the surrounding turgor pressure inside the cell. Furthermore, by manipulation of molecular crowding conditions (entropic forces), we were able to observe in real time fast (approximately 10 s), abrupt, reversible, and repeatable compaction-decompaction cycles of individual chromosomes in confinement. In contrast, we observed much slower dissociation kinetics of a histone-like protein HU from the whole chromosome during its in vivo to in vitro transition. These results for the first time provide quantitative, experimental support for a physical model in which the bacterial chromosome behaves as a loaded entropic spring in vivo.

Surface chemistry to minimize fouling from blood-based fluids

Upon contact with bodily fluids/tissues, exogenous materials spontaneously develop a layer of proteins on their surface. In the case of biomedical implants and equipment, biological processes with deleterious effects may ensue. For biosensing platforms, it is synonymous with an overwhelming background signal that prevents the detection/quantification of target analytes present in considerably lower concentrations. To address this ubiquitous problem, tremendous efforts have been dedicated over the years to engineer protein-resistant coatings. There is now extensive literature available on stealth organic adlayers able to minimize fouling down to a few ng cm(-2), however from technologically irrelevant single-protein buffered solutions. Unfortunately, few coatings have been reported to present such level of performance when exposed to highly complex proteinaceous, real-world media such as blood serum and plasma, even diluted. Herein, we concisely review the surface chemistry developed to date to minimize fouling from these considerably more challenging blood-based fluids. Adsorption dynamics is also discussed.

Lipid-based passivation in nanofluidics

Stretching DNA in nanochannels is a useful tool for direct, visual studies of genomic DNA at the single molecule level. To facilitate the study of the interaction of linear DNA with proteins in nanochannels, we have implemented a highly effective passivation scheme based on lipid bilayers. We demonstrate virtually complete long-term passivation of nanochannel surfaces to a range of relevant reagents, including streptavidin-coated quantum dots, RecA proteins, and RecA-DNA complexes. We show that the performance of the lipid bilayer is significantly better than that of standard bovine serum albumin-based passivation. Finally, we show how the passivated devices allow us to monitor single DNA cleavage events during enzymatic degradation by DNase I. We expect that our approach will open up for detailed, systematic studies of a wide range of protein-DNA interactions with high spatial and temporal resolution.

Micro and nanotechnological tools for study of RNA

Micro and nanotechnologies have originally contributed to engineering, especially in electronics. These technologies enable fabrication and assembly of materials at micrometer and nanometer scales and the manipulation of nano-objects. The power of these technologies has now been exploited in analyzes of biologically relevant molecules. In this review, the use of micro and nanotechnological tools in RNA research is described.

Polyurethane-based microfluidic devices for blood contacting applications

Protein adsorption on PDMS surfaces poses a significant challenge in microfluidic devices that come into contact with biofluids such as blood. Polyurethane (PU) is often used for the construction of medical devices, but despite having several attractive properties for biointerfacing, it has not been widely used in microfluidic devices. In this work we developed two new fabrication processes for making thin, transparent and flexible PU-based microfluidic devices. Methods for the fabrication and bonding of microchannels, the integration of fluidic interconnections and surface modification with hydrophilic polyethylene oxide (PEO) to reduce protein adsorption are detailed. Using these processes, microchannels were produced having high transparency (96% that of glass in visible light), high bond strength (326.4 kPa) and low protein adsorption (80% reduction in fibrinogen adsorption vs. unmodified PDMS), which is critical for prevention of fouling. Our findings indicate that PEO modified PU could serve as an effective alternative to PDMS in blood contacting microfluidic applications.

Enhancing single molecule imaging in optofluidics and microfluidics

Microfluidics and optofluidics have revolutionized high-throughput analysis and chemical synthesis over the past decade. Single molecule imaging has witnessed similar growth, due to its capacity to reveal heterogeneities at high spatial and temporal resolutions. However, both resolution types are dependent on the signal to noise ratio (SNR) of the image. In this paper, we review how the SNR can be enhanced in optofluidics and microfluidics. Starting with optofluidics, we outline integrated photonic structures that increase the signal emitted by single chromophores and minimize the excitation volume. Turning then to microfluidics, we review the compatible functionalization strategies that reduce noise stemming from non-specific interactions and architectures that minimize bleaching and blinking.

Silicate glass coated microchannels through a phase conversion process for glass-like electrokinetic performance

The surface modified polydimethylsiloxane (PDMS) microchannels show a much more inferior performance to the durable and reproducible glass chip. In this paper, a facile approach to preparing a silicate glass modified PDMS microchannel for glass-like performance is presented. This glass-like performance is made possible by a phase conversion of a preceramic polymer--allylhydridopolycarbosilane (AHPCS). The, several hundred nanometer thick, polymer that coats the PDMS channel is hydrolyzed to form hydrophilic silicate glass via phase conversion under an aqueous alkali condition. It is characterized by XPS, FTIR-ATR, AFM, and contact angle measurements. The silicate glass coated PDMS channel from AHPCS has an excellent solvent resistance, delivers a high electroosmotic flow (EOF) that is stable in the long-term (4.9±0.1×10(-4) cm(2) V(-1) s(-1)) and a reliable capillary electrophoresis (CE), which are comparable to those of native glass channels. Moreover, the silicate glass PDMS channel allows easy regeneration of the electrokinetic behavior, just as in a glass channel, by a simple treatment with alkali solution. This coating approach can be applied to other polymer substrates such as polyimide (PI).

Layer-by-layer films made from extracellular matrix macromolecules on silicone substrates

The layer-by-layer (LbL) technique has been widely used to produce nanofilms for biomedical applications. Naturally occurring polymers such as ECM macromolecules are attractive candidates for LbL film preparation. In this study, we assessed the build-up of type I collagen (Col1)/chondroitin sulfate (CS) or Col1/Heparin (HN) on polydimethylsiloxane (PDMS) substrates. The build-up was assessed by quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). Integrin-mediated cell adhesion was assessed by studying the cytoskeletal organization of mammalian primary cells (chondrocytes) seeded on different end layers and number of layers. Data generated from the QCM-D observations showed a consistent build-up of films with more adsorption in the case of Col1/HN. Col1/CS films were stable in media, whereas Col1/HN films were not. AFM analysis showed that the layers were fibrillar in structure for both systems and between 20 and 30 nm thick. The films promoted cell adhesion when compared with tissue culture plastic in serum-free media with cycloheximide. Crosslinking of the films resulted in constrained cell spreading and a ruffled morphology. Finally, beta1 integrin blocking antibodies prevented cell spreading, suggesting that cell adhesion and spreading were mediated mainly by interaction with the collagen fibrils. The ability to construct stable ECM-based films on PDMS has particular relevance in mechanobiology, microfluidics, and other biomedical applications.

Biosensors in microfluidic chips

A biosensor is a sensing device that incorporates a biological sensing element and a transducer to produce electrochemical, optical, mass, or other signals in proportion to quantitative information about the analytes in the given samples. The microfluidic chip is an attractive miniaturized platform with valuable advantages, e.g., low cost analysis requiring low reagent consumption, reduced sample volume, and shortened processing time. Combination of biosensors and microfluidic chips enhances analytical capability so as to widen the scope of possible applications. This review provides an overview of recent research activities in the field of biosensors integrated on microfluidic chips, focusing on the working principles, characteristics, and applicability of the biosensors. Theoretical background and applications in chemical, biological, and clinical analysis are summarized and discussed.

A self-pumping lab-on-a-chip for rapid detection of botulinum toxin

A robust poly(dimethylsiloxane) (PDMS) surface treatment was utilized for the development of a self-pumping lab-on-a-chip (LOC) to rapidly detect minute quantities of toxic substances. One such toxin, botulinum neurotoxin (BoNT), is an extremely lethal substance, which has the potential to cause hundreds of thousands of fatalities if as little as a few grams are released into the environment. To prevent such an outcome, a quick (<45 min) and sensitive detection format is needed. We have developed a self-pumping LOC that can sense down to 1 pg of BoNT type A (in a 1 microL sample) within 15 min in an autonomous manner. The key technologies enabling for such a device are a sensitive electrochemical sensor, an optimized fluidic network and a robust hydrophilic PDMS coating, thereby facilitating autonomous delivery of liquid samples for rapid detection. The stability, simplicity and portability of this device make possible for a storable and distributable system for monitoring bioterrorist attacks.

Hydrophilic biopolymer grafted on poly(dimethylsiloxane) surface for microchip electrophoresis

A novel covalent strategy was developed to modify the poly(dimethylsiloxane) (PDMS) surface. Briefly, dextran was selectively oxidized to aldehyde groups with sodium periodate and subsequently grafted onto amine-functionalized PDMS surface via Schiff base reaction. As expected, the coated PDMS surface efficiently prevented the biomolecules from adsorption. Electro-osmotic flow (EOF) was successfully suppressed compared with that on the native PDMS microchip. Moreover, the stability of EOF was greatly enhanced and the hydrophilicity of PDMS surface was also improved. To apply thus-coated microchip, the separation of peptides, protein and neurotransmitters was investigated in detail. For comparison, these analytes were also measured on the native PDMS microchips. The results demonstrated that these analytes were efficiently separated and detected on the coated PDMS microchips. Furthermore, the relative standard deviations of their migration times for run-to-run, day-to-day, and chip-to-chip reproducibilities were in the range of 0.6-2.7%. In addition, the coated PDMS microchips showed good stability within 1 month.

A novel low-friction surface for biomedical applications: modification of poly(dimethylsiloxane) (PDMS) with polyethylene glycol(PEG)-DOPA-lysine

Aqueous biocompatible tribosystems are desirable for a variety of tissue-contacting medical devices. L-3,4-dihydroxyphenylalanine (DOPA) and lysine (K) peptide mimics of mussel adhesive proteins strongly interact with surfaces and may be useful for surface attachment of lubricating polymers in tribosystems. Here, we describe a significant improvement in lubrication properties of poly (dimethylsiloxane) (PDMS) surfaces when modified with PEG-DOPA-K. Surfaces were characterized by optical and atomic force microscopy, contact angle, PM-IRRAS, and X-ray photoelectron spectroscopy. Such surfaces, tested over the course of 200 rotations ( approximately 8 m in length), maintained an extremely low friction coefficient (mu) (0.03 +/- 0.00) compared to bare PDMS (0.98 +/- 0.02). These results indicate the potential applications of PEG-DOPA-K for the modification of device surfaces. Extremely low mu values were maintained over relatively long length scales and a range of sliding speeds without the need for substrate pre-activation and in the absence of excess polymer in aqueous solution. These results were only obtained when DOPA was bound to lysine (modification with PEG-DOPA did not have an effect on mu) suggesting the critical role of lysine in obtaining a lowered friction coefficient.

Surface molecular property modifications for poly(dimethylsiloxane) (PDMS) based microfluidic devices

Fast advancements of microfabrication processes in past two decades have reached to a fairly matured stage that we can manufacture a wide range of microfluidic devices. At present, the main challenge is the control of nanoscale properties on the surface of lab-on-a-chip to satisfy the need for biomedical applications. For example, poly(dimethylsiloxane) (PDMS) is a commonly used material for microfluidic circuitry, yet the hydrophobic nature of PDMS surface suffers serious nonspecific protein adsorption. Thus the current major efforts are focused on surface molecular property treatments for satisfying specific needs in handling macro functional molecules. Reviewing surface modifications of all types of materials used in microfluidics will be too broad. This review will only summarize recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology and classify them into two main categories: (1) physical approach including physisorption of charged or amphiphilic polymers and copolymers, as well as (2) chemical approach including self assembled monolayer and thick polymer coating. Pros and cons of a collection of available yet fully exploited surface modification methods are briefly compared among subcategories.

Single-chromophore-based photoswitchable nanoparticles enable dual-alternating-color fluorescence for unambiguous live cell imaging

We have developed a class of spiropyran dyes and their fluorescence colors can be reversibly photoswitched from red to green, blue, or nearly dark, thus alternating between two colors. Such individual dyes emit either one color or the other but not both simultaneously. Nanoparticles enabled with these photoswitchable dyes, however, emit either one pure color or a combination of both colors because the nanoparticle fluorescence originates from multiple dyes therein. As a result, the nanoparticle shines >30 times brighter than state-of-the-art organic dyes such as fluorescein. Interestingly, these copolymer nanoparticles exhibit tunable nonspecific interactions with live cells, and nanoparticles containing properly balanced butyl acrylate and acrylamide monomers render essentially very little nonspecific binding to live cells. Decorated with HMGA1 protein, these optically switchable dual-color nanoparticles undergo endocytosis and unambiguously identify themselves from fluorescence interference including autofluorescence, thus enabling a new tool for live cell imaging.

Solvent-dependent friction force response of poly(ethylenimine)-graft-poly(ethylene glycol) brushes investigated by atomic force microscopy

Lateral and normal forces between a surface-bound, brushlike copolymer, poly(ethylenimine)-graft-poly(ethylene glycol) (PEI-g-PEG), and a silica colloidal probe were investigated with atomic force microscopy (AFM) and related to the relative mass of the solvent absorbed within the polymer as measured with the quartz crystal microbalance. PEI-g-PEG was adsorbed onto an oxide-passivated silicon wafer through its exposure to physiologically buffered solutions of the polymer. Frictional forces were measured between the colloidal probe and the substrate by AFM as the polarity of the solvent was systematically varied. Reduced friction forces and greater film thicknesses were encountered under solvents of higher polarity, which are attributed to the extended conformation of the brushlike copolymer under these conditions. Lateral and normal forces detected between the colloidal probe and this surface-bound PEI-g-PEG were found to be similar under certain solvent conditions to those measured for poly(L-lysine)-graft-poly(ethylene glycol), a brushlike copolymer with a different molecular architecture. To this end, friction force studies of both symmetric and asymmetric PEI-g-PEG-coated interfaces served to identify the contributions of conformational and bridging effects in the observed tribological behavior.

Determination of the electron escape depth for NEXAFS spectroscopy

A novel method was developed to determine carbon atom density as a function of depth by analyzing the postedge signal in near-edge X-ray absorption fine structure (NEXAFS) spectra. We show that the common assumption in the analysis of NEXAFS data from polymer films, namely, that the carbon atom density is constant as a function of depth, is not valid. This analysis method is then used to calculate the electron escape depth (EED) for NEXAFS in a model bilayer system that contains a perfluorinated polyether (PFPE) on top of a highly oriented pyrolitic graphite (HOPG) sample. Because the carbon atom densitites of both layers are known, in addition to the PFPE surface layer thickness, the EED is determined to be 1.95 nm. This EED is then used to measure the thickness of the perfluorinated surface layer of poly(4-(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene) (PFPS).

The nanofabrication of polydimethylsiloxane using a focused ion beam

Nanofabrication on insulating and flexible films of polydimethylsiloxane (PDMS) using a focused ion beam (FIB) has been illustrated in this study. The charge accumulation effect, which is inevitable in polymeric fabrication, was shown to be relieved by simultaneously introducing electron beam flooding in the area exposed to FIB. The topography of the fabricated pattern is subsequently characterized by using an atomic force microscope (AFM), by which the dependence of height/depth of the fabricated arrays on ion beam dose could be obtained. In addition, the swelling effect and milling effect relating to focused ion beam dose could be identified in this study.

Adsorption properties of poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) at a hydrophobic interface: influence of tribological stress, pH, salt concentration, and polymer molecular weight

The adsorption properties of a graft copolymer of poly(ethylene glycol) (PEG) with a polycationic backbone, namely, poly( l-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG), onto nonpolar, hydrophobic PDMS surfaces from aqueous solution and the lubrication properties of the self-mated sliding contacts of PDMS surfaces modified with PLL- g-PEG have been investigated. Whereas PLL- g-PEG is spontaneously attracted to negatively charged surfaces as a result of the polycationic PLL backbone, the collective interaction of (CH 2) 4 hydrocarbon moieties on the lysine units in the PLL backbone with nonpolar, hydrophobic surfaces also enables the adsorption of PLL- g-PEG onto hydrophobic surfaces such as PDMS. The adsorption and lubrication properties of PLL- g-PEG have been investigated by varying the aqueous solution parameters, such as pH (2, 7, and 12) and KCl concentration (0, 0.01, 0.1, and 1 M) as well as the length of the PLL backbone of the copolymer (20 vs 375 kDa). In the absence of tribological stress, the adsorption of PLL- g-PEG onto PDMS surfaces was mainly governed by the KCl concentration, whereas the role of pH or the molecular weight of the copolymer was of relatively minor importance; for all pH values, the adsorbed mass decreased with increasing KCl concentration. Under tribological stress, however, a clear dependence of the lubrication properties of PLL- g-PEG on all of the studied parameters, including pH, KCl concentration, and backbone molecular weight, was observed. The adsorption strength of PLL- g-PEG on PDMS surfaces, rather than the adsorbed mass itself, appeared to be the most critical parameter in determining the lubrication properties.

Adhesion forces between Staphylococcus epidermidis and surfaces bearing self-assembled monolayers in the presence of model proteins

Self-assembled monolayers (SAMs) are being developed into coatings to reduce microbial biofilm formation on biomaterials. To test anti-adhesion properties, SAMs can be easily constructed on gold, and used to represent a coated biomaterial. However, coatings that prevent bacterial adhesion must also resist protein adsorption. We explored the competitive effects of bacteria and protein for adsorption to SAMs, choosing fetal bovine serum (FBS) to represent protein non-specific binding, and fibronectin (FN) to evaluate ligand/receptor binding. Staphylococcus epidermidis were immobilized on an atomic force microscope (AFM) tip and used as a force probe to detect the interaction forces between bacteria and gold-coated SAMs. The SAMs tested were alkanethiol molecules terminating in isophthalic acid (IPA) or isophthalic acid with silver (IAG). While S. epidermidis showed weak interactions with FBS, the bacteria showed strong adhesion with FN, due to ligand/receptor binding. Bacterial retention and viability experiments were correlated with the force measurements. S. epidermidis interacting with IAG SAMs showed a loss of viability, due to the mobility of silver ions. For most substrata, there was a link between high adhesion forces with bacteria and a high percentage of dead cells being retained on that substratum (even in the absence of a specific biocidal effect, such as silver). This may suggest that high adhesion forces can cause stress to the bacteria which contributed to their death. The relationship between highly adhesive SAMs and bacterial inactivation may be useful in future biomaterial design. When evaluating coatings for biomaterials, it is important to consider the interplay between bacteria, proteins, and the coating material.