Bacterial Adhesion Is Affected by the Thickness and Stiffness of Poly(ethylene glycol) Hydrogels

Film Formation of Iodinated Latex Dispersions and Its Role in Their Antimicrobial Activity

Waterborne coatings with intrinsic antibacterial attributes have attracted significant attention due to their potential in mitigating microbial contamination while simultaneously addressing the environmental drawbacks of their solvent-based counterparts. Typically, antimicrobial coatings are designed to resist and eliminate microbial threats, encompassing challenges such as biofilm formation, fungal contamination, and proliferation of black mold. Iodine, when solubilized using ethylene glycol and incorporated as a complex into waterborne latex dispersions, has shown remarkable antimicrobial activity. Here, we demonstrate the effect of the film formation process of these iodinated latex dispersions on their antimicrobial properties. The effect of iodine on the surface morphology and mechanical, adhesion, and antimicrobial properties of the generated films was investigated. Complete integration and uniform distribution of iodine in the films were confirmed through UV-vis spectrophotometry and a laser Raman imaging system (LRIS). In terms of properties, iodinated films showed improved mechanical strength and adhesion compared with blank films. Further, the presence of iodine rendered the films rougher, making them susceptible to bacterial adhesion, but interestingly provided enhanced antibiofilm activity. Moreover, thicker films had a lower surface roughness and reduced biofilm growth. These observations are elucidated through the complex interplay among film thickness, surface morphology, and iodine properties. The insights into the interlink between the film formation process and antimicrobial properties of iodinated latex dispersions will facilitate their enhanced application as sustainable alternatives to solvent-based coatings.

Polyethylene glycol hydrogel coatings for protection of electroactive bacteria against chemical shocks

Loss of bioelectrochemical activity in low resource environments or from chemical toxin exposure is a significant limitation in microbial electrochemical cells (MxCs), necessitating the development of materials that can stabilize and protect electroactive biofilms. Here, polyethylene glycol (PEG) hydrogels were designed as protective coatings over anodic biofilms, and the effect of the hydrogel coatings on biofilm viability under oligotrophic conditions and ammonia-N (NH4+-N) shocks was investigated. Hydrogel deposition occurred through polymerization of PEG divinyl sulfone and PEG tetrathiol precursor molecules, generating crosslinked PEG coatings with long-term hydrolytic stability between pH values of 3 and 10. Simultaneous monitoring of coated and uncoated electrodes co-located within the same MxC anode chamber confirmed that the hydrogel did not compromise biofilm viability, while the coated anode sustained nearly a 4 × higher current density (0.44 A/m2) compared to the uncoated anode (0.12 A/m2) under oligotrophic conditions. Chemical interactions between NH4+-N and PEG hydrogels revealed that the hydrogels provided a diffusive barrier to NH4+-N transport. This enabled PEG-coated biofilms to generate higher current densities during NH4+-N shocks and faster recovery afterwards. These results indicate that PEG-based coatings can expand the non-ideal chemical environments that electroactive biofilms can reliably operate in.

Light-sensitive PEG hydrogel with antibacterial performance for pacemaker pocket infection prevention

Prevention of cardiovascular implantable electronic devices (CIED) infection is crucial for successful outcomes. In this study, we report an adhesive and antibacterial hydrogel coating for CIED infection treatment, by immobilizing polyethylene glycol (PEG) and 2'-O-hydroxypropyl trimethyl ammonium chloride chitosan (HAC) on Ti surface. Initial alkali and APTES treatment caused the formation of -NH2 to enhance the adhesion of the hydrogel coating to Ti implants, followed by immobilizing a photo-cross-linkable PEG/2'-O-HTACCS hydrogel on Ti/OH/NH2 surface. Surface characterization of Ti/OH/NH2 sample and adhesion testing of hydrogel on Ti/OH/NH2 surface confirm successful immobilization of hydrogel onto the Ti/OH/NH2 surface. In vitro and in vivo antimicrobial results exhibited that the photo-cross-linkable PEG/HAC composite hydrogel has excellent antimicrobial capabilities against both Grampositive (S. aureus and S. epidermidis) and Gram-negative (P. aeruginosa and E. coli) bacteria. The outcome of this study demonstrates the photo-cross linked PEG/HAC coating hydrogels can be easily formed on the Ti implants, and has great potential in preventing CIED pocket infection.

Dynamics of underwater microparticle adhesion to soft hydrated surfaces: Modeling and analysis by time-dependent AFM force spectroscopy

HYPOTHESIS

Understanding the attachment and detachment of microparticles and living cells to surfaces is crucial for developing antifouling strategies. Hydrogel coatings have shown promise in reducing fouling and particle adhesion due to their softness and high water content, yet the mechanisms involved are dynamic and complex, and relevant parameters are not easily accessible. AFM-based force spectroscopy (FS) experiments with colloidal probe particles is a direct way of evaluating adhesive and mechanical relaxational dynamics, yet their interpretation and modeling has been challenging. The present study proposes and examines several dynamic models, suitable for quantitative analysis of FS results with model probe particle on hydrogels surfaces.

EXPERIMENTS

FS were performed using polyethylene glycol (PEG) hydrogels and polystyrene microspheres including particle attachement to the hydrogel surface (loading), holding the particle on the surface with a constant force for variable times (dwell) and pulling the particle away from the surface (unloading) FINDINGS: It was found that a viscoelastic extension of the classical JKR model with energy of adhesion unevenly distributed over the contact area and vanishing at its circumferences accurately described all FS experiments and yielded physically consistent viscoelastic and adhesive dynamic parameters, steadily changing with dwell time and applied force. The observed time evolution and force dependence were rationalized as combination of osmotic and osmo-mechnical relaxation in the contact region.

Stiffness and Oligomer Content Affect the Initial Adhesion of Staphylococcus aureus to Polydimethylsiloxane Gels

The growing prevalence of methicillin-resistant Staphylococcus aureus (S. aureus) infections necessitates a greater understanding of their initial adhesion to medically relevant surfaces. In this study, the influence of the mechanical properties and oligomer content of polydimethylsiloxane (PDMS) gels on the initial attachment of Gram-positive S. aureus was explored. Small-amplitude oscillatory shear rheological measurements were conducted to verify that by altering the base to curing (B:C) ratio of the commonly used Sylgard 184 silicone elastomer kit (B:C ratios of 60:1, 40:1, 10:1, and 5:1), PDMS gels could be synthesized with Young's moduli across four distinct regimes: ultrasoft (15 kPa), soft (30 kPa), standard (400 kPa), and stiff (1500 kPa). These as-prepared gels (unextracted) were compared to gels prepared from the same B/C ratios that underwent Soxhlet extraction to remove any unreacted oligomers. While the Young's moduli of unextracted and extracted PDMS gels prepared from the same B:C ratio were statistically equivalent, the associated adhesion failure energy statistically decreased for the ultrasoft gels after extraction (from 25 to 8 J/mm2). The interactions of these eight well-characterized gels with bacteria were tested by using S. aureus SH1000, a commonly studied laboratory strain, as well as S. aureus ATCC 12600, which was isolated from a human lung infection. Increased S. aureus inactivation occurred only when the bacteria were incubated directly on top of the unextracted gels prepared at high B:C ratios (40:1 and 60:1), whereas none of the extracted gels (no unreacted oligomers) had significant levels of inactivated bacteria. S. aureus adhered the least to the stiffest extracted PDMS gels (no unreacted oligomers) and the most to soft, unextracted PDMS gels (with ∼17% unreacted oligomers). These findings suggest that both unreacted oligomers and Young's moduli are important material factors to consider when exploring the attachment behavior of Gram-positive S. aureus to hydrophobic elastomer gels.

Synthesis and Characterization of Durable Antibiofilm and Antiviral Silane-Phosphonium Thin Coatings for Medical and Agricultural Applications

Pathogens such as bacteria and viruses cause disease in a range of hosts, from humans to plants. Bacterial biofilms, communities of bacteria, e.g., Staphylococcus aureusand Escherichia coli, attached to the surface, create a protective layer that enhances their survival in harsh environments and resistance to antibiotics and the host's immune system. Biofilms are commonly associated with food spoilage and chronic infections, posing challenges for treatment and prevention. Tomato brown rugose fruit virus (ToBRFV), a newly discovered tobamovirus, infects tomato plants, causing unique symptoms on the fruit, posing a risk for tomato production. The present study focuses on the effectiveness of silane-phosphonium thin coatings on polymeric films, e.g., polypropylene. Phosphonium has significant antibacterial activity and is less susceptible to antibacterial resistance, making it a safer alternative with a reduced environmental impact. We successfully synthesized silane-phosphonium monomers as confirmed by 31P NMR and mass spectrometry. The chemical composition, thickness, morphology, and wetting properties of the coatings were tested by Fourier-transform infrared spectroscopy with attenuated total reflectance, focused ion beam, atomic force microscopy, environmental scanning electron microscope, and contact angle (CA) measurements. The antibiofilm and antibacterial activities of the coatings were tested against S. aureus and E. coli, while the antiviral activity was evaluated against ToBRFV. The significant antibiofilm and antiviral activity suggests applications in various fields including medicine, agriculture, and the food industry.

The accumulation and growth of Pseudomonas aeruginosa on surfaces is modulated by surface mechanics via cyclic-di-GMP signaling

Attachment of bacteria onto a surface, consequent signaling, and accumulation and growth of the surface-bound bacterial population are key initial steps in the formation of pathogenic biofilms. While recent reports have hinted that surface mechanics may affect the accumulation of bacteria on that surface, the processes that underlie bacterial perception of surface mechanics and modulation of accumulation in response to surface mechanics remain largely unknown. We use thin and thick hydrogels coated on glass to create composite materials with different mechanics (higher elasticity for thin composites; lower elasticity for thick composites) but with the same surface adhesivity and chemistry. The mechanical cue stemming from surface mechanics is elucidated using experiments with the opportunistic human pathogen Pseudomonas aeruginosa combined with finite-element modeling. Adhesion to thin composites results in greater changes in mechanical stress and strain in the bacterial envelope than does adhesion to thick composites with identical surface chemistry. Using quantitative microscopy, we find that adhesion to thin composites also results in higher cyclic-di-GMP levels, which in turn result in lower motility and less detachment, and thus greater accumulation of bacteria on the surface than does adhesion to thick composites. Mechanics-dependent c-di-GMP production is mediated by the cell-surface-exposed protein PilY1. The biofilm lag phase, which is longer for bacterial populations on thin composites than on thick composites, is also mediated by PilY1. This study shows clear evidence that bacteria actively regulate differential accumulation on surfaces of different stiffnesses via perceiving varied mechanical stress and strain upon surface engagement.

Mechanobiology as a tool for addressing the genotype-to-phenotype problem in microbiology

The central hypothesis of the genotype-phenotype relationship is that the phenotype of a developing organism (i.e., its set of observable attributes) depends on its genome and the environment. However, as we learn more about the genetics and biochemistry of living systems, our understanding does not fully extend to the complex multiscale nature of how cells move, interact, and organize; this gap in understanding is referred to as the genotype-to-phenotype problem. The physics of soft matter sets the background on which living organisms evolved, and the cell environment is a strong determinant of cell phenotype. This inevitably leads to challenges as the full function of many genes, and the diversity of cellular behaviors cannot be assessed without wide screens of environmental conditions. Cellular mechanobiology is an emerging field that provides methodologies to understand how cells integrate chemical and physical environmental stress and signals, and how they are transduced to control cell function. Biofilm forming bacteria represent an attractive model because they are fast growing, genetically malleable and can display sophisticated self-organizing developmental behaviors similar to those found in higher organisms. Here, we propose mechanobiology as a new area of study in prokaryotic systems and describe its potential for unveiling new links between an organism's genome and phenome.

Quantifiable Relationship Between Antibacterial Efficacy and Electro-Mechanical Intervention on Nanowire Arrays

Physical disruption is an important antibacterial means as it is lethal to bacteria without spurring antimicrobial resistance. However, it is very challenging to establish a quantifiable relationship between antibacterial efficacy and physical interactions such as mechanical and electrical forces. Herein, titanium nitride (TN) nanowires with adjustable orientations and capacitances are prepared to exert gradient electro-mechanical forces on bacteria. While vertical nanowires show the strongest mechanical force resulting in an antibacterial efficiency of 0.62 log reduction (vs 0.22 for tiled and 0.36 for inclined nanowires, respectively), the addition of electrical charges maximizes the electro-mechanical interactions and elevates the antibacterial efficacy to more than 3 log reduction. Biophysical and biochemical analyses indicate that electrostatic attraction by electrical charge narrows the interface. The electro-mechanical intervention more easily stiffens and rips the bacteria membrane, disturbing the electron balance and generating intracellular oxidative stress. The antibacterial ability is maintained in vivo and bacteria-challenged rats are protected from serious infection. The physical bacteria-killing process demonstrated here can be controlled by adjusting the electro-mechanical interactions. Overall, these results revealed important principles for rationally designing high-performance antibacterial interfaces for clinical applications.

Hydrogel Based on Polyhydroxyalkanoate Sulfonate: Control of the Swelling Rate by the Ionic Group Content

Hydrogels based on poly(3-hydroxyalkanoate) (PHA) sulfonate and poly(ethylene glycol) diacrylate, PEGDA, are prepared. First, PHA sulfonate is synthesized from unsaturated PHA by a thiol-ene reaction in the presence of sodium-3-mercapto-1-ethanesulfonate. The hydrophilicity of PHAs is considerably increased by adding sulfonate functions, and three amphiphilic PHAs are synthesized, containing 10, 22, or 29% sulfonate functions. Then, hydrogels are formed in the presence of PEGDA having different molar masses, that is, 575 or 2000 g mol^-1. The hydrogels show fibrillar and porous structures observed in cryo-MEB with pore sizes that vary according to the content of sulfonated groups (10 to 29 mol %) ranging from 50 to more than 150 nm. Furthermore, depending on the proportions of the two polymers, a variable rigidity is observed from 2 to 40 Pa. In fact, the evaluation of the dynamic mechanical properties of the hydrogel determined by DMA reveals that the less rigid hydrogels hinder the adhesion of Pseudomonas aeruginosa PaO1 bacteria. Finally, these hydrogels swelling up to 5000% are noncytotoxic, allowing the adhesion and amplification of immortalized C2C12 cells, and they are therefore seen as promising materials both for repelling PaO1 bacteria and for amplifying myogenic cells.

Multifunctional antibacterial chitosan-based hydrogel coatings on Ti6Al4V biomaterial for biomedical implant applications

Among biomedical community, great efforts have been realized to develop antibacterial coatings that avoid implant-associated infections. To date, conventional mono-functional antibacterial strategies have not been effective enough for successful long-term implantations. Consequently, researchers have recently focused their attention on novel bifunctional or multifunctional antibacterial coatings, in which two or more antibacterial mechanisms interact synergistically. Thus, in this work different chitosan-based (CHI) hydrogel coatings were created on Ti6Al4V surface using genipin (Ti-CHIGP) and polyethylene glycol (Ti-CHIPEG) crosslinking agents. Hydrogel coatings demonstrated an exceptional in vivo biocompatibility plus a remarkable ability to promote cell proliferation and differentiation. Lastly, hydrogel coatings demonstrated an outstanding bacteria-repelling (17-28 % of S. aureus and 33-43 % of E. coli repelled) and contact killing (186-222 % of S. aureus and 72-83 % of E. coli damaged) ability. Such bifunctional antibacterial activity could be further improved by the controlled release of drugs resulting in powerful multifunctional antibacterial coatings.

Biodegradable and Cytocompatible Hydrogel Coating with Antibacterial Activity for the Prevention of Implant-Associated Infection

Implant-associated infection (IAI) caused by pathogens colonizing on the implant surface is a serious issue in the trauma-orthopedic surgery, which often leads to implant failure. The complications of IAI bring a big threat to the clinical practice of implants, accompanied by significant economic cost and long hospitalization time. In this study, we propose an antibiotics-free strategy to address IAI-related challenges by using a biodegradable and cytocompatible hydrogel coating. To achieve this, a novel hydrogel system was developed to combine the synergistic effects of good cell affinity and antibacterial properties. The hydrogel material was prepared by modifying a photocross-linkable gelatin-based polymer (GelMA) with cationic quaternary ammonium salt (QAS) groups via a mild and simple synthesis procedure. By engineering the length of the hydrophobic carbon chain on the QAS group and the degree of functionalization, the resulting GelMA-octylQAS hydrogel exhibited an integration of good mechanical properties, biodegradability, excellent bactericidal activity against various types of bacteria, and high cytocompatibility with mammalian cells. When coated onto the implant via the in situ cross-linking procedure, our hydrogel demonstrated superior antimicrobial ability in the infective model of femoral fracture of rats. Our results suggest that the GelMA-octylQAS hydrogel might provide a promising platform for preventing and treating IAI.

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and β-cyclodextrin-modified hyaluronan

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of β-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and β-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

Bacterial mechanosensing of surface stiffness promotes signaling and growth leading to biofilm formation by Pseudomonas aeruginosa

The attachment of bacteria onto a surface, consequent signaling, and the accumulation and growth of the surface-bound bacterial population are key initial steps in the formation of pathogenic biofilms. While recent reports have hinted that the stiffness of a surface may affect the accumulation of bacteria on that surface, the processes that underlie bacterial perception of and response to surface stiffness are unknown. Furthermore, whether, and how, the surface stiffness impacts biofilm development, after initial accumulation, is not known. We use thin and thick hydrogels to create stiff and soft composite materials, respectively, with the same surface chemistry. Using quantitative microscopy, we find that the accumulation, motility, and growth of the opportunistic human pathogen Pseudomonas aeruginosa respond to surface stiffness, and that these are linked through cyclic-di-GMP signaling that depends on surface stiffness. The mechanical cue stemming from surface stiffness is elucidated using finite-element modeling combined with experiments - adhesion to stiffer surfaces results in greater changes in mechanical stress and strain in the bacterial envelope than does adhesion to softer surfaces with identical surface chemistry. The cell-surface-exposed protein PilY1 acts as a mechanosensor, that upon surface engagement, results in higher cyclic-di-GMP levels, lower motility, and greater accumulation on stiffer surfaces. PilY1 impacts the biofilm lag phase, which is extended for bacteria attaching to stiffer surfaces. This study shows clear evidence that bacteria actively respond to different stiffness of surfaces where they adhere via perceiving varied mechanical stress and strain upon surface engagement.

Effect of surface stiffness in initial adhesion of oral microorganisms under various environmental conditions

Biofilms are three-dimensional structures formed as a result of microorganism's adhesion on a biotic or abiotic surface. Once a biofilm is established, it is onerous to eradicate it or kill the pathogens therein. Thus, targeting the microbial adhesion process, the initial stage of biofilm formation, is a reasonable approach to avoid challenges associated with subsequently formed biofilms. While many properties of interacting material that play significant roles in initial bacterial adhesion have been widely studied, the effect of surface stiffness on bacterial adhesion was relatively underexplored. In this study, we aimed to investigate the effect of surface stiffness on the adhesion of microbial species found in the oral cavity by employing representative oral bacteria, Streptococcus mutans and Streptococcus oralis, and the fungus, Candida albicans. We compared the adhesion behavior of these species alone or in combination toward various surface stiffness (0.06 - 3.01 MPa) by assessing the adhered and planktonic cell numbers at an early (4 h) adhesion stage under various carbon sources and the presence of conditioning film. Our data revealed that in general, a higher amount of microbial cells adhered to softer PDMS surfaces than stiffer ones, which indicates that surface stiffness plays a role in the adhesion of tested species (either single or co-cultured). This pattern was more obvious under sucrose conditions than glucose + fructose conditions. Interestingly, in monospecies, saliva coating did not alter the effect of surface stiffness on S. mutans adhesion while it diminished S. oralis and C. albicans adhesion. However, in the multispecies model, saliva coating rendered the percentage of all adhered microbes to varied PDMS not distinct. The data provide new insights into the role of surface stiffness on microbial mechanosensing and their initial adhesion behavior which may set a scientific foundation for future anti-adhesion strategies.

Microbial Adherence to Contact Lenses and Pseudomonas aeruginosa as a Model Organism for Microbial Keratitis

Microbial keratitis (MK), the infection of the cornea, is a devastating disease and the fifth leading cause of blindness and visual impairment around the world. The overwhelming majority of MK cases are linked to contact lens wear combined with factors which promote infection such as corneal abrasion, an immunocompromised state, improper contact lens use, or failing to routinely disinfect lenses after wear. Contact lens-related MK involves the adherence of microorganisms to the contact lens. Therefore, this review discusses the information currently available regarding the disease pathophysiology, the common types of microorganisms causing MK, physical and organic mechanisms of adhesion, material properties which are involved in adhesion, and current antimicrobial strategies. This review also concludes that Pseudomonas aeruginosa is a model organism for the investigation of contact lens microbial adherence due to its prevalence in MK cases, its extremely robust adhesion, antimicrobial-resistant properties, and the severity of the disease it causes.

Multifunctional Composite Hydrogels for Bacterial Capture, Growth/Elimination, and Sensing Applications

Hydrogels are cross-linked networks of hydrophilic polymer chains with a three-dimensional structure. Owing to their unique features, the application of hydrogels for bacterial/antibacterial studies and bacterial infection management has grown in importance in recent years. This trend is likely to continue due to the rise in bacterial infections and antimicrobial resistance. By exploiting their physicochemical characteristics and inherent nature, hydrogels have been developed to achieve bacterial capture and detection, bacterial growth or elimination, antibiotic delivery, or bacterial sensing. Traditionally, the development of hydrogels for bacterial/antibacterial studies has focused on achieving a single function such as antibiotic delivery, antibacterial activity, bacterial growth, or bacterial detection. However, recent studies demonstrate the fabrication of multifunctional hydrogels, where a single hydrogel is capable of performing more than one bacterial/antibacterial function, or composite hydrogels consisting of a number of single functionalized hydrogels, which exhibit bacterial/antibacterial function synergistically. In this review, we first highlight the hydrogel features critical for bacterial studies and infection management. Then, we specifically address unique hydrogel properties, their surface/network functionalization, and their mode of action for bacterial capture, adhesion/growth, antibacterial activity, and bacterial sensing, respectively. Finally, we provide insights into different strategies for developing multifunctional hydrogels and how such systems can help tackle, manage, and understand bacterial infections and antimicrobial resistance. We also note that the strategies highlighted in this review can be adapted to other cell types and are therefore likely to find applications beyond the field of microbiology.

Synthesis and Properties of Hydrogels on Medical Titanium Alloy Surface by Modified Dopamine Adhesion

Medical titanium alloy Ti-6Al-4V (TC4) is an ideal surgical implant material for human tissue repair and replacement. TC4 implantation will be in close contact with human soft tissue and has mechanical compatibility problems. In order to solve this problem, the hydrogel was formed on the surface of TC4 by utilizing the adhesion of dopamine, and the storage modulus of the formed hydrogel matched that of human soft tissue. In this paper, the surface of TC4 was first modified with dopamine (DA) and 2-bromoisobutyryl bromide (BIBB). 2-(2-methoxyethoxy) ethyl methacrylate (MEO₂MA), oligo (ethylene oxide) methacrylate (OEGMA) and 2-methacryloyloxyethyl phosphorylcholine (MPC) are used as monomers, and methylenebisacrylamide (MBA) is used as cross-linking agent. Thermosensitive hydrogels were formed on the surface of modified TC4 by the ATRP technique. The successful synthesis of initiator and hydrogels on TC4 was demonstrated by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). The morphology of the hydrogel was observed by the scanning electron microscope (SEM), and the water absorption and temperature sensitivity were investigated by the swelling property. The thermal and mechanical properties of these gels were measured using thermal analysis system (TAS) and dynamic mechanical analyzer (DMA). The results show that the hydrogel on TC4 has good thermal stability and storage modulus that matches human soft tissue.

Manipulating Bacterial Biofilms Using Materiobiology and Synthetic Biology Approaches

Bacteria form biofilms on material surfaces within hours. Biofilms are often considered problematic substances in the fields such as biomedical devices and the food industry; however, they are beneficial in other fields such as fermentation, water remediation, and civil engineering. Biofilm properties depend on their genome and the extracellular environment, including pH, shear stress, and matrices topography, stiffness, wettability, and charges during biofilm formation. These surface properties have feedback effects on biofilm formation at different stages. Due to emerging technology such as synthetic biology and genome editing, many studies have focused on functionalizing biofilm for specific applications. Nevertheless, few studies combine these two approaches to produce or modify biofilms. This review summarizes up-to-date materials science and synthetic biology approaches to controlling biofilms. The review proposed a potential research direction in the future that can gain better control of bacteria and biofilms.

Measuring the elastic modulus of soft biomaterials using nanoindentation

The measurement of the elastic modulus of soft biomaterials via nanoindentation relies on the accurate determination of the zero-point of the tip-sample interaction on which the depth of penetration into the sample is based. Non-cantilever based nanoindentation systems were originally designed for hard materials, and therefore monitoring the zero-point contact presents a significant challenge for the characterisation of very soft biomaterials. This study investigates the ability of non-cantilever based nanoindentation to differentiate between hydrogels with elastic moduli on the order of single kiloPascals (kPa) using a bespoke soft contact protocol and low flexural stiffness of instrument. Polyethylene glycol (PEG) hydrogels were fabricated as a model system with a range of elastic moduli by varying the polymer concentration and degree of crosslinking. Elastic modulus values were calculated using the Oliver-Pharr method, Hertzian contact model, as well as a viscoelastic model to account for the time-dependent behaviour of the gels. The stiffness measurements were validated by measuring cantilever beams with the equivalent flexural stiffness to that of the PEG hydrogels being tested. The results demonstrated a high repeatability of the measurements, enabling differentiation between hydrogels with elastic moduli in the single kPa to hundreds of kPa range.

Investigation of heparin-loaded poly(ethylene glycol)-based hydrogels as anti-thrombogenic surface coatings for extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO), a critical life-sustaining tool, faces significant challenges for the maintenance of normal haemostasis due to the large volume of circulating blood continuously in contact with artificial surfaces, hyperoxia and excessive shear stresses of the extracorporeal circuit. From a biomaterials perspective, it has been hypothesised that drug eluting coatings composed of haemocompatible hydrogels loaded with an anticoagulant drug could potentially enhance the haemocompatibility of the circuit. Poly(ethylene glycol) (PEG) has been well established as a biocompatible and anti-fouling material with wide biomedical application. Unfractionated heparin is the most commonly used anticoagulant for ECMO. In the present study, the feasibility of using heparin-loaded PEG-based hydrogels as anti-thrombogenic surface coatings for ECMO was investigated. The hydrogels were synthesised by photopolymerisation using poly(ethylene glycol) diacrylate (PEGDA) as the crosslinking monomer and poly(ethylene glycol) methacrylate (PEGMA) as the hydrophilic monomer, with heparin loaded into the pre-gel solution. Factors which could affect the release of heparin were investigated, including the ratio of PEGDA/PEGMA, water content, loading level of heparin and the flow of fluid past the hydrogel. Our results showed that increased crosslinker content and decreased water content led to slower heparin release. The hydrogels with water contents of 60 wt% and 70 wt% could achieve a sustained heparin release by adjusting the ratio of PEGDA/PEGMA. The anticoagulation efficacy of the released heparin was evaluated by measuring the activated clotting time of whole blood. The hydrogels with desirable heparin release profiles were prepared onto poly(4-methyl-1-pentene) (PMP) films with the same chemical composition as the PMP ECMO membranes. The coatings showed sustained heparin release with a cumulative release of 70-80% after 7 days. Haemocompatibility tests demonstrated that PEG hydrogel coatings significantly reduced platelet adhesion and prolonged plasma recalcification time. These results suggest that heparin-loaded PEG hydrogels are potential anti-thrombogenic coatings for ECMO.

Interplay of physico-chemical and mechanical bacteria-surface interactions with transport processes controls early biofilm growth: A review

Biofilms initiate when bacteria encounter and are retained on surfaces. The surface orchestrates biofilm growth through direct physico-chemical and mechanical interactions with different structures on bacterial cells and, in turn, through its influence on cell-cell interactions. Individual cells respond directly to a surface through mechanical or chemical means, initiating "surface sensing" pathways that regulate gene expression, for instance producing extra cellular matrix or altering phenotypes. The surface can also physically direct the evolving colony morphology as cells divide and grow. In either case, the physico-chemistry of the surface influences cells and cell communities through mechanisms that involve additional factors. For instance the numbers of cells arriving on a surface from solution relative to the generation of new cells by division depends on adhesion and transport kinetics, affecting early colony density and composition. Separately, the forces experienced by adhering cells depend on hydrodynamics, gravity, and the relative stiffnesses and viscoelasticity of the cells and substrate materials, affecting mechanosensing pathways. Physical chemistry and surface functionality, along with interfacial mechanics also influence cell-surface friction and control colony morphology, in particular 2D and 3D shape. This review focuses on the current understanding of the mechanisms in which physico-chemical interactions, deriving from surface functionality, impact individual cells and cell community behavior through their coupling with other interfacial processes.

Uncoupling bacterial attachment on and detachment from polydimethylsiloxane surfaces through empirical and simulation studies

Bacterial infections related to medical devices can cause severe problems, whose solution requires in-depth understanding of the interactions between bacteria and surfaces. This work investigates the influence of surface physicochemistry on bacterial attachment and detachment under flow through both empirical and simulation studies. We employed polydimethylsiloxane (PDMS) substrates having different degrees of crosslinking as the model material and the extended Derjaguin - Landau - Verwey - Overbeek model as the simulation method. Experimentally, the different PDMS materials led to similar numbers of attached bacteria, which can be rationalized by the identical energy barriers simulated between bacteria and the different materials. However, different numbers of residual bacteria after detachment were observed, which was suggested by simulation that the detachment process is determined by the interfacial physicochemistry rather than the mechanical property of a material. This finding is further supported by analyzing the bacteria detachment from PDMS substrates from which non-crosslinked polymer chains had been removed: similar numbers of residual bacteria were found on the extracted PDMS substrates. The knowledge gained in this work can facilitate the projection of bacterial colonization on a given surface.

Spreading rates of bacterial colonies depend on substrate stiffness and permeability

The ability of bacteria to colonize and grow on different surfaces is an essential process for biofilm development. Here, we report the use of synthetic hydrogels with tunable stiffness and porosity to assess physical effects of the substrate on biofilm development. Using time-lapse microscopy to track the growth of expanding Serratia marcescens colonies, we find that biofilm colony growth can increase with increasing substrate stiffness, unlike what is found on traditional agar substrates. Using traction force microscopy-based techniques, we find that biofilms exert transient stresses correlated over length scales much larger than a single bacterium, and that the magnitude of these forces also increases with increasing substrate stiffness. Our results are consistent with a model of biofilm development in which the interplay between osmotic pressure arising from the biofilm and the poroelastic response of the underlying substrate controls biofilm growth and morphology.

Recent advancement in Bi5O7I-based nanocomposites for high performance photocatalysts

Bi₅O₇I belongs to the family of bismuth oxyhalides (BiOX, X = Cl, Br, I), having a unique layered structure with an internal electrostatic field that promotes the separation and transfer of photo-generated charge carriers. Interestingly, Bi₅O₇I exhibits higher thermal stability compared to its other BiOX member compounds and absorption spectrum extended to the visible region. Bi₅O₇I has demonstrated applications in diverse fields such as photocatalytic degradation of various organic pollutants, marine antifouling, etc. Unfortunately, owing to its wide band gap of ∼2.9 eV, its absorption lies mainly in the ultraviolet region, and a tiny portion of absorption lies in the visible region. Due to limited absorption, the photocatalytic performance of pure Bi₅O₇I is still facing challenges. In order to reduce the band gap and increase the light absorption capability of Bi₅O₇I, doping and formation of heterostructure strategies have been employed, which showed promising results in the photocatalytic performance. In addition, the plasmonic heterostructures of Bi₅O₇I were also developed to further boost the efficiency of Bi₅O₇I as a photocatalyst. Here, in this review article, we present such recent efforts made for the advanced development of Bi₅O₇I regarding its synthesis, properties and applications. The strategies for photocatalytic performance enhancement have been discussed in detail. Moreover, in the conclusion section, we have presented the current challenges and discussed possible prospective developments in this field.

Past and Current Progress in the Development of Antiviral/Antimicrobial Polymer Coating towards COVID-19 Prevention: A Review

The astonishing outbreak of SARS-CoV-2 coronavirus, known as COVID-19, has attracted numerous research interests, particularly regarding fabricating antimicrobial surface coatings. This initiative is aimed at overcoming and minimizing viral and bacterial transmission to the human. When contaminated droplets from an infected individual land onto common surfaces, SARS-CoV-2 coronavirus is able to survive on various surfaces for up to 9 days. Thus, the possibility of virus transmission increases after touching or being in contact with contaminated surfaces. Herein, we aim to provide overviews of various types of antiviral and antimicrobial coating agents, such as antimicrobial polymer-based coating, metal-based coating, functional nanomaterial, and nanocomposite-based coating. The action mode for each type of antimicrobial agent against pathogens is elaborated. In addition, surface properties of the designed antiviral and antimicrobial polymer coating with their influencing factors are discussed in this review. This paper also exhibits several techniques on surface modification to improve surface properties. Various developed research on the development of antiviral/antimicrobial polymer coating to curb the COVID-19 pandemic are also presented in this review.

Surface Modification to Modulate Microbial Biofilms-Applications in Dental Medicine

Recent progress in materials science and nanotechnology has led to the development of advanced materials with multifunctional properties. Dental medicine has benefited from the design of such materials and coatings in providing patients with tailored implants and improved materials for restorative and functional use. Such materials and coatings allow for better acceptance by the host body, promote successful implantation and determine a reduced inflammatory response after contact with the materials. Since numerous dental pathologies are influenced by the presence and activity of some pathogenic microorganisms, novel materials are needed to overcome this challenge as well. This paper aimed to reveal and discuss the most recent and innovative progress made in the field of materials surface modification in terms of microbial attachment inhibition and biofilm formation, with a direct impact on dental medicine.

Mussel-inspired self-assembly engineered implant coatings for synergistic anti-infection and osteogenesis acceleration

Implant associated infections (IAI) and poor osseointegration are the two major causes for titanium implant failure, leading to subsequent financial burden and physical sufferings. Therefore, advanced implants with excellent anti-infection and osseointegration performance are needed. In this work, mussel-inspired tannic acid (TA) mediated layer-by-layer (LbL) self-assembly was used for fabricating bonded polyethylene glycol (PEG) and 8DSS (8 repeating units of aspartate-serine-serine) coatings (Ti/8DSS/PEG) on the surface of titanium implants. The coating is designed to simultaneously reduce bacterial adhesion through the super-hydrophilic effect of PEG and promote osseointegration through the effective biomineralization of 8DSS. The obtained Ti/8DSS/PEG implant exhibits superior anti-biofouling capabilities (anti-protein adhesion and anti-bacterial adhesion against S. aureus and E. coli) and excellent biocompatibility. Meanwhile, the Ti/8DSS/PEG implant accelerates osteoblast differentiation and presents significantly better osteogenic ability than bare titanium implants in vivo. This mussel-inspired TA mediated LbL self-assembly method is expected to provide a multifunctional and robust platform for surface engineering in bone repair.

The Mammalian Membrane Microenvironment Regulates the Sequential Attachment of Bacteria to Host Cells

Pathogen attachment to host tissue is critical in the progress of many infections. Bacteria use adhesion in vivo to stabilize colonization and subsequently regulate the deployment of contact-dependent virulence traits. To specifically target host cells, they decorate themselves with adhesins, proteins that bind to mammalian cell surface receptors. One common assumption is that adhesin-receptor interactions entirely govern bacterial attachment. However, how adhesins engage with their receptors in an in vivo-like context remains unclear, in particular under the influence of a heterogeneous mechanical microenvironment. We here investigate the biophysical processes governing bacterial adhesion to host cells using a tunable adhesin-receptor system. By dynamically visualizing attachment, we found that bacterial adhesion to host cell surface, unlike adhesion to inert surfaces, involves two consecutive steps. Bacteria initially attach to their host without engaging adhesins. This step lasts about 1 min, during which bacteria can easily detach. We found that at this stage, the glycocalyx, a layer of glycosylated proteins and lipids, shields the host cell by keeping adhesins away from their receptor ligand. In a second step, adhesins engage with their target receptors to strengthen attachment for minutes to hours. The active properties of the membrane, endowed by the actin cytoskeleton, strengthen specific adhesion. Altogether, our results demonstrate that adhesin-ligand binding is not the sole regulator of bacterial adhesion. In fact, the host cell’s surface mechanical microenvironment mediates the physical interactions between host and bacteria, thereby playing an essential role in the onset of infection.

Engineered topographies and hydrodynamics in relation to biofouling control-a review

Biofouling, the unwanted growth of microorganisms on submerged surfaces, has appeared as a significant impediment for underwater structures, water vessels, and medical devices. For fixing the biofouling issue, modification of the submerged surface is being experimented as a non-toxic approach worldwide. This technique necessitated altering the surface topography and roughness and developing a surface with a nano- to micro-structured pattern. The main objective of this study is to review the recent advancements in surface modification and hydrodynamic analysis concerning biofouling control. This study described the occurrence of the biofouling process, techniques suitable for biofouling control, and current state of research advancements comprehensively. Different biofilms under various hydrodynamic conditions have also been outlined in this study. Scenarios of biomimetic surfaces and underwater super-hydrophobicity, locomotion of microorganisms, nano- and micro-hydrodynamics on various surfaces around microorganisms, and material stiffness were explained thoroughly. The review also documented the approaches to inhibit the initial settlement of microorganisms and prolong the subsequent biofilm formation process for patterned surfaces. Though it is well documented that biofouling can be controlled to various degrees with different nano- and micro-structured patterned surfaces, the understanding of the underlying mechanism is still imprecise. Therefore, this review strived to present the possibilities of implementing the patterned surfaces as a physical deterrent against the settlement of fouling organisms and developing an active microfluidic environment to inhibit the initial bacterial settlement process. In general, microtopography equivalent to that of bacterial cells influences attachment via hydrodynamics, topography-induced cell placement, and air-entrapment, whereas nanotopography influences physicochemical forces through macromolecular conditioning.

Tuning surface topographies on biomaterials to control bacterial infection

Microbial contamination and subsequent formation of biofilms frequently cause failure of surgical implants and a good understanding of the bacteria-surface interactions is vital to the design and safety of biomaterials. In this review, the physical and chemical factors that are involved in the various stages of implant-associated bacterial infection are described. In particular, topographical modification strategies that have been employed to mitigate bacterial adhesion via topographical mechanisms are summarized and discussed comprehensively. Recent advances have improved our understanding about bacteria-surface interactions and have enabled biomedical engineers and researchers to develop better and more effective antibacterial surfaces. The related interdisciplinary efforts are expected to continue in the quest for next-generation medical devices to attain the ultimate goal of improved clinical outcomes and reduced number of revision surgeries.

Ultra-Conformable Ionic Skin with Multi-Modal Sensing, Broad-Spectrum Antimicrobial and Regenerative Capabilities for Smart and Expedited Wound Care

While rapid wound healing is essential yet challenging, there is also an unmet need for functional restoration of sensation. Inspired by natural skin, an ultra-conformable, adhesive multi-functional ionic skin (MiS) with multi-modal sensing capability is devised for smart and expedited wound care. The base of MiS is a unique skin-like, conductive and self-adaptive adhesive polyacrylamide/starch double-network hydrogel (PSH) and self-powered, flexible, triboelectric sensor(s) is integrated on top of PSH for multi-tactile sensing. MiS could enhance wound contraction, collagen deposition, angiogenesis, and epidermis formation in a full-thickness skin defect wound model in vivo, while significantly inhibiting the biofilm formation of a wide range of microorganisms. MiS also exhibits multi-modal sensing capability for smart and instant therapeutics and diagnostics, including skin displacement or joint motion, temperature, pressure and tissue exudate changes of wound bed, and locally releasing drugs in a pH-responsive manner. More importantly, MiS could restore the skin-mimicking tactile sensing function of both touch location and intensity, and thus could be used as a human-machine interface for accurate external robotic control. MiS demonstrates a new comprehensive paradigm of combining wound diagnosis and healing, broad-spectrum anti-microbial capability and restoration of multi-tactile sensing for the reparation of severe wound.

Impacts of Mechanical Stiffness of Bacteriophage-Loaded Hydrogels on Their Antibacterial Activity

The elaboration of efficient hydrogel-based materials with antimicrobial properties requires a refined control of defining their physicochemical features, which includes mechanical stiffness, so as to properly mediate their antibacterial activity. In this work, we design hydrogels consisting of polyelectrolyte multilayer films for the loading of T4 and φX174 bacteria-killing viruses, also called bacteriophages. We investigate the antiadhesion and bactericidal performances of this biomaterial against Escherichia coli, with a specific focus on the effects of chemical cross-linking of the hydrogel matrix, which, in turn, mediates the hydrogel stiffness. Depending on the latter and on phage replication features, it is found that the hydrogels loaded with the bacteria-killing viruses make both contact killing (targeted bacteria are those adhered at the hydrogel surface) and release killing (planktonic bacteria are the targets) possible with ca. 20-80% efficiency after only 4 h of incubation at 25 °C as compared to cases where hydrogels are free of viruses. We further demonstrate the lack of dependence of virus diffusion within the hydrogel and of the maximal viral storage capacity on the hydrogel mechanical properties. In addition to the evidenced bacteriolytic activity of the phages loaded in the hydrogels, the antimicrobial property of the phage-loaded materials is shown to be partly controlled by the chemistry of the hydrogel skeleton and, more specifically, by the mobility of the peripheral free polycationic components, known for their ability to weaken and permeabilize membranes of bacteria, the latter then becoming "easier" targets for the viruses.

Surface Coating with Hyaluronic Acid-Gelatin-Crosslinked Hydrogel on Gelatin-Conjugated Poly(dimethylsiloxane) for Implantable Medical Device-Induced Fibrosis

Polydimethylsiloxane (PDMS) is a biocompatible polymer that has been applied in many fields. However, the surface hydrophobicity of PDMS can limit successful implementation, and this must be reduced by surface modification to improve biocompatibility. In this study, we modified the PDMS surface with a hydrogel and investigated the effect of this on hydrophilicity, bacterial adhesion, cell viability, immune response, and biocompatibility of PDMS. Hydrogels were created from hyaluronic acid and gelatin using a Schiff-base reaction. The PDMS surface and hydrogel were characterized using nuclear magnetic resonance, X-ray photoelectron spectroscopy, attenuated total reflection Fourier-transform infrared spectroscopy, and scanning electron microscopy. The hydrophilicity of the surface was confirmed via a decrease in the water contact angle. Bacterial anti-adhesion was demonstrated for Pseudomonas aeruginosa, Ralstonia pickettii, and Staphylococcus epidermidis, and viability and improved distribution of human-derived adipose stem cells were also confirmed. Decreased capsular tissue responses were observed in vivo with looser collagen distribution and reduced cytokine expression on the hydrogel-coated surface. Hydrogel coating on treated PDMS is a promising method to improve the surface hydrophilicity and biocompatibility for surface modification of biomedical applications.

Implication of Surface Properties, Bacterial Motility, and Hydrodynamic Conditions on Bacterial Surface Sensing and Their Initial Adhesion

Biofilms are structured microbial communities attached to surfaces, which play a significant role in the persistence of biofoulings in both medical and industrial settings. Bacteria in biofilms are mostly embedded in a complex matrix comprised of extracellular polymeric substances that provide mechanical stability and protection against environmental adversities. Once the biofilm is matured, it becomes extremely difficult to kill bacteria or mechanically remove biofilms from solid surfaces. Therefore, interrupting the bacterial surface sensing mechanism and subsequent initial binding process of bacteria to surfaces is essential to effectively prevent biofilm-associated problems. Noting that the process of bacterial adhesion is influenced by many factors, including material surface properties, this review summarizes recent works dedicated to understanding the influences of surface charge, surface wettability, roughness, topography, stiffness, and combination of properties on bacterial adhesion. This review also highlights other factors that are often neglected in bacterial adhesion studies such as bacterial motility and the effect of hydrodynamic flow. Lastly, the present review features recent innovations in nanotechnology-based antifouling systems to engineer new concepts of antibiofilm surfaces.

Biomimetic mineralized hybrid scaffolds with antimicrobial peptides

Infection in hard tissue regeneration is a clinically-relevant challenge. Development of scaffolds with dual function for promoting bone/dental tissue growth and preventing bacterial infections is a critical need in the field. Here we fabricated hybrid scaffolds by intrafibrillar-mineralization of collagen using a biomimetic process and subsequently coating the scaffold with an antimicrobial designer peptide with cationic and amphipathic properties. The highly hydrophilic mineralized collagen scaffolds provided an ideal substrate to form a dense and stable coating of the antimicrobial peptides. The amount of hydroxyapatite in the mineralized fibers modulated the rheological behavior of the scaffolds with no influence on the amount of recruited peptides and the resulting increase in hydrophobicity. The developed scaffolds were potent by contact killing of Gram-negative Escherichia coli and Gram-positive Streptococcus gordonii as well as cytocompatible to human bone marrow-derived mesenchymal stromal cells. The process of scaffold fabrication is versatile and can be used to control mineral load and/or intrafibrillar-mineralized scaffolds made of other biopolymers.

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A biomimetic intrafibrillar-mineralized scaffold was prepared using a non-classical pathway for mineralization.

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The mineralized scaffold was stably coated with designer antimicrobial peptide GL13K.

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The hybrid scaffold was cytocompatible and potent against biofilms of model Gram-positive and Gram-negative bacteria.

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The mineral content affected the rheological properties of the scaffolds, but not the loading of antimicrobial peptides.

Quantitative confocal microscopy and calibration for measuring differences in cyclic-di-GMP signalling by bacteria on biomedical hydrogels

The growth of bacterial biofilms on implanted medical devices causes harmful infections and device failure. Biofilm development initiates when bacteria attach to and sense a surface. For the common nosocomial pathogen Pseudomonas aeruginosa and many others, the transition to the biofilm phenotype is controlled by the intracellular signal and second messenger cyclic-di-GMP (c-di-GMP). It is not known how biomedical materials might be adjusted to impede c-di-GMP signalling, and there are few extant methods for conducting such studies. Here, we develop such a method. We allowed P. aeruginosa to attach to the surfaces of poly(ethylene glycol) diacrylate (PEGDA) hydrogels. These bacteria contained a plasmid for a green fluorescent protein (GFP) reporter for c-di-GMP. We used laser-scanning confocal microscopy to measure the dynamics of the GFP reporter for 3 h, beginning 1 h after introducing bacteria to the hydrogel. We controlled for the effects of changes in bacterial metabolism using a promoterless plasmid for GFP, and for the effects of light passing through different hydrogels being differently attenuated by using fluorescent plastic beads as 'standard candles' for calibration. We demonstrate that this method can measure statistically significant differences in c-di-GMP signalling associated with different PEGDA gel types and with the surface-exposed protein PilY1.

A scope at antifouling strategies to prevent catheter-associated infections

The use of invasive medical devices is becoming more common nowadays, with catheters representing one of the most used medical devices. However, there is a risk of infection associated with the use of these devices, since they are made of materials that are prone to bacterial adhesion with biofilm formation, often requiring catheter removal as the only therapeutic option. Catheter-related urinary tract infections (CAUTIs) and central line-associated bloodstream infections (CLABSIs) are among the most common causes of healthcare-associated infections (HAIs) worldwide while endotracheal intubation is responsible for ventilator-associated pneumonia (VAP). Therefore, to avoid the use of biocides due to the potential risk of bacterial resistance development, antifouling strategies aiming at the prevention of bacterial adherence and colonization of catheter surfaces represent important alternative measures. This review is focused on the main strategies that are able to modify the physical or chemical properties of biomaterials, leading to the creation of antiadhesive surfaces. The most promising approaches include coating the surfaces with hydrophilic polymers, such as poly(ethylene glycol) (PEG), poly(acrylamide) and poly(acrylates), betaine-based zwitterionic polymers and amphiphilic polymers or the use of bulk-modified poly(urethanes). Natural polysaccharides and its modifications with heparin, have also been used to improve hemocompatibility. Recently developed bioinspired techniques yielding very promising results in the prevention of bacterial adhesion and colonization of surfaces include slippery liquid-infused porous surfaces (SLIPS) based on the superhydrophilic rim of the pitcher plant and the Sharklet topography inspired by the shark skin, which are potential candidates as surface-modifying approaches for biomedical devices. Concerning the potential application of most of these strategies in catheters, more in vivo studies and clinical trials are needed to assure their efficacy and safety for possible future use.

Role of adhesion forces in mechanosensitive channel gating in Staphylococcus aureus adhering to surfaces

Mechanosensitive channels in bacterial membranes open or close in response to environmental changes to allow transmembrane transport, including antibiotic uptake and solute efflux. In this paper, we hypothesize that gating of mechanosensitive channels is stimulated by forces through which bacteria adhere to surfaces. Hereto, channel gating is related with adhesion forces to different surfaces of a Staphylococcus aureus strain and its isogenic ΔmscL mutant, deficient in MscL (large) channel gating. Staphylococci becoming fluorescent due to uptake of calcein, increased with adhesion force and were higher in the parent strain (66% when adhering with an adhesion force above 4.0 nN) than in the ΔmscL mutant (40% above 1.2 nN). This suggests that MscL channels open at a higher critical adhesion force than at which physically different, MscS (small) channels open and contribute to transmembrane transport. Uptake of the antibiotic dihydrostreptomycin was monitored by staphylococcal killing. The parent strain exposed to dihydrostreptomycin yielded a CFU reduction of 2.3 log-units when adhering with an adhesion force above 3.5 nN, but CFU reduction remained low (1.0 log-unit) in the mutant, independent of adhesion force. This confirms that large channels open at a higher critical adhesion-force than small channels, as also concluded from calcein transmembrane transport. Collectively, these observations support our hypothesis that adhesion forces to surfaces play an important role, next to other established driving forces, in staphylococcal channel gating. This provides an interesting extension of our understanding of transmembrane antibiotic uptake and solute efflux in infectious staphylococcal biofilms in which bacteria experience adhesion forces from a wide variety of surfaces, like those of other bacteria, tissue cells, or implanted biomaterials.

Surface Design for Immobilization of an Antimicrobial Peptide Mimic for Efficient Anti-Biofouling

Microbial surface attachment negatively impacts a wide range of devices from water purification membranes to biomedical implants. Mimics of antimicrobial peptides (AMPs) constituted from poly(N-substituted glycine) "peptoids" are of great interest as they resist proteolysis and can inhibit a wide spectrum of microbes. We investigate how terminal modification of a peptoid AMP-mimic and its surface immobilization affect antimicrobial activity. We also demonstrate a convenient surface modification strategy for enabling alkyne-azide "click" coupling on amino-functionalized surfaces. Our results verified that the N- and C-terminal peptoid structures are not required for antimicrobial activity. Moreover, our peptoid immobilization density and choice of PEG tether resulted in a "volumetric" spatial separation between AMPs that, compared to past studies, enabled the highest AMP surface activity relative to bacterial attachment. Our analysis suggests the importance of spatial flexibility for membrane activity and that AMP separation may be a controlling parameter for optimizing surface anti-biofouling.

Magnetically driven active topography for long-term biofilm control

Microbial biofilm formation on indwelling medical devices causes persistent infections that cannot be cured with conventional antibiotics. To address this unmet challenge, we engineer tunable active surface topographies with micron-sized pillars that can beat at a programmable frequency and force level in an electromagnetic field. Compared to the flat and static controls, active topographies with the optimized design prevent biofilm formation and remove established biofilms of uropathogenic Escherichia coli (UPEC), Pseudomonas aeruginosa, and Staphylococcus aureus, with up to 3.7 logs of biomass reduction. In addition, the detached biofilm cells are found sensitized to bactericidal antibiotics to the level comparable to exponential-phase planktonic cells. Based on these findings, a prototype catheter is engineered and found to remain clean for at least 30 days under the flow of artificial urine medium, while the control catheters are blocked by UPEC biofilms within 5 days.

Patterning non-equilibrium morphologies in stimuli-responsive gels through topographical confinement

Stimuli-responsive "smart" polymers have generated significant interest for introducing dynamic control into the properties of antifouling coatings, smart membranes, switchable adhesives and cell manipulation substrates. Switchable surface morphologies formed by confining stimuli-responsive gels to topographically structured substrates have shown potential for a variety of interfacial applications. Beyond patterning the equilibrium swelling behavior of gels, subjecting stimuli-responsive gels to topographical confinement could also introduce spatial gradients in the various timescales associated with gel deformation, giving rise to novel non-equilibrium morphologies. Here we show how by curing poly(N-isopropylacrylamide) (pNIPAAm)-based gel under confinement to a rigid, bumpy substrate, we can not only induce the surface curvature to invert with temperature, but also program the transient, non-equilibrium morphologies that emerge during the inversion process through changing the heating path. Finite element simulations show that the emergence of these transient morphologies is correlated with confinement-induced gradients in polymer concentration and position-dependent hydrostatic pressure within the gel. To illustrate the relevance of such morphologies in interfacial applications, we show how they enable us to control the gravity-induced assembly of colloidal particles and microalgae. Finally, we show how more complex arrangements in particle assembly can be created through controlling the thickness of the temperature-responsive gel over the bumps. Patterning stimuli-responsive gels on topographically-structured surfaces not only enables switching between two invertible topographies, but could also create opportunities for stimuli ramp-dependent control over the local curvature of the surface and emergence of unique transient morphologies. Harnessing these features could have potential in the design of multifunctional, actuatable materials for switchable adhesion, antifouling, cell manipulation, and liquid and particle transport surfaces.