Salt-induced Fmoc-tripeptide supramolecular hydrogels: a combined experimental and computational study of the self-assembly

Delving into the mechanism behind the molecular interactions at the atomic level of short-sequence peptides plays a key role in the development of nanomaterials with specific structure-property-function relationships from a bottom-up perspective. Due to their poor water solubility, the self-assembly of Fmoc-bearing peptides is usually induced by dissolution in an organic solvent, followed by a dilution step in water, pH changes, and/or a heating-cooling process. Herein, we report a straightforward methodology for the gelation of Fmoc-FFpY (F: phenylalanine; Y: tyrosine; and p: PO42-), a negatively charged tripeptide, in NaCl solution. The electrostatic interactions between Fmoc-FFpY and Na+ ions give rise to different nanofibrillar hydrogels with rheological properties and nanofiber sizes modulated by the NaCl concentration in pure aqueous media. Initiated by the electrostatic interactions between the peptide phosphate groups and the Na+ ions, the peptide self-assembly is stabilized thanks to hydrogen bonds between the peptide backbones and the π-π stacking of aromatic Fmoc and phenyl units. The hydrogels showed self-healing and thermo-responsive properties for potential biomedical applications. Molecular dynamics simulations from systems devoid of prior training not only confirm the aggregation of peptides at a critical salt concentration and the different interactions involved, but also corroborate the secondary structure of the hydrogels at the microsecond timescale. It is worth highlighting the remarkable achievement of reproducing the morphological behavior of the hydrogels using atomistic simulations. To our knowledge, this study is the first to report such a correspondence.

Glucono-Delta-Lactone-Induced Alginate Gelation: New Insights into the Effect of the Cross-Linker Carrier Type on the Hydrogel Mechanics

Physical alginate hydrogels commonly rely on "internal gelation" to introduce the cross-linker, e.g., calcium (Ca(II)) ions. These are released in a homogeneous manner by using a pH-sensitive Ca(II) carrier and glucono-delta-lactone (GDL) as the acidifier. Yet, it remains unclear how the carrier of the cross-linker affects the gelation process and final hydrogel properties. We therefore investigate two internal gelation methods using either Ca(II)-chelating ligand complexes or insoluble Ca(II)-based salts. Ionometry coupled with pH measurements reveals the release process of Ca(II) ions upon acidification, which is well described by simulations using the Hyperquad Simulation and Speciation program. We show that these findings correlate well with the evolution of the mechanical properties of the hydrogels. Although the two pH-triggered gelation methods appear to be similar, we demonstrate their differences in terms of the gelation kinetics and final cross-link density. The nature of the ligand or the salt significantly affects the fraction of the released Ca(II) ions and, hence, the mechanical properties of the final hydrogel for a given GDL concentration. Furthermore, for the first time, we demonstrate the competition between GDL and alginate in binding with Ca(II) ions. This study therefore provides different tools for the efficient formulation of alginate hydrogels.

Tuning the underwater adhesiveness of antibacterial polysaccharides complex coacervates

HYPOTHESIS

Adjusting the water content and mechanical properties of polyelectrolyte coacervates for optimal underwater adhesion requires simultaneous control of the macromolecular design and the type and concentration of the salt used. Using synthetic or bio-inspired polymers to make coacervates often involves complicated chemistries and large variations in salt concentration. The underwater adhesiveness of simple, bio-sourced coacervates can be tuned with relatively small variations in salt concentration. Bio-sourced polymers can also impart beneficial biological activities to the final material.

EXPERIMENTS

We made complex coacervates from charged chitosan (CHI) and hyaluronic acid (HA) with NaCl as the salt. Their water content and viscoelastic properties were investigated to identify the formulation with optimal underwater adhesion in physiological conditions. The coacervates were also studied in antibacterial and cytotoxicity experiments.

FINDINGS

As predicted by linear rheology, the CHI-HA coacervates at 0.1 and 0.2 M NaCl had the highest pull-off adhesion strengths of 44.4 and 40.3 kPa in their respective supernatants. In-situ physical hardening of the 0.2 M coacervate upon a salt switch in 0.1 M NaCl resulted in a pull-off adhesion strength of 62.9 kPa. This material maintained its adhesive properties in physiological conditions. Finally, the optimal adhesive was found to be non-cytotoxic and inherently antimicrobial through a chitosan release-killing mechanism.

Interfacial rheology of linearly growing polyelectrolyte multilayers at the water-air interface: from liquid to solid viscoelasticity

Despite the long history of investigations of polyelectrolyte multilayer formation on solid or liquid surfaces, important questions remain open concerning the construction of the first set of layers. These are generally deposited on a first anchoring layer of different chemistry, influencing their construction and properties. We propose here an in-depth investigation of the formation of NaPSS/PAH multilayers at the air/water interface in the absence of a chemically different anchoring layer, profiting from the surface activity of NaPSS. To analyse the mechanical properties of the different layers, we combine recently established analysis techniques of an inflating/deflating bubble exploiting simultaneous shape and pressure measurement: bubble shape elastometry, general stress decomposition and capillary meniscus dynanometry. We complement these measurements by interfacial shear rheology. The obtained results allow us to confirm, first of all, the strength of the aforementioned techniques to characterize complex interfaces with non-linear viscoelastic properties. Furthermore, their sensitivity allows us to show that the multilayer properties are highly sensitive to the temporal and mechanical conditions under which they are constructed and manipulated. We nevertheless identify a robust trend showing a clear transition from a liquid-like viscoelastic membrane to a solid-like viscoelastic membrane after the deposition of 5 layers. We interpret this as the number of layers required to create a fully connected multilayer, which is consistent with previous results obtained on solid or liquid interfaces.

An increase in Wharton's jelly membrane osteocompatibility by a genipin-cross-link

Wharton's Jelly (WJ) has attracted significant interest in the field of tissue healing thanks to its biological properties, including antibacterial activity and immunomodulation. However, due to the fast degradation and poor mechanical behavior in biological environment, its application in bone regeneration is compromised. Here, we proposed to use genipin as an efficient cross-linking agent to significantly improve the elasticity and the enzymatical stability of the WJ matrix. The degree of cross-linking, linear elastic moduli, and collagenase resistance varied over a wide range depending on genipin concentration. Furthermore, our results highlighted that an increase in genipin concentration led to a decreased surface wettability, therefore impairing cell attachment and proliferation. The genipin cross-linking prevented rapid in vitro and in vivo degradation, but led to an adverse host reaction and calcification. When implanted in the parietal bone defect, a limited parietal bone regeneration to the dura was observed. We conclude that genipin-cross-linked WJ is a versatile medical device however, a careful selection is required with regards to the genipin concentration.

Electrochemical Assembly Strategies of Polymer and Hybrid Thin Films for (Bio)sensors, Charge Storage, and Triggered Release

In the context of functional and hierarchical materials, electrode reactions coupled with one or more chemical reactions constitute the most powerful bottom-up process for the electrosynthesis of film components and their electrodeposition, enabling the localized functionalization of conductive surfaces using an electrical stimulus. In analogy with developmental biological processes, our group introduced the concept of morphogen-driven film buildup. In this approach, the gradient of a diffusing reactive molecule or ion (called a morphogen) is controlled by an electrical stimulus to locally induce a chemical process (solubility change, hydrolysis, complexation, and covalent reaction) that induces a film assembly. One of the prominent advantages of this technique is the conformal nature of the deposits toward the electrode. This Feature Article presents the contributions made by our group and other researchers to develop strategies for the assembly of different polymer and nanoparticle/polymer hybrid films by using electrochemically generated reagents and/or catalysts. The main electrochemical-chemical approaches for conformal films are described in the case where (i) the products are noncovalent aggregates that spontaneously precipitate on the electrode (film electrodeposition) or (ii) new chemical compounds are generated, which do not necessarily spontaneously precipitate and enable the formation of covalent or noncovalent films (film electrosynthesis). The applications of those electrogenerated films will be described with a focus on charge storage/transport, (bio)sensing, and stimuli-responsive cargo delivery systems.

Brush-Induced Orientation of Collagen Fibers in Layer-by-Layer Nanofilms: A Simple Method for the Development of Human Muscle Fibers

The engineering of skeletal muscle tissue, a highly organized structure of myotubes, is promising for the treatment of muscle injuries and muscle diseases, for replacement, or for pharmacology research. Muscle tissue development involves differentiation of myoblasts into myotubes with parallel orientation, to ultimately form aligned myofibers, which is challenging to achieve on flat surfaces. In this work, we designed hydrogen-bonded tannic acid/collagen layer-by-layer (TA/COL LbL) nanofilms using a simple brushing method to address this issue. In comparison to films obtained by dipping, brushed TA/COL films showed oriented COL fibers of 60 nm diameter along the brushing direction. Built at acidic pH due to COL solubility, TA/COL films released TA in physiological conditions with a minor loss of thickness. After characterization of COL fibers' orientation, human myoblasts (C25CL48) were seeded on the oriented TA/COL film, ended by COL. After 12 days in a differentiation medium without any other supplement, human myoblasts were able to align on brushed TA/COL films and to differentiate into long aligned myotubes (from hundreds of μm up to 1.7 mm length) thanks to two distinct properties: (i) the orientation of COL fibers guiding myoblasts' alignment and (ii) the TA release favoring the differentiation. This simple and potent brushing process allows the development of anisotropic tissues in vitro which can be used for studies of drug discovery and screening or the replacement of damaged tissue.

Optimization of the Rheological Properties of Self-Assembled Tripeptide/Alginate/Cellulose Hydrogels for 3D Printing

3D printing is an emerging and powerful technique to create shape-defined three-dimensional structures for tissue engineering applications. Herein, different alginate-cellulose formulations were optimized to be used as printable inks. Alginate (Alg) was chosen as the main component of the scaffold due to its tunable mechanical properties, rapid gelation, and non-toxicity, whereas microcrystalline cellulose (MCC) was added to the hydrogel to modulate its mechanical properties for printing. Additionally, Fmoc-FFY (Fmoc: 9-fluorenylmethoxycarbonyl; F: phenylalanine; Y: tyrosine), a self-assembled peptide that promotes cell adhesion was incorporated into the ink without modifying its rheological properties and shear-thinning behavior. Then, 3D-printed scaffolds made of Alg, 40% of MCC inks and Fmoc-FFY peptide were characterized by scanning electron microscopy and infrared spectroscopy, confirming the morphological microstructure of the hydrogel scaffolds with edged particles of MCC homogeneously distributed within the alginate matrix and the self-assembly of the peptide in a β-sheet conformation. Finally, the cytocompatibility of the scaffolds was tested in contact with the MG63 osteosarcoma cells, confirming the absence of cytotoxic components that may compromise their viability. Interestingly, MG63 cell growth was retarded in the scaffolds containing the peptide, but cells were more likely to promote adhesive interactions with the material rather than with the other cells, indicating the benefits of the peptide in promoting biological functionality to alginate-based biomaterials.

Optimization of Nanohybrid Biosensors Based on Electro-Crosslinked Tannic Acid Capped Nanoparticles/Enzyme

Enzymes/Nanoparticles (NPs) bioconjugates are massively used nowadays to develop thin films for optical and electrochemical biosensors. Nevertheless, their full characterization as a thin coating onto electrodes remains little discussed, in particular the influence of NPs size and enzyme/NPs ratio used in the electrodeposition solution. In this study, GOx (160 kDa) and HRP (44 kDa) were used in association with tannic acid capped gold NPs (a series with sizes from 7 to 40 nm) to electrodeposit biosensor coatings, sensitive towards glucose and H2O2, respectively. The electrodeposition process was based on a mussel-inspired electro-crosslinking between gallol moieties of tannic acid (at the surface of NPs) and amine moieties of the enzymes. On one hand, the sensitivity of the GOx/NPs coatings depends strongly on the NP size and the enzyme/NPs molar ratio of the electrodeposition solution. An optimal sensitivity was obtained by electrodeposition of 11 nm NPs at a GOx/NPs molar ratio close to the theoretical value of the enzyme monolayer. On the other hand, a modest influence of the NPs size was found on the sensitivity in the case of the electrodeposited HRP/NPs coatings, reaching a plateau at the HRP/NPs molar ratio close to the value of the theoretical enzyme monolayer. In both cases, the enzyme/NPs molar ratio played a role in the sensitivity. To fully understand the parameters driving the biosensor sensitivity, a comprehensive evaluation of the colloidal state of the bioconjugates is proposed here.

Injectable Tripeptide/Polymer Nanoparticles Supramolecular Hydrogel: A Candidate for the Treatment of Inflammatory Pathologies

Supramolecular peptide-based hydrogels attract great attention in several fields, i.e., biomedicine, catalysis, energy, and materials chemistry, due to the noncovalent nature of the self-assembly and functional tunable properties defined by the amino acid sequence. In this work, we developed an injectable hybrid supramolecular hydrogel whose formation was triggered by electrostatic interactions between a phosphorylated tripeptide, Fmoc-FFpY (F: phenylalanine, pY: phosphorylated tyrosine), and cationic polymer nanoparticles made of vinylimidazole and ketoprofen (poly(HKT-co-VI) NPs). Hydrogel formation was assessed through inverted tube tests, and its fibrillary structure, around polymer NPs, was observed by transmission electron microscopy. Interestingly, peptide self-assembly yields the formation of nontwisted and twisted fibers, which could be attributed to β-sheets and α-helix structures, respectively, as characterized by circular dichroism and infrared spectroscopies. An increase of the elastic modulus of the Fmoc-FFpY/polymer NPs hybrid hydrogels was observed with peptide concentration as well as its injectability property, due to its shear thinning behavior and self-healing ability. After checking their stability under physiological conditions, the cytotoxicity properties of these hybrid hydrogels were evaluated in contact with human dermal fibroblasts (FBH) and murine macrophages (RAW 264.7). Finally, the Fmoc-FFpY/polymer NPs hybrid hydrogels exhibited a great nitric oxide reduction (∼67%) up to basal values of pro-inflammatory RAW 264.7 cells, thus confirming their excellent anti-inflammatory properties for the treatment of localized inflammatory pathologies.

Study of Membrane Protein Monolayers Using Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS): Critical Dependence of Nanostructured Gold Surface Morphology

Surface-enhanced infrared absorption spectroscopy (SEIRAS) is a powerful tool that allows studying the reactivity of protein monolayers at very low concentrations and independent from the protein size. In this study, we probe the surface's morphology of electroless gold deposition for optimum enhancement using two different types of immobilization adapted to two proteins. Independently from the mode of measurement (i.e., transmission or reflection) or type of protein immobilization (i.e., through electrostatic interactions or nickel-HisTag), the enhancement and reproducibility of protein signals in the infrared spectra critically depended on the gold nanostructured surface morphology deposited on silicon. Just a few seconds deviation from the optimum time in the nanoparticle deposition led to a significantly weaker enhancement. Scanning electron microscopy and atomic force microscopy measurements revealed the evolution of the nanostructured surface when comparing different deposition times. The optimal deposition time led to isolated gold nanostructures on the silicon crystal. Importantly, in the case of the immobilization using nickel-HisTag, the surface morphology is rearranged upon immobilization of linker and the protein. A complex three-dimensional (3D) network of nanoparticles decorated with the protein could be observed leading to the optimal enhancement. The electroless deposition of gold is a simple technique, which can be adapted to flow cells and used in analytical approaches.

Supramolecular tripeptide self-assembly initiated at the surface of coacervates by polyelectrolyte exchange

Spatial control of supramolecular self-assembly can yield compartmentalized structures, a key feature for the design of artificial cells. Inducing self-assembly from and on compartments is still a challenge. Polyelectrolyte complex coacervates are simple model droplet systems able to reproduce the basic features of membrane-less organelles, appearing in cells. Here, we demonstrate the supramolecular self-assembly of a phosphorylated tripeptide, Fmoc-FFpY (Fmoc: fluorenyl-methoxycarbonyl; F: phenyl alanine, pY: phosphorylated tyrosine), on the surface of poly(l-glutamic acid)/poly(allylamine hydrochloride) (PGA/PAH) complex coacervate microdroplets. The phosphorylated peptides self-assemble, without dephosphorylation, through ion pairing between the phosphate groups of Fmoc-FFpY and the amine groups of PAH. This process provides spontaneous capsules formed by an amorphous polyelectrolyte complex core surrounded by a structured peptide/PAH shell. Similar fibrillar Fmoc-FFpY self-assembled structures are obtained at the interface between the peptide solution and a PGA/PAH polyelectrolyte multilayer, a complex coacervate in the thin film or "multilayer" format. In contact with the peptide solution, PAH chains diffuse out of the coacervate or multilayer film and complex with Fmoc-FFpY at the solution interface, exchanging any PGA with which they were associated. Self-assembly of Fmoc-FFpY, now concentrated by complexation with PAH, follows quickly.

Biopolymers-calcium phosphate antibacterial coating reduces the pathogenicity of internalized bacteria by mesenchymal stromal cells

A multifunctional material system that kills bacteria and drives bone healing is urgently sought to improve bone prosthesis. Herein, the osteoinductive coating made of calcium phosphate/chitosan/hyaluronic acid, named Hybrid, was proposed as an antibacterial substrate for stromal cell adhesion. This Hybrid coating possesses a contact-killing effect reducing by 90% the viability of Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Pseudomonas aeruginosa (P. aeruginosa) strains after 48 h of contact. In addition to the production of immunomodulatory mediators, Wharton's jelly (WJ-SCs), dental pulp (DPSCs) and bone marrow (BM-MSCs) derived stromal cells were able to release antibacterial and antibiofilm agents effective against S. aureus and P. aeruginosa strains, respectively. Studying the effect of the Hybrid coating on the internalization of S. aureus by the stromal cells, in acute-mimicking bone infection, highlighted an increase in the bacteria internalization by DPSCs and BM-MSCs when cultured on the Hybrid coating versus uncoated glass. Despite the internalization, Hybrid coating showed a beneficial effect by reducing the pathogenicity of the internalized bacteria. The formation of biofilm was reduced by at least 50% in comparison to internalized bacteria by stromal cells on uncoated glass. This work opens the route for the development of innovative antibacterial coatings by taking into account the internalization of bacteria by stromal cells.

Surface Triggered Self-Assembly of Fmoc-Tripeptide as an Antibacterial Coating

In western countries, one patient on twenty will develop a nosocomial infection during his hospitalization at health care facilities. Classical antibiotics being less and less effective, this phenomenon is expanding year after year. Prevention of bacteria colonization of implantable medical devices constitutes a major medical and financial issue. In this study, we developed an antibacterial coating based on self-assembled Fmoc-tripeptide. Fmoc-FFpY peptides (F: phenylalanine; Y: tyrosine; p: PO4 2-) are dephosphorylated enzymatically into Fmoc-FFY by action of alkaline phosphatase functionalized silica nanoparticles (NPs@AP), previously deposited on a surface. Fmoc-FFY peptides then self-assemble through π-π stacking interactions, hydrogen bonds and hydrophobic interactions adopting β-sheets secondary structures. The obtained hydrogel coatings show fibrillary structures observed by cryo-scanning electron microscopy with a thickness of few micrometers. At low concentration (≤0.5 mg.mL-1), self-assembled Fmoc-FFY has a superior antibacterial activity than Fmoc-FFpY peptide in solution. After 24 h of incubation, Fmoc-FFY hydrogel coatings fully inhibit the development of Gram-positive Staphylococcus aureus (S. aureus). The antibacterial effect is maintained on an in vitro model of repetitive infection in the case of S. aureus. This coating could serve in infections were Gram positive bacteria are prevalent, e.g., intravascular catheter infections. This work gives new insights toward the design of an alternative antimicrobial coating.

Effect of the Buffer on the Buildup and Stability of Tannic Acid/Collagen Multilayer Films Applied as Antibacterial Coatings

The deposition of polyelectrolyte multilayers, obtained by the layer-by-layer (LbL) method, is a well-established technology to design biocompatible and antibacterial coatings aimed at preventing implant-associated infections. Several types of LbL films have been reported to exhibit antiadhesive and/or antibacterial (contact-killing or release-killing) properties governed not only by the incorporated compounds but also by their buildup conditions or their postbuildup treatments. Tannic acid (TA), a natural polyphenol, is known to inhibit the growth of several bacterial strains. In this work, we developed TA/collagen (TA/COL) LbL films built in acetate or citrate buffers at pH 4. Surprisingly, the used buffer impacts not only the physicochemical but also the antibacterial properties of the films. When incubated in physiological conditions, both types of TA/COL films released almost the same amount of TA depending on the last layer and showed an antibacterial effect against Staphylococcus aureus only for citrate-built films. Because of their granular topography, TA/COL citrate films exhibited an efficient release-killing effect with no cytotoxicity toward human gingival fibroblasts. Emphasis is put on a comprehensive evaluation of the physicochemical parameters driving the buildup and the antibacterial property of citrate films. Specifically, complexation strengths between TA and COL are different in the presence of the two buffers affecting the LbL deposition. This work constitutes an important step toward the use of polyphenols as an antibacterial agent when incorporated in LbL films.

Polyanionic Hydrogels as Reservoirs for Polycationic Antibiotic Substitutes Providing Prolonged Antibacterial Activity

Implantation of biomedical devices is often followed by bacterial infections that may seriously affect implant functionalities and lead to their failure. In the context of bacterial resistance to antibiotics, which is a growing problem worldwide, new strategies that are able to overcome these problems are needed. In this work, we introduce a new formulation of hyaluronic acid (HA)-based antimicrobial material: HA hydrogels loaded with polyarginine (PAR), a polycationic antibiotic substitute. The loading is possible through electrostatic interactions between negatively charged HA and positively charged PAR. Such hydrogels absorb high quantities of PAR, which are then gradually released from the hydrogel. This original system provides a long-lasting antibacterial effect on an in vitro model of repetitive infection, thus demonstrating a strong potential to fight multiple rounds of infections that are resistant to antibiotic treatment. In addition, HA-PAR hydrogels could be deposited onto/into medical devices such as wound dressings and mesh prostheses used in clinical applications. Finally, we performed first in vivo tests of hydrogel-coated mesh materials to verify their biocompatibility in a rat model, which show no difference between control HA hydrogel and PAR-loaded hydrogel in terms of inflammation.

Enzyme assisted peptide self-assemblies trigger cell adhesion in high density oxime based host gels

Peptide supramolecular self-assemblies are recognized as important components in responsive hydrogel based materials with applications in tissue engineering and regenerative medicine. Studying the influence of hydrogel matrices on the self-assembly behavior of peptides and interaction with cells is essential to guide the future development of engineered biomaterials. In this contribution, we present a PEG based host hydrogel material generated by oxime click chemistry that shows cellular adhesion behavior in response to enzyme assisted peptide self-assembly (EASA) within the host gel. This hydrogel prepared from poly(dimethylacrylamide-co-diacetoneacrylamide), poly(DMA-DAAM) with high molar fractions (49%) of DAAM and dialkoxyamine PEG cross-linker, was studied in the presence of embedded enzyme alkaline phosphatase (AP) and a non-adhesive cell behavior towards NIH 3T3 fibroblasts was observed. When brought into contact with a Fmoc-FFpY peptide solution (pY: phosphorylated tyrosine), the gel forms intercalated Fmoc-FFY peptide self-assemblies upon diffusion of Fmoc-FFpY into the cross-linked hydrogel network as was confirmed by circular dichroism, fluorescence emission spectroscopy and confocal microscopy. Nevertheless, the mechanical properties do not change significantly after the peptide self-assembly in the host gel. This enzyme assisted peptide self-assembly promotes fibroblast cell adhesion that can be enhanced if Fmoc-F-RGD peptides are added to the pre-gelator Fmoc-FFpY peptide solution. Cell adhesion results mainly from interactions of cells with the non-covalent peptide self-assemblies present in the gel despite the fact that the mechanical properties are very close to those of the native host gel. This result is in contrast to numerous studies which showed that the mechanical properties of a substrate are key parameters of cell adhesion. It opens up the possibility to develop a diverse set of hybrid materials to control cell fate in culture due to tailored self-assemblies of peptides responding to the environment provided by the host guest gel.

A Pilot Study on Continuous Infusion of 4% Albumin in Critically Ill Patients: Impact on Nosocomial Infection via a Reduction Mechanism for Oxidized Substrates

Care-related infections affect up to 11% of ICU patients. Running therapeutic albumin is sometimes associated to less infection: whether a specific method of its infusion is of any interest to modulate innate defense is unknown. Our objectives were: 1) to test whether the method for albumin infusion is important to prevent care-related infections and 2) to analyze in vitro the antioxidative role of albumin on host defense proteins during shock (using vasostatin-I as an example).

Design

In a prospective, randomized, open-label trial, shock patients were allocated to receive either continuously 4% albumin or intermittently 20% albumin, as long as they were infused with norepinephrine. A translational study including in vivo and in vitro analyses of albumin-vasostatin-I interactions is reported.

Setting

A tertiary ICU caring for 1,000 patients per year.

Patients

Fifty shock patients with serum albumin less than 20 g/L.

Interventions

In vivo colonization and nosocomial infections were recorded and time-dependent changes in serum albumin, chromogranin A, and vasostatin-I concentrations as well. In vitro, we studied biochemical albumin-vasostatin-I relationship using biochemical methods.

Measurements and Main Results

Over 18 days, we recorded a decrease in colonization (four vs 12 episodes; p = 0.035) and nosocomial infection frequency (two vs 13 episodes; p = 0.002) in patients infused continuously 4% albumin versus controls. In vitro, albumin interacts with the disulfide loop vasostatin-I (residues 17-40) and continuous 4% albumin infusion restores its oxidative status required for antimicrobial activity.

Conclusions

Continuous 4% albumin is effective in reducing care-related infections in shock patients by increasing the availability of antimicrobial vasostatin-I. This might guide future care of shock patients.

Mechanisms of Fibrinogen Adsorption on Silica Sensors at Various pHs: Experiments and Theoretical Modeling

The adsorption kinetics of human serum fibrinogen at silica substrates was studied using optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance (QCM) techniques. Measurements were performed at pH 3.5, 4, and 7.4 for various ionic strengths. The experimental data were interpreted in terms of a hybrid random sequential adsorption model. This allowed the mass transfer rate coefficient for the OWLS cell and maximum coverages to be determined at various pHs. The appearance of different, pH-dependent mechanisms of fibrinogen adsorption on silica substrates was confirmed. At pH 3.5 the molecules mostly adsorb in the side-on orientation that produces a low maximum coverage of ca. 1 mg m^-2. At this pH, the kinetics derived from the OWLS measurements agree with those theoretically predicted using the convective-diffusion theory. In consequence, a comparison of the OWLS and QCM results allows the water factor and the dynamic hydration of fibrinogen molecules to be determined. At pH 7.4, the OWLS method gives inaccurate kinetic data for the low coverage range. However, the maximum coverage that was equal to ca. 4 mg m^-2 agrees with the QCM results and with previous literature results. It is postulated that the limited accuracy of the OWLS method for lower coverage stems from a heterogeneous structure of fibrinogen monolayers, which consist of side-on and end-on adsorbed molecules. One can expect that the results acquired in this work allow development of a robust procedure for preparing fibrinogen monolayers of well-controlled coverage and molecule orientation, which can be exploited for efficient immunosensing purposes.

Photoresponsive Nanometer-Scale Iron Alginate Hydrogels: A Study of Gel-Sol Transition Using a Quartz Crystal Microbalance

Alginate/Fe³⁺ hydrogels were fabricated on hyaluronic acid (HA) and poly(allylamine hydrochloride) (PAH) multilayers to yield photoresponsive nanometer-scale hydrogels. Light irradiation of the resulting hydrogels induced the photoreduction of "hard" Fe³⁺ to "soft" Fe²⁺ cations, leading to changes in the mechanical properties of the hydrogels related to their cross-linking behavior. The buildup and the phototriggered response of the supported alginate hydrogels were followed in situ with a quartz crystal microbalance (QCM) using an open cell allowing light irradiation from an LED source on top of the hydrogel. The results were correlated to the release profiles of folic acid, employed herein as a drug model, obtained from light-irradiated supported iron alginate hydrogels.

Phase Separation in Supramolecular Hydrogels Based on Peptide Self-Assembly from Enzyme-Coated Nanoparticles

Spatial localization of biocatalysts, such as enzymes, has recently proven to be an effective process to direct supramolecular self-assemblies in a spatiotemporal way. In this work, silica nanoparticles (NPs) functionalized covalently by alkaline phosphatase (NPs@AP) induce the localized growth of self-assembled peptide nanofibers from NPs by dephosphorylation of Fmoc-FFpY peptides (Fmoc: fluorenylmethyloxycarbonyl; F: phenylalanine; Y: tyrosine; p: phosphate group). The fibrillary nanoarchitecture around NPs@AP underpins a homogeneous hydrogel, which unexpectedly undergoes a macroscopic shape change over time. This macroscopic change is due to a phase separation leading to a dense phase (in NPs and nanofibers) in the center of the vial and surrounded by a dilute one, which still contains NPs and peptide self-assemblies. We thus hypothesize that the phase separation is not a syneresis process. Such a change is only observed when the enzymes are localized on the NPs. The dense phase contracts with time until reaching a constant volume after several days. For a given phosphorylated peptide concentration, the dense phase contracts faster when the NPs@AP concentration is increased. For a given NPs@AP concentration, it condenses faster when the peptide concentration increases. We hypothesize that the appearance of a dense phase is not only due to attractive interactions between NPs@AP but also to the strong interactions of self-assembled peptide nanofibers with the enzymes, covalently fixed on the NPs.

Boosting mesenchymal stem cells regenerative activities on biopolymers-calcium phosphate functionalized collagen membrane

The regeneration of bone-soft tissue interface, using functional membranes, remains challenging and can be promoted by improving mesenchymal stem cells (MSCs) paracrine function. Herein, a collagen membrane, used as guided bone regeneration membrane, was functionalized by calcium phosphate, chitosan and hyaluronic acid hybrid coating by simultaneous spray of interacting species process. Composed of brushite, octacalcium phosphate and hydroxyapatite, the hybrid coating increased the membrane stiffness by 50%. After 7 days of MSCs culture on the hybrid coated polymeric membrane, biological studies were marked by a lack of osteoblastic commitment. However, MSCs showed an enhanced proliferation along with the secretion of cytokines and growth factors that could block bone resorption and favour endothelial cell recruitment without exacerbating polynuclear neutrophils infiltration. These data shed light on the great potential of inorganic/organic coated collagen membranes as an alternative bioactive factor-like platform to improve MSCs regenerative capacity, in particular to support bone tissue vascularization and to modulate inflammatory infiltrates.

Modulation of Cellular Colonization of Porous Polyurethane Scaffolds via the Control of Pore Interconnection Size and Nanoscale Surface Modifications

Full-scale cell penetration within porous scaffolds is required to obtain functional connective tissue components in tissue engineering applications. For this aim, we produced porous polyurethane structures with well-controlled pore and interconnection sizes. Although the influence of the pore size on cellular behavior is widely studied, we focused on the impact of the size of the interconnections on the colonization by NIH 3T3 fibroblasts and Wharton's jelly-derived mesenchymal stem cells (WJMSCs). To render the material hydrophilic and allow good material wettability, we treated the material either by plasma or by polydopamine (PDA) coating. We show that cells weakly adhere on these surfaces. Keeping the average pore diameter constant at 133 μm, we compare two structures, one with LARGE (52 μm) and one with SMALL (27 μm) interconnection diameters. DNA quantification and extracellular matrix (ECM) production reveal that larger interconnections is more suitable for cells to move across the scaffold and form a three-dimensional cellular network. We argue that LARGE interconnections favor cell communication between different pores, which then favors the production of the ECM. Moreover, PDA treatment shows a truly beneficial effect on fibroblast viability and on matrix production, whereas plasma treatment shows the same effect for WJMSCs. We, therefore, claim that both pore interconnection size and surface treatment play a significant role to improve the quality of integration of tissue engineering scaffolds.

Protein-induced low molecular weight hydrogelator self-assembly through a self-sustaining process

Controlling how, when and where a self-assembly process occurs is essential for the design of the next generation of smart materials. Along this route, enzyme-assisted self-assembly is a powerful tool developed during the last decade. Here we introduce another strategy allowing for spatiotemporal control over peptide self-assemblies. We use a Fmoc-peptide precursor in dynamic equilibrium with its low molecular weight hydrogelator (LMWH) through a reversible disulfide bond. In the absence of proteins, no self-assembly of the hydrogelator is observed. In the presence of proteins, their interactions with the precursor initiate a self-assembly process of the hydrogelator around them. This self-assembly displaces the equilibrium between precursor and LMWH according to Le Chatelier's principle, producing new hydrogelators available to pursue the self-assembly growth. One thus establishes a self-sustaining cycle fuelled by the self-assembly itself until full consumption of the LMWH. For proteins in solutions this process can lead to a supramolecular hydrogel whereas for proteins deposited on a surface, the gel growth is initiated exclusively from the surface.

Enzyme-assisted self-assembly within a hydrogel induced by peptide diffusion

Peptide diffusion into an enzymatically active hydrogel induces the formation of a self-assembled network, changing the mechanical and chemical properties.

Control of Interfacial Diels-Alder Reactivity by Tuning the Plasma Polymer Properties

Functionalizing the surface of a material with a smart plasma polymer coating is an interesting alternative strategy to obtain a thermoresponsive material without changing its formulation. On the basis of a low-pressure plasma polymerization process, the present work first aims to fabricate polymer thin films that react via the well-known thermoreversible Diels-Alder (DA) reaction (diene/dienophile cycloaddition). A two-step surface modification process based on (pulsed) plasma polymerization enables the design of functional coatings that contain furan (diene) groups. The reactivity of these surfaces with maleic anhydride (dienophile) in solution is thoroughly investigated, mainly by studying the kinetics of the DA reaction by advancing contact angle measurements. The determination of rate constants of reactions at various temperatures leads to the quantification of thermodynamic parameters such as the activation energy of the reaction as well as the enthalpy and entropy of activation related to the formation of the transition-state complex involved in the DA reaction. More interestingly, the design of furan-functionalized coatings with various physicochemical properties enables the understanding of the role played by the density of functional groups and the cross-linking rate of the polymer on the interfacial reactivity. Thus, we show in this work how to control the interfacial DA reaction on plasma coatings by tailoring the operating conditions of plasma polymerization.

Electrochemistry on Stretchable Nanocomposite Electrodes: Dependence on Strain

Stretchable nanocomposite conductors are essential for engineering of bio-inspired deformable electronics, human-machine interfaces, and energy storage devices. While the effect of strain on conductivity for stretchable conductors has been thoroughly investigated, the strain dependence of multiple other electrical-transport processes and parameters that determine the functionalities and biocompatibility of deformable electrodes has received virtually no attention. The constancy of electrochemical parameters at electrode-fluid interfaces such as redox potentials, impedances, and charge-transfer rate constants on strain is often tacitly assumed. However, it remains unknown whether these foundational assumptions actually hold true for deformable electrodes. Furthermore, it is also unknown whether the previously used charge-transport circuits describing electrochemical processes on rigid electrodes are applicable to deformable electrodes. Here, we investigate the validity of the strain invariability assumptions for an elastic composite electrode based on gold nanoparticles (AuNPs). A comprehensive model of electrode reactions that accurately describes electrochemical processes taking place on nanocomposite electrodes for ferro-/ferricyanide electrochemicals pair at different strains is developed. Unlike rigid gold electrodes, the model circuit for stretchable electrodes is comprised of two parallel impedance segments describing (a) diffusion and redox processes taking place on the open surface of the composite electrode and (b) redox processes that occur in nanopores. AuNPs forming the open-surface circuit support the redox process, whereas those forming the nanopores only increase the double-layer capacitance. The redox potential was found to be strain-independent for tensile deformations as high as 40%. Other parameters, however, display strong strain dependence, exemplified by the 2-2.5 and 27 times increases of active area of the open and nanopore surface area, respectively, after application of 40% strain. Gaining better understanding of the strain-dependent and -independent electrochemical parameters enables both fundamental and practical advances in technologies based on deformable electrodes.

Mussel-Inspired Electro-Cross-Linking of Enzymes for the Development of Biosensors

In medical diagnosis and environmental monitoring, enzymatic biosensors are widely applied because of their high sensitivity, potential selectivity, and their possibility of miniaturization/automation. Enzyme immobilization is a critical process in the development of this type of biosensors with the necessity to avoid the denaturation of the enzymes and ensuring their accessibility toward the analyte. Electrodeposition of macromolecules is increasingly considered to be the most suitable method for the design of biosensors. Being simple and attractive, it finely controls the immobilization of enzymes on electrode surfaces, usually by entrapment or adsorption, using an electrical stimulus. Performed manually, enzyme immobilization by cross-linking prevents enzyme leaching and was never done using an electrochemical stimulus. In this work, we present a mussel-inspired electro-cross-linking process using glucose oxidase (GOX) and a homobifunctionalized catechol ethylene oxide spacer as a cross-linker in the presence of ferrocene methanol (FC) acting as a mediator of the buildup. Performed in one pot, the process takes place in three steps: (i) electro-oxidation of FC, by the application of cyclic voltammetry, creating a gradient of ferrocenium (FC⁺); (ii) oxidation of bis-catechol into a bis-quinone molecule by reaction with the electrogenerated FC⁺; and (iii) a chemical reaction of bis-quinone with free amino moieties of GOX through Michael addition and a Schiff's base condensation reaction. Employed for the design of a second-generation glucose biosensor using ferrocene methanol (FC) as a mediator, this new enzyme immobilization process presents several advantages. The cross-linked enzymatic film (i) is obtained in a one-pot process with nonmodified GOX, (ii) is strongly linked to the metallic electrode surface thanks to catechol moieties, and (iii) presents no leakage issues. The developed GOX/bis-catechol film shows a good response to glucose with a quite wide linear range from 1.0 to 12.5 mM as well as a good sensitivity (0.66 μA/mM cm²) and a high selectivity to glucose. These films would distinguish between healthy (3.8 and 6.5 mM) and hyperglycemic subjects (>7 mM). Finally, we show that this electro-cross-linking process allows the development of miniaturized biosensors through the functionalization of a single electrode out of a microelectrode array. Elegant and versatile, this electro-cross-linking process can also be used for the development of enzymatic biofuel cells.

Surface-Assisted Self-Assembly Strategies Leading to Supramolecular Hydrogels

Localized molecular self-assembly processes leading to the growth of nanostructures exclusively from the surface of a material is one of the great challenges in surface chemistry. In the last decade, several works have been reported on the ability of modified or unmodified surfaces to manage the self-assembly of low-molecular-weight hydrogelators (LMWH) resulting in localized supramolecular hydrogel coatings mainly based on nanofiber architectures. This Minireview highlights all strategies that have emerged recently to initiate and localize LMWH supramolecular hydrogel formation, their related fundamental issues and applications.

Step-by-step build-up of covalent poly(ethylene oxide) nanogel films

Hydrogels based on poly(ethylene glycol) (PEG) are commonly used for studies related to cell fate and tissue engineering. Here we present a new covalent layer-by-layer build-up process leading to PEG coatings of nanometer size called "nanogel films". Compared to macroscopic hydrogels, such nanogels should provide a fine control over the structure and the thickness of the coating. Alternated deposition of bifunctional and tetra functional PEG molecules reacting through thiol/maleimide click chemistry is evaluated by quartz crystal microbalance. We first study parameters influencing the build-up process of such coatings and demonstrate the importance of (i) the nature of the first deposited layer, (ii) the PEG concentrations and (iii) the length of the PEG chains that appears to be the most significant parameter influencing film growth. The build-up process can be extended to a large variety of substrates like SiO₂ or polymers by using an appropriate anchoring layer. Covalent functionalization of these nanogel films by proteins or enzymes is suited by modifying the biomolecules with thiol or maleimide groups and immobilizing them during the build-up process. Activity of the embedded enzymes can be maintained. Moreover ligands like biotin can be incorporated into the film and recognition by streptavidin can be modulated by playing with the number of PEG layers covering biotin. Compared to well-known PEG hydrogels, these new coatings are promising as they allow to (i) build thin nanometric coatings, (ii) finely control the amount of deposited PEG and (iii) organize the position of the embedded biomolecules inside the film layers.

Localized Supramolecular Peptide Self-Assembly Directed by Enzyme-Induced Proton Gradients

Electrodes are ideal substrates for surface localized self-assembly processes. Spatiotemporal control over such processes is generally directed through the release of ions generated by redox reactions occurring specifically at the electrode. The so-used gradients of ions proved their effectiveness over the last decade but are in essence limited to material-based electrodes, considerably reducing the scope of applications. Herein is described a strategy to enzymatically generate proton gradients from non-conductive surfaces. In the presence of oxygen, immobilization of glucose oxidase (GOx) on a multilayer film provides a flow of protons through enzymatic oxidation of glucose by GOx. The confined acidic environment located at the solid-liquid interface allows the self-assembly of Fmoc-AA-OH (Fmoc=fluorenylmethyloxycarbonyl and A=alanine) dipeptides into β-sheet nanofibers exclusively from and near the surface. In the absence of oxygen, a multilayer nanoreactor containing GOx and horseradish peroxidase (HRP) similarly induces Fmoc-AA-OH self-assembly.

Review of Electrochemically Triggered Macromolecular Film Buildup Processes and Their Biomedical Applications

Macromolecular coatings play an important role in many technological areas, ranging from the car industry to biosensors. Among the different coating technologies, electrochemically triggered processes are extremely powerful because they allow in particular spatial confinement of the film buildup up to the micrometer scale on microelectrodes. Here, we review the latest advances in the field of electrochemically triggered macromolecular film buildup processes performed in aqueous solutions. All these processes will be discussed and related to their several applications such as corrosion prevention, biosensors, antimicrobial coatings, drug-release, barrier properties and cell encapsulation. Special emphasis will be put on applications in the rapidly growing field of biosensors. Using polymers or proteins, the electrochemical buildup of the films can result from a local change of macromolecules solubility, self-assembly of polyelectrolytes through electrostatic/ionic interactions or covalent cross-linking between different macromolecules. The assembly process can be in one step or performed step-by-step based on an electrical trigger affecting directly the interacting macromolecules or generating ionic species.

Control of Surface-Localized, Enzyme-Assisted Self-Assembly of Peptides through Catalyzed Oligomerization

Localized self-assembly allowing both spatial and temporal control over the assembly process is essential in many biological systems. This can be achieved through localized enzyme-assisted self-assembly (LEASA), also called enzyme-instructed self-assembly, where enzymes present on a substrate catalyze a reaction that transforms noninteracting species into self-assembling ones. Very few LEASA systems have been reported so far, and the control of the self-assembly process through the surface properties represents one essential step toward their use, for example, in artificial cell mimicry. Here, we describe a new type of LEASA system based on α-chymotrypsin adsorbed on a surface, which catalyzes the production of (KL)_nOEt oligopeptides from a KLOEt (K: lysine; L: leucine; OEt ethyl ester) solution. When a critical concentration of the formed oligopeptides is reached near the surface, they self-assemble into β-sheets resulting in a fibrillar network localized at the interface that can extend over several micrometers. One significant feature of this process is the existence of a lag time before the self-assembly process starts. We investigate, in particular, the effect of the α-chymotrypsin surface density and KLOEt concentration on the self-assembly kinetics. We find that the lag time can be finely tuned through the surface density in α-chymotrypsin and KLOEt concentration. For a given surface enzyme concentration, a critical KLOEt concentration exists below which no self-assembly takes place. This concentration increases when the surface density in enzyme decreases.

Bioinspired Nanofeatured Substrates: Suitable Environment for Bone Regeneration

Bone mimicking coatings provide a complex microenvironment in which material, through its inherent properties (such as nanostructure and composition), affects the commitment of stem cells into bone lineage and the production of bone tissue regulating factors required for bone healing and regeneration. Herein, a bioactive mineral/biopolymer composite made of calcium phosphate/chitosan and hyaluronic acid (CaP-CHI-HA) was elaborated using a versatile simultaneous spray coating of interacting species. The resulting CaP-CHI-HA coating was mainly constituted of bioactive, carbonated and crystalline hydroxyapatite with 277 ± 98 nm of roughness, 1 μm of thickness, and 2.3 ± 1 GPa of stiffness. After five days of culture, CaP-CHI-HA suggested a synergistic effect of intrinsic biophysical features and biopolymers on stem cell mechanobiology and nuclear organization, leading to the expression of an early osteoblast-like phenotype and the production of bone tissue regulating factors such as osteoprotegerin and vascular endothelial growth factor. More interestingly, amalgamation with biopolymers conferred to the mineral a bacterial antiadhesive property. These significant data shed light on the potential regenerative application of CaP-CHI-HA bioinspired coating in providing a suitable environment for stem cell bone regeneration and an ideal strategy to prevent implant-associated infections.

Harnessing Wharton's jelly stem cell differentiation into bone-like nodule on calcium phosphate substrate without osteoinductive factors

An important aim of bone regenerative medicine is to design biomaterials with controlled chemical and topographical features to guide stem cell fate towards osteoblasts without addition of specific osteogenic factors. Herein, we find that sprayed bioactive and biocompatible calcium phosphate substrates (CaP) with controlled topography induce, in a well-orchestrated manner, Wharton's jelly stem cells (WJ-SCs) differentiation into osteoblastic lineage without any osteogenic supplements. The resulting WJ-SCs commitment exhibits features of native bone, through the formation of three-dimensional bone-like nodule with osteocyte-like cells embedded into a mineralized type I collagen. To our knowledge, these results present the first observation of a whole differentiation process from stem cell to osteocytes-like on a synthetic material. This suggests a great potential of sprayed CaP and WJ-SCs in bone tissue engineering. These unique features may facilitate the transition from bench to bedside and the development of successful engineered bone.

STATEMENT OF SIGNIFICANCE

Designing materials to direct stem cell fate has a relevant impact on stem cell biology and provides insights facilitating their clinical application in regenerative medicine. Inspired by natural bone compositions, a friendly automated spray-assisted system was used to build calcium phosphate substrate (CaP). Sprayed biomimetic solutions using mild conditions led to the formation of CaP with controlled physical properties, good bioactivity and biocompatibility. Herein, we show that via optimization of physical properties, CaP substrate induce osteogenic differentiation of Wharton's jelly stem cells (WJ-SCs) without adding osteogenic supplement factors. These results suggest a great potential of sprayed CaP and WJ-SCs in bone tissue engineering and may facilitate the transition from bench to beside and the development of clinically successful engineered bone.

Quantitative Nanomechanical Properties of Multilayer Films Made of Polysaccharides through Spray Assisted Layer-by-Layer Assembly

Nanomechanical properties of alginate/chitosan (Alg/Chi) multilayer films, obtained through spray assisted layer-by-layer assembly, were studied by means of PeakForce quantitative nanomechanical mapping atomic force microscopy (PF-QNM AFM). Prepared at two different alginate concentrations (1.0 and 2.5 mg/mL) and a fixed chitosan concentration (1.0 mg/mL), Alg/Chi films have an exponential growth in thickness with a transition to a linear growth toward a plateau by increasing the number of deposited bilayers. Height, elastic modulus, deformation, and adhesion maps were simultaneously recorded depending on the number of deposited bilayers. The elastic modulus of Alg/Chi films was found to be related to the mechanism of growth in contrast to the adhesion and deformation. A comparison of the nanomechanical properties obtained for non-cross-linked and thermally cross-linked Alg/Chi films revealed an increase of the elastic modulus after cross-linking regardless alginate concentration. The incorporation of iron oxide nanoparticles (NPs), during the spray preparation of the films, gave rise to nanocomposite Alg/Chi films with increased elastic moduli with the number of incorporated NPs layers. Deformation maps of the films strongly suggested the presence of empty spaces associated with the method of preparation. Finally, adhesion measurements point out to a significant role of NPs on the increase of the adhesion values found for nanocomposite films.

Upregulation of endothelial gene markers in Wharton's jelly mesenchymal stem cells cultured on polyelectrolyte multilayers

Designing convenient substrates is a pertinent parameter that can guide stem cell differentiation. Current research is directed toward differentiating mesenchymal stem cells (MSCs) into endothelial cells (ECs). It is generally accepted that MSCs cannot be easily differentiated into ECs without high concentrations of proangiogenic factors. To guide either bone marrow-derived mesenchymal stem cells (BM-MSCs) and Wharton's jelly-derived mesenchymal stem cells (WJ-MSCs) into ECs-like phenotype, poly(allylamine-hydrochloride)/poly(styrene-sulfonate) multilayers film (PAH/PSS) was used as culture coating and compared to type I collagen (as control coating). After 2 weeks of culture and in absence of angiogenic growth factors, PAH/PSS upregulated KDR, PECAM-1, and CDH5 genes, whereas combining PAH/PSS with endothelial growth media (EGM-2^® ) led to the production of respective proteins by WJ-MSCs. In contrast, not fully EC-like phenotype is obtained from the differentiation of BM- or WJ-MSCs cultured on type I collagen. Thus, using PAH/PSS coating in synergy with EGM-2^® appears as an ideal condition promoting WJ-MSCs differentiation into ECs-like. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 292-300, 2017.

Soft-Mechanochemistry: Mechanochemistry Inspired by Nature

Cells and bacteria use mechanotransduction processes to transform a mechanical force into a chemical/biochemical response. The area of chemistry where chemical reactions are induced by mechanical forces is called mechanochemistry. Over the last few years, chemists developed force-induced reactions affecting covalent bonds in molecules under tension which requires high energy input and/or high intensity forces. In contrast, in nature, mechanotransduction processes take place with forces of much weaker intensity and much less demanding energy. They are mainly based on protein conformational changes or changes in supramacromolecular architectures. Mechanochemistry based on such low-energy-demanding processes and which does not affect chemical bonds can be called soft-mechanochemistry. In this feature article, we first discuss some examples of soft-mechanochemistry processes encountered in nature, in particular, cryptic sites, allowing us to define more precisely the concepts underlying soft-mechanochemistry. A series of examples, developed over the past few years, of chemomechanoresponsive systems based on soft-mechanochemistry principles are given. We describe, in particular, cryptic site surfaces, enzymatically active films whose activity can be modulated by stretching and films where stretching induces changes in their fluorescence properties. Finally, we give our view of the future of soft-mechanochemistry.

Stretch-Induced Helical Conformations in Poly(l-lysine)/Hyaluronic Acid Multilayers

We investigate the effect of stretching on the secondary structure of cross-linked poly(l-lysine)/hyaluronic acid (PLL/HA) multilayers. We show that stretching these films induces changes in the secondary structure of PLL chains. Our results suggest that not only α- but also 310-helices might form in the film under stretching. Such 310-helices have never been observed for PLL so far. These changes of the secondary structure of PLL are reversible, i.e., when returning to the nonstretched state one recovers the initial film structure. Using molecular dynamics simulations of chains composed of 20 l-lysine residues (PLL20), we find that these chains never adopt a helical conformation in water. In contrast, when the end-to-end distance of the chains is restrained to values smaller than the mean end-to-end distance of free chains, a distance domain rarely explored by the free chains, helical conformations become accessible. Moreover, the formation of not only α- but also 310-helices is predicted by the simulations. These results suggest that the change of the end-to-end distance of PLL chains in the stretched film is at the origin of the helix formation.

A new biomimetic route to engineer enzymatically active mechano-responsive materials

Using modified β-galactosidase covalently linked to cross-linked polyelectrolyte multilayers (PEM), catalytically active materials have been designed. Their enzymatic activity can be modulated, partially in a reversible way, simply by stretching. This strategy, based on enzyme conformational changes, constitutes a new tool for the development of biocatalytic mechano-responsive materials.

Morphogen Electrochemically Triggered Self-Construction of Polymeric Films Based on Mussel-Inspired Chemistry

Inspired by the strong chemical adhesion mechanism of mussels, we designed a catechol-based electrochemically triggered self-assembly of films based on ethylene glycol molecules bearing catechol groups on both sides and denoted as bis-catechol molecules. These molecules play the role of morphogens and, in contrast to previously investigated systems, they are also one of the constituents, after reaction, of the film. Unable to interact together, commercially available poly(allylamine hydrochloride) (PAH) chains and bis-catechol molecules are mixed in an aqueous solution and brought in contact with an electrode. By application of defined potential cycles, bis-catechol molecules undergo oxidation leading to molecules bearing "reactive" quinone groups which diffuse toward the solution. In this active state, the quinones react with amino groups of PAH through Michael addition and Schiff's base condensation reaction. The application of cyclic voltammetry (CV) between 0 and 500 mV (vs Ag/AgCl, scan rate of 50 mV/s) of a PAH/bis-catechol solution results in a fast self-construction of a film that reaches a thickness of 40 nm after 60 min. The films present a spiky structure which is attributed to the use of bis-functionalized molecules as one component of the films. XPS measurements show the presence of both PAH and bis-catechol cross-linked together in a covalent way. We show that the amine/catechol ratio is an important parameter which governs the film buildup. For a given amine/catechol ratio, it does exist an optimum CV scan rate leading to a maximum of the film thickness as a function of the scan rate.

Polyelectrolyte Multilayers: A Versatile Tool for Preparing Antimicrobial Coatings

The prevention of pathogen colonization of medical implants represents a major medical and financial issue. The development of antimicrobial coatings aimed at protecting against such infections has thus become a major field of scientific and technological research. Three main strategies are developed to design such coatings: (i) the prevention of microorganisms adhesion and the killing of microorganisms (ii) by contact and (iii) by the release of active compounds in the vicinity of the implant. Polyelectrolyte multilayer (PEM) technology alone covers the entire widespread spectrum of functionalization possibilities. PEMs are obtained through the alternating deposition of polyanions and polycations on a substrate, and the great advantages of PEMs are that (i) they can be applied to almost any type of substrate whatever its shape and composition; (ii) various chemical, physicochemical, and mechanical properties of the coatings can be obtained; and (iii) active compounds can be embedded and released in a controlled manner. In this article we will give an overview of the field of PEMs applied to the design of antimicrobial coatings, illustrating the large versatility of the PEM technology.