Acrylate monomer polymerization triggered by iron oxide magnetic nanoparticles and catechol containing microgels.

Phenol and its derivatives are the most used polymerization inhibitors for vinyl-based monomers. Here, we reported a novel catalytic system composed of mussel inspired adhesive moiety, catechol, in combination with iron oxide nanoparticles (IONPs) to generate hydroxyl radical (•OH) at pH 7.4. Catechol-containing microgel (DHM) was prepared by copolymerizing dopamine methacrylamide (DMA) and N-hydroxyethyl acrylamide (HEAA), which generated superoxide (•O2-) and hydrogen peroxide (H2O2) as a result of catechol oxidation. In the presence of IONPs, the generated reactive oxygen species were further converted to •OH, which initiated free radical polymerization of various water-soluble acrylate-based monomers including neutral (acrylamide, methyl acrylamide, etc.), anionic (2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), cationic ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), and zwitterionic (2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide) monomers. Compared with the typical free radical initiating systems, the reported system does not require the addition of extra initiators for polymerization. During the process of polymerization, a bilayer hydrogel was formed in situ and exhibited the ability to bend during the process of swelling. The incorporation of IONPs significantly enhanced magnetic property of the hydrogel and the combination of DHM and IONPs also improved the mechanical properties of these hydrogels.

pH Responsive Antibacterial Hydrogel Utilizing Catechol-Boronate Complexation Chemistry.

Bacteria such as Methicillin-resistant Staphylococcus aureus (MRSA) causes acidic microenvironment during infection. A biomaterial that exhibits tunable antimicrobial property in a pH dependent manner is potentially attractive. Herein, we presented a novel antibacterial hydrogel consisting of pH responsive and reversible catechol-boronate linkage formed between intrinsically bactericidal chlorinated catechol (catechol-Cl) and phenylboronic acid. Fourier transformed infrared spectroscopy (FTIR), oscillatory rheometry, and Johnson Kendall Roberts (JKR) contact mechanics testing confirmed the formation and dissociation of the complex in a pH dependent manner. When the hydrogel was treated with an acidic buffer (pH 3), the hydrogel exhibited excellent antimicrobial property against multiple strains of Gram-positive and negative bacteria including MRSA (up to 4 log10 reduction from 108 colony forming units/mL). At an acidic pH, catechol-Cl was unbound from the phenylboronic acid and available for killing bacteria. Conversely, when the hydrogel was treated with a basic buffer (pH 8.5), the hydrogel lost its antimicrobial property but also became non-cytotoxic. At a basic pH, the formation of catechol-boronate complex effectively reduce the exposure of the cytotoxic catechol-Cl to the surrounding. When further incubating the hydrogel in an acidic pH, the reversible complex dissociated to re-expose catechol-Cl and the hydrogel recovered its antibacterial property. Overall, the combination of catechol-Cl and phenylboronic acid provide a new strategy for designing hydrogels with pH responsive antibacterial activity and reduced cytotoxicity.

Thermomagnetic-Responsive Self-Folding Microgrippers for Improving Minimally Invasive Surgical Techniques and Biopsies.

Traditional open surgery complications are typically due to trauma caused by accessing the procedural site rather than the procedure itself. Minimally invasive surgery allows for fewer complications as microdevices operate through small incisions or natural orifices. However, current minimally invasive tools typically have restricted maneuverability, accessibility, and positional control of microdevices. Thermomagnetic-responsive microgrippers are microscopic multi-fingered devices that respond to temperature changes due to the presence of thermal-responsive polymers. Polymeric devices, made of poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM-AAc) and polypropylene fumarate (PPF), self-fold due to swelling and contracting of the hydrogel layer. In comparison, soft metallic devices feature a pre-stressed metal bilayer and polymer hinges that soften with increased temperature. Both types of microdevices can self-actuate when exposed to the elevated temperature of a cancerous tumor region, allowing for direct targeting for biopsies. Microgrippers can also be doped to become magnetically responsive, allowing for direction without tethers and the retrieval of microdevices containing excised tissue. The smaller size of stimuli-responsive microgrippers allows for their movement through hard-to-reach areas within the body and the successful extraction of intact cells, RNA and DNA. This review discusses the mechanisms of thermal- and magnetic-responsive microdevices and recent advances in microgripper technology to improve minimally invasive surgical techniques.

Effect of Conductivity on In Situ Deactivation of Catechol-Boronate Complexation-Based Reversible Smart Adhesive.

To reduce the need for elevated electrical potential to deactivate catechol-based smart adhesive and preserve its reversibility, conductive 1-pyrenemethyl methacrylate (PyMA) was incorporated into a catechol and phenylboronic acid-containing adhesive coating immobilized on aluminum (Al) discs. Electrochemical impedance spectroscopy (EIS) indicated that incorporation of 26 mol % of PyMA reduced ionic resistance (Rs) and charge-transfer resistance (Rc) of the coating from over 22 Ω/mm2 to 5.9 and 1.2 Ω/mm2, respectively. A custom-built Johnson-Kendall-Roberts (JKR) contact mechanics test setup was used to evaluate the adhesive property of the coating with in situ applied electricity using a titanium (Ti) sphere both as a test substrate as well as the cathode for application of electricity and the Al disc as the anode. The adhesive coating demonstrated over 95% reduction in the adhesive property when electricity (1-2 V) was applied while the adhesive was in direct contact with the Ti surface. The addition of PyMA enables the deactivation of the adhesive using a voltage as low as 1 V. Both cyclic voltammetry (CV) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra confirmed the formation of catechol-boronate complexation through electrochemical stimulation. Breaking the complex with an acidic buffer (pH 3) recovered the catechol for strong wet adhesion and the coating could be repeatedly deactivated and reactivated using low electrical potential for up to five cycles. Incorporation of both conductive PyMA and boronic acid as the temporary protecting group was required to achieve rapidly switchable adhesive that could be deactivated with low applied voltage.

Antimicrobial Property of Halogenated Catechols.

Bacterial infection associated with multidrug resistance (MDR) bacteria is increasingly becoming a significant public health risk. Herein, we synthesized a series of halogenated dopamine methacrylamide (DMA), which contains a catechol side chain modified with either chloro-, bromo-, or iodo-functional group. Catechol is a widely used adhesive moiety for designing bioadhesives and coating. However, the intrinsic antimicrobial property of catechol has not been demonstrated before. These halogenated DMA were incorporated into hydrogels, copolymers, and coatings and exhibited more than 99% killing efficiencies against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. More importantly, hydrogel containing chlorinated DMA demonstrated broad-spectrum antimicrobial activities towards multiple MDR bacteria, which included methicillin resistant S. aureus (MRSA), vancomycin resistant enterococci (VRE), multi antibiotics resistant Pseudomonas aeruginosa (PAER), multi antibiotics resistant Acinetobacter baumannii (AB) and carbapenem resistant Klebsiella pneumoniae (CRKP). These hydrogels also demonstrated the ability to kill bacteria in a biofilm while exhibiting low cytotoxic. Based on molecular docking and molecular dynamics simulation, Cl-functionalized catechol can potentially inhibit bacterial fatty acid synthesis at the enoyl-acyl carrier protein reductase (FabI) step. The combination of moisture-resistant adhesive property, inherent antimicrobial property, and the versatility of incorporating halogenated DMA into different polymeric materials greatly enhanced the potential for using these monomers for designing multifunctional bioadhesives and coatings.

Catechol-functionalized hydrogels: biomimetic design, adhesion mechanism, and biomedical applications.

Hydrogels are a unique class of polymeric materials that possess an interconnected porous network across various length scales from nano- to macroscopic dimensions and exhibit remarkable structure-derived properties, including high surface area, an accommodating matrix, inherent flexibility, controllable mechanical strength, and excellent biocompatibility. Strong and robust adhesion between hydrogels and substrates is highly desirable for their integration into and subsequent performance in biomedical devices and systems. However, the adhesive behavior of hydrogels is severely weakened by the large amount of water that interacts with the adhesive groups reducing the interfacial interactions. The challenges of developing tough hydrogel-solid interfaces and robust bonding in wet conditions are analogous to the adhesion problems solved by marine organisms. Inspired by mussel adhesion, a variety of catechol-functionalized adhesive hydrogels have been developed, opening a door for the design of multi-functional platforms. This review is structured to give a comprehensive overview of adhesive hydrogels starting with the fundamental challenges of underwater adhesion, followed by synthetic approaches and fabrication techniques, as well as characterization methods, and finally their practical applications in tissue repair and regeneration, antifouling and antimicrobial applications, drug delivery, and cell encapsulation and delivery. Insights on these topics will provide rational guidelines for using nature's blueprints to develop hydrogel materials with advanced functionalities and uncompromised adhesive properties.

Catechol-Based Antimicrobial Polymers.

Catechol is a key constituent in mussel adhesive proteins and is responsible for strong adhesive property and crosslinking formation. Plant-based polyphenols are also capable of chemical interactions similar to those of catechol and are inherently antimicrobial. This review reports a series of catechol-based antimicrobial polymers classified according to their antimicrobial mechanisms. Catechol is utilized as a surface anchoring group for adhering monomers and polymers of known antimicrobial properties onto various types of surfaces. Additionally, catechol's ability to form strong complexes with metal ions and nanoparticles was utilized to sequester these antimicrobial agents into coatings and polymer matrices. During catechol oxidation, reactive oxygen species (ROS) is generated as a byproduct, and the use of the generated ROS for antimicrobial applications was also introduced. Finally, polymers that utilized the innate antimicrobial property of halogenated catechols and polyphenols were reviewed.

In Situ Deactivation of Catechol-Containing Adhesive Using Electrochemistry.

Marine mussels secret catechol-containing adhesive proteins that enable these organisms to bind to various surfaces underwater. Synthetic mimics of these proteins have been created to function as adhesives and coatings for a wide range of applications. Here, we demonstrated the use of in situ electrical field stimulation to deactivate the adhesive property of catechol-containing adhesive that is in direct contact with a surface. Johnson-Kendall-Roberts (JKR) contact mechanics test was performed using a titanium (Ti) sphere in the presence of a pH 7.5 aqueous buffer. The Ti sphere also served as a conductive electrode for applying electricity to the adhesive, while a platinum (Pt) wire served as the counter electrode. Work of adhesion (Wadh) decreased with increased levels of applied voltage and current, exposure time to the applied electricity, and salt concentration of the interfacial buffer. Application of 9 V for 1 min completely deactivated the adhesive. UV-vis diffuse reflectance spectra and tracking of catechol oxidation byproduct, hydrogen peroxide, confirmed that catechol was oxidized as a result of applied electricity. Contact mechanics testing further confirmed that the Young's modulus of the adhesive increased by nearly 4 folds at the interface as a result of oxidative cross-linking, even though the modulus of the bulk of the adhesive was unaffected by applied electricity. The accumulation of hydroxyl ions near the cathode increased the local solution pH, which promoted oxidation-induced cross-linking of catechol and subsequently decreased its adhesive property. Tuning adhesive properties through in situ electrochemical oxidation provides on-demand control over the adhesive, which will potentially add another dimension in designing synthetic mimics of mussel adhesive proteins.

Design of pH-responsive SAP polymer for pore solution chemistry regulation and crack sealing in cementitious materials.

The crack development is considered to be one of the most severe threats to the durability of concrete infrastructure. This study aims to enhance the durability performance of cementitious material with the pH-responsive Superabsorbent Polymer (SAP). The SAP was synthesized with acrylic acid (AA)-methyl acrylate (MA) precursors, and three type samples with different crosslinking levels were prepared. The examination on the pH sensitivity indicated that the swelling capacity of the prepared SAP would first increase and then decrease with solution alkalinity, and the peak swelling potential was achieved around pH value of 12 for all the three type SAP with solution/gel mass ratio of 500. Further examination indicated the alkalinity of the buffer solution was reduced during the adsorption test, which can be caused by the hydrolysis of the amide groups and the crosslinker. Besides that, it was also found the solution/gel ratio and the Ca(OH)2 content could affect the swelling potential of the SAP. After that, the performance tests were conducted for the evaluation of concrete with SAP. A wax-coating protocol for the SAP was designed by using the hot-water method to prevent its swelling during mixing process. It was found that the strength reduction for samples with wax-coated SAP was insignificant compared to that of the control samples. Furthermore, durability tests supported the wax-shell could be broken by the crack propagation in concrete. And further experimental studies are needed to optimize the wax-size and shell thickness for enhanced self-sealing efficiency.

Rapidly responsive smart adhesive-coated micropillars utilizing catechol-boronate complexation chemistry.

Smart adhesive hydrogels containing 10 mol% each of dopamine methacrylamide (DMA) and 3-acrylamido phenylboronic acid (APBA) were polymerized in situ onto polydimethylsiloxane (PMDS) micropillars with different aspect ratios (AR = 0.4, 1 and 2). Using Johnson-Kendall-Roberts (JKR) contact mechanics tests, the adhesive-coated pillars demonstrated strong wet adhesion at pH 3 (Wadh = 420 mJ m-2) and can be repeatedly deactivated and reactivated by changing the pH value (pH 9 and 3, respectively). When compared to the bulk adhesive hydrogel of the same composition, the adhesive-coated pillars exhibited a significantly faster rate of transition (1 min) between strong and weak adhesion. This was attributed to an increased surface area to volume ratio of the adhesive hydrogel-coated pillars, which permitted rapid diffusion of ions into the adhesive matrix to form or break the catechol-boronate complex.

Incorporation of Anionic Monomer to Tune the Reversible Catechol-Boronate Complex for pH-Responsive, Reversible Adhesion.

Up to 30 mol % of acrylic acid (AAc) was incorporated into a pH-responsive smart adhesive consisting of dopamine methacrylamide and 3-acrylamido phenylboronic acid. Fourier transform infrared spectroscopy and rheometry confirmed that the incorporation of AAc shifted the pH of catechol-boronate complexation to a more basic pH. Correspondingly, adhesive formulations with elevated AAc contents demonstrated strong adhesion to quartz substrate at a neutral to mildly basic pH (7.5-8.5) based on Johnson-Kendall-Roberts contact mechanics test. When pH was further increased to 9.0, there was a drastic reduction in the measured work of adhesion (18- and 7-fold reduction compared to values measured at pHs 7.5 and 8.5, respectively) due to the formation of catechol-boronate complex. The complex remained reversible, and the interfacial binding property of the adhesive was successfully tuned with changing pH in successive contact cycles. However, an acidic pH (3.0) was required to break the catechol-boronate complex to recover the elevated adhesive property. Adding AAc enables the smart adhesive to function in physiological or marine pH ranges.

Effect of Ionic Functional Groups on the Oxidation State and Interfacial Binding Property of Catechol-Based Adhesive.

Adhesive hydrogels were prepared by copolymerizing dopamine methacrylamide with either acrylic acid (AAc) or N-(3-aminopropyl)methacrylamide hydrochloride (APMH). The effect of incorporating the anionic and cationic side chains on the oxidation state of catechol was characterized using the FOX assay to track the production of hydrogen peroxide byproduct generated during the autoxidation of catechol, and the interfacial binding property of the adhesive was determined by performing Johnson-Kendall-Roberts contact mechanics tests tested over a wide range of pH values (pH 3.0-9.0). The ionic species contributed to interfacial binding to surfaces with the opposite charge with measured work of adhesion values that were comparable to or in some cases higher than those of catechol. Addition of AAc minimized the oxidation of catechol even at a pH of 8.5 and correspondingly preserved the elevated adhesive property of catechol to both quartz and amine-functionalized surfaces. However, AAc lost its buffering capacity at pH 9.0, and catechol was oxidized at this pH. On the other hand, catechol formed a cohesive covalent bond with the network-bound amine side chain of APMH at basic pH, which interfered with the interfacial binding capability of APMH and the catechol.

A Moldable Nanocomposite Hydrogel Composed of a Mussel-Inspired Polymer and a Nanosilicate as a Fit-to-Shape Tissue Sealant.

The engineering of bioadhesives to bind and conform to the complex contour of tissue surfaces remains a challenge. We have developed a novel moldable nanocomposite hydrogel by combining dopamine-modified poly(ethylene glycol) and the nanosilicate Laponite, without the use of cytotoxic oxidants. The hydrogel transitioned from a reversibly cross-linked network formed by dopamine-Laponite interfacial interactions to a covalently cross-linked network through the slow autoxidation and cross-linking of catechol moieties. Initially, the hydrogel could be remolded to different shapes, could recover from large strain deformation, and could be injected through a syringe to adhere to the convex contour of a tissue surface. With time, the hydrogel solidified to adopt the new shape and sealed defects on the tissue. This fit-to-shape sealant has potential in sealing tissues with non-flat geometries, such as a sutured anastomosis.

Effect of nitro-functionalization on the cross-linking and bioadhesion of biomimetic adhesive moiety.

Dopamine mimics the exceptional moisture-resistant adhesive properties of the amino acid, DOPA, found in adhesive proteins secreted by marine mussels. The catechol side chain of dopamine was functionalized with a nitro-group, and the effect of the electron withdrawing group modification on the cross-linking chemistry and bioadhesive properties of the adhesive moiety was evaluated. Both nitrodopamine and dopamine were covalently attached as a terminal group onto an inert, 4-armed poly(ethylene glygol) (PEG-ND and PEG-D, respectively). PEG-ND and PEG-D exhibited different dependence on the concentration of NaIO4 and pH, which affected the curing rate, mechanical properties, and adhesive performance of these biomimetic adhesives differently. PEG-ND cured instantly and its bioadhesive properties were minimally affected by the change in pH (5.7-8) within the physiological range. Under mildly acidic conditions (pH 5.7 and 6.7), PEG-ND outperformed PEG-D in lap shear adhesion testing using wetted pericardium tissues. However, nitrodopamine only formed dimers, which resulted in the formation of loosely cross-linked network and adhesive with reduced cohesive properties. UV-vis spectroscopy further confirmed nitrodopamine's ability for rapid dimer formation. The ability for nitrodopamine to rapidly cure and adhere to biological substrates in an acidic pH make it suitable for designing adhesive biomaterials targeted at tissues that are more acidic (i.e., subcutaneous, dysoxic, or tumor tissues).

Fibrin gel as an injectable biodegradable scaffold and cell carrier for tissue engineering.

Due to the increasing needs for organ transplantation and a universal shortage of donated tissues, tissue engineering emerges as a useful approach to engineer functional tissues. Although different synthetic materials have been used to fabricate tissue engineering scaffolds, they have many limitations such as the biocompatibility concerns, the inability to support cell attachment, and undesirable degradation rate. Fibrin gel, a biopolymeric material, provides numerous advantages over synthetic materials in functioning as a tissue engineering scaffold and a cell carrier. Fibrin gel exhibits excellent biocompatibility, promotes cell attachment, and can degrade in a controllable manner. Additionally, fibrin gel mimics the natural blood-clotting process and self-assembles into a polymer network. The ability for fibrin to cure in situ has been exploited to develop injectable scaffolds for the repair of damaged cardiac and cartilage tissues. Additionally, fibrin gel has been utilized as a cell carrier to protect cells from the forces during the application and cell delivery processes while enhancing the cell viability and tissue regeneration. Here, we review the recent advancement in developing fibrin-based biomaterials for the development of injectable tissue engineering scaffold and cell carriers.

Effect of pH on the rate of curing and bioadhesive properties of dopamine functionalized poly(ethylene glycol) hydrogels.

The remarkable underwater adhesion strategy employed by mussels has inspired bioadhesives that have demonstrated promise in connective tissue repair, wound closure, and local delivery of therapeutic cells and drugs. While the pH of oxygenated blood and internal tissues is typically around 7.4, skin and tumor tissues are significantly more acidic. Additionally, blood loss during surgery and ischemia can lead to dysoxia, which lowers pH levels of internal tissues and organs. Using 4-armed PEG end-capped with dopamine (PEG-D) as a model adhesive polymer, the effect of pH on the rate of intermolecular cross-linking and adhesion to biological substrates of catechol-containing adhesives was determined. Adhesive formulated at an acidic pH (pH 5.7-6.7) demonstrated reduced curing rate, mechanical properties, and adhesive performance to pericardium tissues. Although a faster curing rate was observed at pH 8, these adhesives also demonstrated reduced mechanical and bioadhesive properties when compared to adhesives buffered at pH 7.4. Adhesives formulated at pH 7.4 demonstrated a good balance of fast curing rate, elevated mechanical properties and interfacial binding ability. UV-vis spectroscopy evaluation revealed that the stability of the transient oxidation intermediate of dopamine was increased under acidic conditions, which likely reduced the rate of intermolecular cross-linking and bulk cohesive properties for hydrogels formulated at these pH levels. At pH 8, competing cross-linking reaction mechanisms and reduced concentration of dopamine catechol due to auto-oxidation likely reduced the degree of dopamine polymerization and adhesive strength for these hydrogels. pH plays an important role in the adhesive performance of mussel-inspired bioadhesives and the pH of the adhesive formulation needs to be adjusted for the intended application.

pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli.

Growing evidence supports a critical role of metal-ligand coordination in many attributes of biological materials including adhesion, self-assembly, toughness, and hardness without mineralization [Rubin DJ, Miserez A, Waite JH (2010) Advances in Insect Physiology: Insect Integument and Color, eds Jérôme C, Stephen JS (Academic Press, London), pp 75-133]. Coordination between Fe and catechol ligands has recently been correlated to the hardness and high extensibility of the cuticle of mussel byssal threads and proposed to endow self-healing properties [Harrington MJ, Masic A, Holten-Andersen N, Waite JH, Fratzl P (2010) Science 328:216-220]. Inspired by the pH jump experienced by proteins during maturation of a mussel byssus secretion, we have developed a simple method to control catechol-Fe(3+) interpolymer cross-linking via pH. The resonance Raman signature of catechol-Fe(3+) cross-linked polymer gels at high pH was similar to that from native mussel thread cuticle and the gels displayed elastic moduli (G') that approach covalently cross-linked gels as well as self-healing properties.

Utilizing Rapid Hydrogen Peroxide Generation from 6-Hydroxycatechol to Design Moisture-Activated, Self-Disinfecting Coating.

A coating that can be activated by moisture found in respiratory droplets could be a convenient and effective way to control the spread of airborne pathogens and reduce fomite transmission. Here, the ability of a novel 6-hydroxycatechol-containing polymer to function as a self-disinfecting coating on the surface of polypropylene (PP) fabric was explored. Catechol is the main adhesive molecule found in mussel adhesive proteins. Molecular oxygen found in an aqueous solution can oxidize catechol and generate a known disinfectant, hydrogen peroxide (H2O2), as a byproduct. However, given the limited amount of moisture found in respiratory droplets, there is a need to enhance the rate of catechol autoxidation to generate antipathogenic levels of H2O2. 6-Hydroxycatechol contains an electron donating hydroxyl group on the 6-position of the benzene ring, which makes catechol more susceptible to autoxidation. 6-Hydroxycatechol-coated PP generated over 3000 µM of H2O2 within 1 h when hydrated with a small amount of aqueous solution (100 µL of PBS). The generated H2O2 was three orders of magnitude higher when compared to the amount generated by unmodified catechol. 6-Hydroxycatechol-containing coating demonstrated a more effective antimicrobial effect against both Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative (Pseudomonas aeruginosa and Escherichia coli) bacteria when compared to unmodified catechol. Similarly, the self-disinfecting coating reduced the infectivity of both bovine viral diarrhea virus and human coronavirus 229E by as much as a 2.5 log reduction value (a 99.7% reduction in viral load). Coatings containing unmodified catechol did not generate sufficient H2O2 to demonstrate significant virucidal effects. 6-Hydroxycatechol-containing coating can potentially function as a self-disinfecting coating that can be activated by the moisture present in respiratory droplets to generate H2O2 for disinfecting a broad range of pathogens.

Pair of Functional Polyesters That Are Photo-Cross-Linkable and Electrospinnable to Engineer Elastomeric Scaffolds with Tunable Structure and Properties.

A pair of alkyne- and thiol-functionalized polyesters are designed to engineer elastomeric scaffolds with a wide range of tunable material properties (e.g., thermal, degradation, and mechanical properties) for different tissues, given their different host responses, mechanics, and regenerative capacities. The two prepolymers are quickly photo-cross-linkable through thiol-yne click chemistry to form robust elastomers with small permanent deformations. The elastic moduli can be easily tuned between 0.96 ± 0.18 and 7.5 ± 2.0 MPa, and in vitro degradation is mediated from hours up to days by adjusting the prepolymer weight ratios. These elastomers bear free hydroxyl and thiol groups with a water contact angle of less than 85.6 ± 3.58 degrees, indicating a hydrophilic nature. The elastomer is compatible with NIH/3T3 fibroblast cells with cell viability reaching 88 ± 8.7% relative to the TCPS control at 48 h incubation. Differing from prior soft elastomers, a mixture of the two prepolymers without a carrying polymer is electrospinnable and UV-cross-linkable to fabricate elastic fibrous scaffolds for soft tissues. The designed prepolymer pair can thus ease the fabrication of elastic fibrous conduits, leading to potential use as a resorbable synthetic graft. The elastomers could find use in other tissue engineering applications as well.

A reversible wet/dry adhesive inspired by mussels and geckos.

The adhesive strategy of the gecko relies on foot pads composed of specialized keratinous foot-hairs called setae, which are subdivided into terminal spatulae of approximately 200 nm (ref. 1). Contact between the gecko foot and an opposing surface generates adhesive forces that are sufficient to allow the gecko to cling onto vertical and even inverted surfaces. Although strong, the adhesion is temporary, permitting rapid detachment and reattachment of the gecko foot during locomotion. Researchers have attempted to capture these properties of gecko adhesive in synthetic mimics with nanoscale surface features reminiscent of setae; however, maintenance of adhesive performance over many cycles has been elusive, and gecko adhesion is greatly diminished upon full immersion in water. Here we report a hybrid biologically inspired adhesive consisting of an array of nanofabricated polymer pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts. Wet adhesion of the nanostructured polymer pillar arrays increased nearly 15-fold when coated with mussel-mimetic polymer. The system maintains its adhesive performance for over a thousand contact cycles in both dry and wet environments. This hybrid adhesive, which combines the salient design elements of both gecko and mussel adhesives, should be useful for reversible attachment to a variety of surfaces in any environment.