Tougher Bioadhesives through Dual Stimulation Strategies.

Carbene-based bioadhesives have favourable attributes for tissue adhesion, including non-specific bonding to wet and dry tissues, but suffer from relatively weak fracture strength after photocuring. Light irradiation of carbene-precursor (diazirine) also creates inert side products that are absent under thermal activation. Herein, a dual activation method combines light irradiation at elevated temperatures for the evaluation of diazirine depletion and effects on cohesive properties. A customized photo/thermal-rheometer evaluates viscoelastic properties, correlated to the kinetics of carbene:diazoalkane ratios via 19 F NMR). The latter exploits the sensitive -CF3 functional group to determine joule-based light/temperature kinetics on trifluoroaryl diazirine consumption. The combination of heat and photoactivation produced bioadhesives that are 3× tougher compared to control. Dual thermal/light irradiation may be a strategy to improve viscoelastic dissipation and toughness of photo-activated adhesive resins.

Synthesis and Evaluation of Metal Lipoate Adhesives.

The development of new bioadhesives with integrated properties remains an unmet clinical need to replace staples or sutures. Current bioadhesives do not allow electronic activation, which would allow expansion into laparoscopic and robotic surgeries. To address this deficiency, voltage-activated adhesives have been developed on both carbene- and catechol-based chemical precursors. Herein, a third platform of voltage-activated adhesive is evaluated based on lipoic acid, a non-toxic dithiolane found in aerobic metabolism and capable of ring-opening polymerization. The electro-rheological and adhesive properties of lithium, sodium, and potassium salts of lipoic acid are applied for wet tissue adhesion. At ambient conditions, potassium lipoate displays higher storage modulus than lithium or sodium salt under similar conditions. Voltage stimulation significantly improves gelation kinetics to Na- and K-lipoates, while Li-lipoate is found to not require voltage stimulation for gelation. Lap shear adhesion strength on wetted collagen substrates reveals that the synthetic metal lipoates have comparable adhesion strength to fibrin sealants without viral or ethical risks.

Bacterial cellulose adhesive patches designed for soft mucosal interfaces.

The wet environment in the oral cavity is challenging for topical disease management approaches. The compromised material properties leading to weak adhesion and short retention (<8 h) in such environment result in frequent reapplication of the therapeutics. Composites of bacterial cellulose (BC) and carbene-based bioadhesives attempt to address these shortcomings. Previous designs comprised of aqueous formulations. The current design, for the first time, presents dry, shelf-stable cellulose patches for convenient ready-to-use application. The dry patches simultaneously remove tissue surface hydration while retaining carbene-based photocuring and offers on-demand adhesion. The dry patch prototypes are optimized by controlling BC/adhesive mole ratios and dehydration technique. The adhesion strength is higher than commercial denture adhesives on soft mucosal tissues. The structural integrity is maintained for a minimum of 7 days in aqueous environment. The patches act as selective nanoporous barrier against bacteria while allowing permeation of proteins. The results support the application of BC-based adhesive patches as a flexible platform for wound dressings, drug depots, or combination thereof.

Fibrillated bacterial cellulose liquid carbene bioadhesives for mimicking and bonding oral cavity surfaces.

Topical treatments for oral wounds and infections exhibit weak adhesion to wet surfaces which results in short retention duration (6-8 hours), frequent dosing requirement and patient incompatibility. To address these limitations, aqueous composites made of fibrillated bacterial cellulose and photoactive bioadhesives are designed for soft epithelial surfaces. The aqueous composites crosslink upon photocuring within a minute and exhibit a transition from viscous to elastic adhesive hydrogels. The light-cured composites have shear moduli mimicking oral mucosa and other soft tissues. The tunable adhesion strength ranges from 3 to 35 kPa on hydrated tissue-mimicking surfaces (collagen film). The results support the application of bacterial cellulose hydrogel systems for potential treatment of mucosal wounds.

A Fluorescence Kinetic-Based Aptasensor Employing Stilbene Isomerization for Detection of Thrombin.

It is important to detect thrombin due to its physiological and pathological roles, where rapid and simple analytical approaches are needed. In this study, an aptasensor based on fluorescence attenuation kinetics for the detection of thrombin is presented, which incorporates the features of stilbene and aptamer. We designed and synthesized an aptasensor by one-step coupling of stilbene compound and aptamer, which employed the adaptive binding of the aptamer with thrombin to cause a change in stilbene fluorescence attenuation kinetics. The sensor realized detection of thrombin by monitoring the variation in apparent fluorescence attenuation rate constant (kapp), which could be further used for probing of enzyme-aptamer binding. In comprehensive studies, the developed aptasensor presented satisfactory performance on repeatability, specificity, and regeneration capacity, which realized rapid sensing (10 s) with a limit of detection (LOD) of 0.205 µM. The strategy was successful across seven variants of thrombin aptasensors, with tunable kapp depending on the SITS (4-Acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid disodium salt hydrate) grafting site. Analyte detection mode was demonstrated in diluted serum, requiring no separation or washing steps. The new sensing mode for thrombin detection paves a way for high-throughput kinetic-based sensors for exploiting aptamers targeted at clinically relevant proteins.

Bacterial cellulose adhesive composites for oral cavity applications.

Topical approaches to oral diseases require frequent dosing due to limited retention time. A mucoadhesive drug delivery platform with extended soft tissue adhesion capability of up to 7 days is proposed for on-site management of oral wound. Bacterial cellulose (BC) and photoactivated carbene-based bioadhesives (PDz) are combined to yield flexible film platform for interfacing soft tissues in dynamic, wet environments. Structure-activity relationships evaluate UV dose and hydration state with respect to adhesive strength on soft tissue mimics. The bioadhesive composite has an adhesion strength ranging from 7 to 17 kPa and duration exceeding 48 h in wet conditions under sustained shear forces, while other mucoadhesives based on hydrophilic macromolecules exhibit adhesion strength of 0.5-5 kPa and last only a few hours. The work highlights the first evaluation of BC composites for mucoadhesive treatments in the buccal cavity.

Rapid Activation of Diazirine Biomaterials with the Blue Light Photocatalyst.

Carbene-based macromolecules are an emerging new stimuli-sensitive class of biomaterials that avoid the impediments of free radical polymerization but maintain a rapid liquid-to-biorubber transition. Activation of diazirine-grafted polycaprolactone polyol (CaproGlu) is limited to UVA wavelengths that have tissue exposure constraints and limited light intensities. For the first time, UVA is circumvented with visible light-emitting diodes at 445 nm (blue) to rapidly activate diazirine-to-carbene covalent cross-linking. Iridium photocatalysts serve to initiate diazirine, despite having little to no absorption at 445 nm. CaproGlu's liquid organic matrix dissolves the photocatalyst with no solvents required, creating a light transparent matrix. Considerable differences in cross-linking chemistry are observed in UVA vs visible/photocatalyst formulations. Empirical analysis and theoretical calculations reveal a more efficient conversion of diazirine directly to carbene with no diazoalkane intermediate detected. Photorheometry results demonstrate a correlation between shear moduli, joules light dose, and the lower limits of photocatalyst concentration required for the liquid-to-biorubber transition. Adhesion strength on ex vivo hydrated tissues exceeds that of cyanoacrylates, with a fixation strength of up to 20 kg·f·cm2. Preliminary toxicity assessment on leachates and materials directly in contact with mammalian fibroblast cells displays no signs of fibroblast cytotoxicity.

Fixation of Transparent Bone Pins with Photocuring Biocomposites.

Bone fractures are in need of rapid fixation methods, but the current strategies are limited to metal pins and screws, which necessitate secondary surgeries upon removal. New techniques are sought to avoid surgical revisions, while maintaining or improving the fixation speed. Herein, a method of bone fixation is proposed with transparent biopolymers anchored in place via light-activated biocomposites based on expanding CaproGlu bioadhesives. The transparent biopolymers serve as a UV light guide for the activation of CaproGlu biocomposites, which results in evolution of molecular nitrogen (from diazirine photolysis), simultaneously expanding the covalently cross-linked matrix. Osseointegration additives of hydroxyapatite or Bioglass 45S5 yield a biocomposite matrix with increased stiffness and pullout strength. The structure-property relationships of UV joules dose, pin diameter, and biocomposite additives are assessed with respect to the apparent viscosity, shear modulus, spatiotemporal pin curing, and lap-shear adhesion. Finally, a model system is proposed based on ex vivo investigation with bone tissue for the exploration and optimization of UV-active transparent biopolymer fixation.

Sunlight activated film forming adhesive polymers.

Stimuli-sensitive biomaterials that are activated by light are in need of formulations that are stable under indoor lighting yet can be activated under direct sunlight. Carbene-based bioadhesives are a new generation of film-forming polymers that are stable under indoor lighting yet are rapidly activated with low-energy UVA light, but have never been evaluated under sunlight exposure. Previous investigations have evolved two flexible carbene-based platforms, where aryl-diazirine is grafted on to polyamidoamine dendrimers (PAMAM-NH2; generation-5) or hydrophobic liquid polycaprolactone tetrol to yield G5-Dzx and CaproGlu, respectively. For the first time the activation of G5-Dzx and CaproGlu is investigated by natural sunlight with intensities up to 10 mW·cm-2. Structure-property relationships of bioadhesion are investigated by: (1) joules dose of sunlight; (2) bioadhesive polymer structure; and (3) optical concentrators of magnifying glass and Fresnel lens. Using only natural sunlight, adhesion strength could be tuned from 20 to 150 kPa with crosslinking achieved in under 1 min. The results show that carbene-based polymers are a class of stimuli-sensitive biomaterials that are stable to indoor lighting, yet can be rapidly activated under direct sunlight, which may be useful for topical film forming polymers or as active ingredients in sunscreen formulations.

Minimally invasive electroceutical catheter for endoluminal defect sealing.

Surgical repair of lumen defects is associated with periprocedural morbidity and mortality. Endovascular repair with tissue adhesives may reduce host tissue damage, but current bioadhesive designs do not support minimally invasive deployment. Voltage-activated tissue adhesives offer a new strategy for endoluminal repair. To facilitate the clinical translation of voltage-activated adhesives, an electroceutical patch (ePATCH) paired with a minimally invasive catheter with retractable electrodes (CATRE) is challenged against the repair of in vivo and ex vivo lumen defects. The ePATCH/CATRE platform demonstrates the sealing of lumen defects up to 2 millimeters in diameter on wet tissue substrates. Water-tight seals are flexible and resilient, withstanding over 20,000 physiological relevant stress/strain cycles. No disruption to electrical signals was observed when the ePATCH was electrically activated on the beating heart. The ePATCH/CATRE platform has diverse potential applications ranging from endovascular treatment of pseudo-aneurysms/fistulas to bioelectrodes toward electrophysiological mapping.

Hydrophobic Bioadhesive Composites for Human Motion Detection.

Conductive hydrogels are rapidly rising as sensing materials for bioelectronics applications, but lack mechanical and adhesion strength due to their excess water content. We propose a diazirine-grafted polycaprolactone adhesive (CaproGlu)/carbon nanotubes (CNTs) composite that can provide wet adhesion and strong mechanical properties at the tissue-machine interface. The introduced CNTs not only reinforced the CaproGlu, but also formed electrically conducting pathways. The CaproGlu composites exhibited conductivity of 0.1 S m-1 and a charge storage capacity of 5 µC cm-2. The resulting composites are biocompatible and can be used as strain sensors to detect mechanical deformations.

An organosynthetic dynamic heart model with enhanced biomimicry guided by cardiac diffusion tensor imaging.

The complex motion of the beating heart is accomplished by the spatial arrangement of contracting cardiomyocytes with varying orientation across the transmural layers, which is difficult to imitate in organic or synthetic models. High-fidelity testing of intracardiac devices requires anthropomorphic, dynamic cardiac models that represent this complex motion while maintaining the intricate anatomical structures inside the heart. In this work, we introduce a biorobotic hybrid heart that preserves organic intracardiac structures and mimics cardiac motion by replicating the cardiac myofiber architecture of the left ventricle. The heart model is composed of organic endocardial tissue from a preserved explanted heart with intact intracardiac structures and an active synthetic myocardium that drives the motion of the heart. Inspired by the helical ventricular myocardial band theory, we used diffusion tensor magnetic resonance imaging and tractography of an unraveled organic myocardial band to guide the design of individual soft robotic actuators in a synthetic myocardial band. The active soft tissue mimic was adhered to the organic endocardial tissue in a helical fashion using a custom-designed adhesive to form a flexible, conformable, and watertight organosynthetic interface. The resulting biorobotic hybrid heart simulates the contractile motion of the native heart, compared with in vivo and in silico heart models. In summary, we demonstrate a unique approach fabricating a biomimetic heart model with faithful representation of cardiac motion and endocardial tissue anatomy. These innovations represent important advances toward the unmet need for a high-fidelity in vitro cardiac simulator for preclinical testing of intracardiac devices.

In Vitro Biocompatibility of Diazirine-Grafted Biomaterials.

Photoactivation of aryl-diazirines is an emerging method of rapid, covalent crosslinking under ambient conditions. These attributes make those compounds candidates for grafting onto inert polymer backbones in order to produce stimuli-sensitive biomaterials. However, no risk assessments are available to gauge the toxicity of the leachable components after crosslinking activation. Herein, a stimuli-sensitive biomaterial is formulated from diazirine-grafted polycaprolactone tetrol. Also known as CaproGlu, this biomaterial undergoes UVA-activated crosslinking, with many positive attributes toward bioadhesive applications; hydrophobic, solvent-free, liquid at room temperature, and transitions into a foam biorubber after mild UVA illumination. As a model diazirine-grafted biomaterial, hydrolyzed CaproGlu leachates are evaluated for genotoxicity and skin sensitization, namely, Ames test, direct peptide reactivity, and ARE-Nrf2 luciferase assays. The degradation products of diazirine-mediated crosslinking observe little to no risk of in vitro genotoxicity or skin sensitization.

Photocurable platelet rich plasma bioadhesives.

Closure of wounds with tissue adhesives has many advantages over sutures, but existing synthetic adhesives are toxic and have poor workability. Blood-derived adhesives display complete resorption but have adhesion too weak for reliable wound dressings. We propose a semi-synthetic design that combines the positive attributes of synthetic and blood-derived tissue adhesives. PAMAM-g-diazirine (PDz) is a rapidly gelling bioadhesive miscible in both aqueous and organic solvents. PDz blended with platelet-rich plasma (PRP) forms PDz/PRP composite, a semi-synthetic formulation that combines PDz's wet tissue adhesion with PRP's potent wound healing properties. Light-activated PDz/PRP bioadhesive composite has similar elasticity to soft tissues and behaves as an induced hemostat-an unmet clinical need for rapid wound dressings. PDz/PRP composite applied to in-vivo full-thickness wounds observed a 25% reduction in inflammation, as assessed by the host-cell response.

CaproGlu: Multifunctional tissue adhesive platform.

Driven by the clinical need for a strong tissue adhesive with elastomeric material properties, a departure from legacy crosslinking chemistries was sought as a multipurpose platform for tissue mending. A fresh approach to bonding wet substrates has yielded a synthetic biomaterial that overcomes the drawbacks of free-radical and nature-inspired bioadhesives. A food-grade liquid polycaprolactone grafted with carbene precursors yields CaproGlu. The first-of-its-kind low-viscosity prepolymer is VOC-free and requires no photoinitiators. Grafted diazirine end-groups form carbene diradicals upon low energy UVA (365 nm) activation that immediately crosslink tissue surfaces; no pre-heating or animal-derived components are required. The hydrophobic polymeric environment enables metastable functional groups not possible in formulations requiring solvents or water. Activated diazirine within CaproGlu is uniquely capable of crosslinking all amino acids, even on wet tissue substrates. CaproGlu undergoes rapid liquid-to-biorubber transition within seconds of UVA exposure-features not found in any other bioadhesive. The exceptional shelf stability of CaproGlu allows gamma sterilization with no change in material properties. CaproGlu wet adhesiveness is challenged against current unmet clinical needs: anastomosis of spliced blood vessels, anesthetic muscle patches, and human platelet-mediating coatings. The versatility of CaproGlu enables both organic and inorganic composites for future bioadhesive platforms.

Analysis of the role played by ligand-induced folding of the cocaine-binding aptamer in the photochrome aptamer switch assay.

The Photochrome Aptamer Switch Assay (PHASA) relies on ligand binding by an aptamer to alter the local environment of a stilbene compound covalently attached to the 5' end of the aptamer. We used the PHASA with both structure switching and non-structure switching versions of the cocaine-binding aptamer. We show that the largest change in fluorescence intensity and the lowest concentration limit of detection (CLooD) is obtained using the structure-switching cocaine-binding aptamer. Fluorescence anisotropy measurements were used to quantify the affinity of the conjugated aptamer to cocaine. We also used thermal melt analysis and Nuclear Magnetic Resonance (NMR) spectroscopy to show that the addition of the stilbene to the aptamer increases the melt temperature of the cocaine-bound structure-switching aptamer by (6.4 ± 0.3) °C compared to the unconjugated aptamer while the free form of the structure-switching aptamer-stilbene conjugate remains unfolded.

Structure-Activity Relationships of Voltaglue Organic Blends.

Voltage-activated, one-pot adhesives are an emerging platform with many potential advantages, but require multicomponent grafting of electrochemical donors and acceptors for operation in organic environments. This formulation strategy reduces throughput efficiency, organic solubility, and requires additional purification of the grafted dendrimers. A more advanced strategy is proposed for setting up the donor-acceptor conductive network by exploiting a flexible blending design, providing faster throughput of structure-activity analyses with less synthetic investment. The blend method investigates the ampere-dependent storage modulus and gelation time as a function of both donor and acceptor concentration. This blend strategy allows a rapid evaluation of donor-acceptor parameters involved in voltage-activated adhesive formulations.

A self-adhesive microneedle patch with drug loading capability through swelling effect.

Microneedles (MNs) offer a rapid method of transdermal drug delivery through penetration of the stratum corneum. However, commercial translation has been limited by fabrication techniques unique to each drug. Herein, a broadly applicable platform is explored by drug-loading via swelling effect of a hydrogel MN patch. A range of small molecule hydrophilic, hydrophobic, and biomacromolecule therapeutics demonstrate successful loading and burst release from hydrogel MNs fabricated from methacrylated hyaluronic acid (MeHA). The post-fabrication drug loading process allows MeHA MN patches with drug loadings of 10 µg cm-2. Additional post-fabrication processes are explored with dendrimer bioadhesives that increase work of adhesion, ensuring stable fixation on skin, and allow for additional drug loading strategies.

Light-Activated, Bioadhesive, Poly(2-hydroxyethyl methacrylate) Brush Coatings.

Rapid adhesion between tissue and synthetic materials is relevant to accelerate wound healing and to facilitate the integration of implantable medical devices. Most frequently, tissue adhesives are applied as a gel or a liquid formulation. This manuscript presents an alternative approach to mediate adhesion between synthetic surfaces and tissue. The strategy presented here is based on the modification of the surface of interest with a thin polymer film that can be transformed on-demand, using UV-light as a trigger, from a nonadhesive into a reactive and tissue adhesive state. As a first proof-of-concept, the feasibility of two photoreactive, thin polymer film platforms has been explored. Both of these films, colloquially referred to as polymer brushes, have been prepared using surface-initiated atom transfer radical polymerization (SI-ATRP) of 2-hydroxyethyl methacrylate (HEMA). In the first part of this study, it is shown that direct UV-light irradiation of PHEMA brushes generates tissue-reactive aldehyde groups and facilitates adhesion to meniscus tissue. While this strategy is very straightforward from an experimental point of view, a main drawback is that the generation of the tissue reactive aldehyde groups uses the 250 nm wavelength region of the UV spectrum, which simultaneously leads to extensive photodegradation of the polymer brush. The second part of this report outlines the synthesis of PHEMA brushes that are modified with 4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzoic acid (TFMDA) moieties. UV-irradiation of the TFMDA containing brushes transforms the diazirine moieties into reactive carbenes that can insert into C-H, N-H, and O-H bonds and mediate the formation of covalent bonds between the brush surface and meniscus tissue. The advantage of the TFMDA-modified polymer brushes is that these can be activated with 365 nm wavelength UV light, which does not cause photodegradation of the polymer films. While the work presented in this manuscript has used silicon wafers and fused silica substrates as a first proof-of-concept, the versatility of SI-ATRP should enable the application of this strategy to a broad range of biomedically relevant surfaces.

Tertiary blends of PAMAM/PEG/PEG tissue bioadhesives.

Lack of wet adhesion and biomechanical mismatch at tissue interfaces are the major challenges related to surgical adhesive formulations. Carbene-based bioadhesives seek to address those limitations, due to their ability to covalently bond to wet tissue surfaces. Herein, diazirine-grafted polyamidoamine (PAMAM) dendrimers (PDz) dissolved in various liquid polyethylene glycols (PEG) are reported. Non-aqueous liquid PEG 400 reinforced with high molecular weight PEGs (2 kDa, 6 kDA and 10 kDa) dissolved PDz to form tertiary blends for optimization of viscosity and shear storage modulus. Dynamic photorheometry correlated PEG molar mass and tertiary ratios to adhesion strength, swelling-in-water profile and potential weight loss in aqueous medium. Tertiary blended adhesives demonstrate an improvement in elongation and adhesion strength compared to the binary blends used as control.