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.

pH-Universal Catechol-Amine Chemistry for Versatile Hyaluronic Acid Bioadhesives

Catechol, a major mussel-inspired underwater adhesive moiety, has been used to develop functional adhesive hydrogels for biomedical applications. However, oxidative catechol chemistry for interpolymer crosslinking and adhesion is exclusively effective under alkaline conditions, with limited applications in non-alkaline conditions. To overcome this limitation, pH-universal catechol-amine chemistry to recapitulate naturally occurring biochemical events induced by pH variation in the mussel foot is suggested. Aldehyde moieties are introduced to hyaluronic acid (HA) by partial oxidation, which enables dual-mode catechol tethering to the HA via both stable amide and reactive secondary amine bonds. Because of the presence of additional reactive amine groups, the resultant aldehyde-modified HA conjugated with catechol (AH-CA) is effectively crosslinked in acidic and neutral pH conditions. The AH-CA hydrogel exhibits not only fast gelation via active crosslinking regardless of pH conditions, but also strong adhesion and excellent biocompatibility. The hydrogel enables rapid and robust wound sealing and hemostasis in neutral and alkaline conditions. The hydrogel also mediates effective therapeutic stem cell and drug delivery even in dynamic and harsh environments, such as a motile heart and acidic stomach. Therefore, the AH-CA hydrogel can serve as a versatile biomaterial in a wide range of pH conditions in vivo.

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.

Light-Activated Adhesion and Debonding of Underwater Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSAs) such as sticky notes and labels are a ubiquitous part of modern society. PSAs with a wide range of peel adhesion strength are designed by tailoring the bulk and surface properties of the adhesive. However, designing an adhesive with strong initial adhesion but showing an on-demand decrease in adhesion has been an enduring challenge in the design of PSAs. To address this challenge, we designed alkoxyphenacyl-based polyurethane (APPU) PSAs that show a photoactivated increase and decrease in peel strength. With increasing time of light exposure, the failure mode of our PSAs shifted from cohesive to adhesive failure, providing residue-free removal with up to 83% decrease in peel strength. The APPU-PSAs also adhere to substrates submerged underwater and show a similar photoinduced decrease in adhesion strength.

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.

Oxidation Chemistry of Catechol Utilized in Designing Stimuli-Responsive Adhesives and Antipathogenic Biomaterials

Mussel foot proteins (Mfps) contain a large amount of the catecholic amino acid, DOPA, allowing the marine organism to anchor themselves onto various surfaces in a turbulent and wet environment. Modification of polymers with catechol imparts these materials with a strong, wet adhesive property. The oxidation chemistry and oxidation state of catechol are critical to the design of synthetic adhesives and biomaterials. In this Mini-Review, the effect of catechol oxidation state on adhesion, oxidation-mediated catechol cross-linking, and the generation of reactive oxygen species (ROS) during catechol oxidation are reviewed. Finally, the tuning of catechol oxidation state in designing stimuli-responsive adhesives and the utilization of ROS byproducts for antimicrobial and antiviral applications are reviewed.

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.

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.