Adhesive curing through low-voltage activation

Electronically optimized diazirine-based polymer crosslinkers

Electronically optimized bis-diazirine crosslinkers allow aliphatic polymers to be crosslinked with up to 10-fold improved efficacy, relative to earlier designs. Activation is achieved using modest temperatures or through UV or visible light.

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.

Structure-function relationships in aryl diazirines reveal optimal design features to maximize C-H insertion

Diazirine reagents allow for the ready generation of carbenes upon photochemical, thermal, or electrical stimulation. Because carbenes formed in this way can undergo rapid insertion into any nearby C-H, O-H or N-H bond, molecules that encode diazirine functions have emerged as privileged tools in applications ranging from biological target identification and proteomics through to polymer crosslinking and adhesion. Here we use a combination of experimental and computational methods to complete the first comprehensive survey of diazirine structure-function relationships, with a particular focus on thermal activation methods. We reveal a striking ability to vary the activation energy and activation temperature of aryl diazirines through the rational manipulation of electronic properties. Significantly, we show that electron-rich diazirines have greatly enhanced efficacy toward C-H insertion, under both thermal and photochemical activation conditions. We expect these results to lead to significant improvements in diazirine-based chemical probes and polymer crosslinkers.

Rational engineering and applications of functional bioadhesives in biomedical engineering

For the past decades, several bioadhesives have been developed to replace conventional wound closure medical tools such as sutures, staples, and clips. The bioadhesives are easy to use and can minimize tissue damage. They are designed to provide strong adhesion with stable mechanical support on tissue surfaces. However, this monofunctionality of the bioadhesives hinders their practical applications. In particular, a bioadhesive can lose its intended function under harsh tissue environments or delay tissue regeneration during wound healing. Based on several natural and synthetic biomaterials, functional bioadhesives have been developed to overcome the aforementioned limitations. The functional bioadhesives are designed to have specific characteristics such as antimicrobial, cell infiltrative, stimuli-responsive, electrically conductive, and self-healing to ensure stability under harsh tissue conditions, facilitate tissue regeneration, and effectively monitor biosignals. Herein, we thoroughly review the functional bioadhesives from their fundamental background to recent progress with their practical applications for the enhancement of tissue healing and effective biosignal sensing. Furthermore, the future perspectives on the applications of functional bioadhesives and current challenges in their commercialization are also discussed.

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.

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-Dz_x and CaproGlu, respectively. For the first time the activation of G5-Dz_x 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.

Fabrication of PAMAM antimicrobial monolayer via UV induced grafting on the surface of polyethylene terephthalate

Poly (amidoamine) (PAMAM) with 3rd and 5th generation was covalently grafted as the contact active biocidal agent on the surface of polyethylene terephthalate (PET) with the help of UV induced carbene chemistry (PAMAM-g-PET). The graft density and the surface roughness were controlled by turning UV irradiation time and the PAMAM generation. The PAMAM graft monolayer was characterized via the contact angle, XPS, nanoIR, SEM and AFM. The antibacterial ability of PAMAM-g-PET was evaluated ex-vivo with the help of laser scanning confocal microscope (CLSM), and the results indicated that the decorated PET was able to kill both S. aureus and E. coli in the aqueous environment. Increasing the surface graft concentration and using the dendrimer with higher generation enhanced the lethality towards the bacterial. The decorated film was still able to kill the contact bacterial strain when the cationic primary amine groups were shielded by acetyl chloride, however, the bacterial in the suspension was hardly affected in this case. The un-selectivity and instantaneity of carbene chemistry endowed this grafting strategy the potential to be extended to other organic substances.

Flexible polyfluorinated bis-diazirines as molecular adhesives

Motivated by a desire to develop flexible covalent adhesives that afford some of the same malleability in the adhesive layer as traditional polymer-based adhesives, we designed and synthesized two flexible, highly fluorinated bis-diazirines. Both molecules are shown to function as effective crosslinkers for polymer materials, and to act as strong adhesives when painted between two polymer objects of low surface energy, prior to thermal activation. Data obtained from lap-shear experiments suggests that greater molecular flexibility is correlated with improved mechanical compliance in the adhesive layer.

Design of Multifunctional Composites: New Strategy to Save Energy and Improve Mechanical Performance

In this paper, an alternative curing strategy, based on the application of an electric field, is proposed to harden nano-filled multifunctional resins. The resin is obtained through the dispersion of carbon nanotubes, which act as nanometric heater elements in the epoxy matrix. The electro-curing is activated by applying an external electric voltage, which allows tunable cross-linking within the epoxy matrix entrapped between the nanotubes. The electro-curing method allows reaching higher curing degrees with respect to the conventional ones and, consequently, higher glass transition temperatures. This is a direct consequence of the fact that the curing reactions start directly in the regions at the interphase between carbon nanotubes, acting as heater nano-filaments, and the polymeric matrix. The proposed method is able to give composites better properties, making the curing process fast and energy-saving.

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.

Multifunctional Biomedical Adhesives

Currently available biomedical adhesives are mainly engineered to have one function (i.e., providing mechanical support for the repaired tissue). To improve the performance of existing bioadhesives and broaden their applications in medicine, numerous multifunctional bioadhesives are reported in the literature. These adhesives can be categorized as passive or active by design. Passive multifunctional bioadhesives contain inherent compositions and structural designs that can carry out additional functions without added external influences. These adhesives exhibit new functionalities such as antimicrobial properties, self-healing abilities, the ability to promote cellular ingrowth, and the ability to be reshaped. Conversely, active multifunctional bioadhesives respond to environmental changes (e.g., pH, temperature, electricity, light, and biomolecule concentration), which initiate a change in the adhesive to release encapsulated drugs or to activate or deactivate the bioadhesive for interfacial binding. This review article highlights recent advances in multifunctional bioadhesives.

Self curing and voltage activated catechol adhesives

Catechol adhesives are designed for curing with a low voltage signal—no oxidants or metal chelators are required.

Voltaglue Bioadhesives Energized with Interdigitated 3D-Graphene Electrodes

Soft tissue fixation of implant and bioelectrodes relies on mechanical means (e.g., sutures, staples, and screws), with associated complications of tissue perforation, scarring, and interfacial stress concentrations. Adhesive bioelectrodes address these shortcomings with voltage cured carbene-based bioadhesives, locally energized through graphene interdigitated electrodes. Electrorheometry and adhesion structure activity relationships are explored with respect to voltage and electrolyte on bioelectrodes synthesized from graphene 3D-printed onto resorbable polyester substrates. Adhesive leachates effects on in vitro metabolism and human-derived platelet-rich plasma response serves to qualitatively assess biological response. The voltage activated bioadhesives are found to have gelation times of 60 s or less with maximum shear storage modulus (G') of 3 kPa. Shear modulus mimics reported values for human soft tissues (0.1-10 kPa). The maximum adhesion strength achieved for the ≈50 mg bioelectrode films is 170 g cm^-2 (17 kPa), which exceeds the force required for tethering of electrodes on dynamic soft tissues. The method provides the groundwork for implantable bio/electrodes that may be permanently incorporated into soft tissues, vis-à-vis graphene backscattering wireless electronics since all components are bioresorbable.

On-Demand Bioadhesive Dendrimers with Reduced Cytotoxicity

Tissue adhesives based on polyamidoamine (PAMAM) dendrimer, grafted with UV-sensitive aryldiazirine (PAMAM-g-diazirine) are promising new candidates for light active adhesion on soft tissues. Diazirine carbene precursors form interfacial and intermolecular covalent crosslinks with tissues after UV light activation that requires no premixing or inclusion of free radical initiators. However, primary amines on the PAMAM dendrimer surface present a potential risk due to their cytotoxic and immunological effects. PAMAM-g-diazirine formulations with cationic pendant amines converted into neutral amide groups were evaluated. In vitro toxicity is reduced by an order of magnitude upon amine capping while retaining bioadhesive properties. The in vivo immunological response to PAMAM-g-diazirine formulations was found to be optimal in comparison to standard poly(lactic-co-glycolic acid) (PLGA) thin films.

Magnetically activated adhesives: towards on-demand magnetic triggering of selected polymerisation reactions

We demonstrate a new strategy to inhibit and trigger polymerisation of an adhesive formulation, utilising colloidal core@shell CoFe₂O₄@MnO₂ magnetic nanoparticles.

On-demand initiation of chemical reactions is becoming increasingly popular in many areas. The use of a magnetic field to trigger reactions is an intriguing concept, with vast potential in both research and industrial settings, though it remains a challenge as yet unsolved. Here we report the first example of on-demand magnetic activation of a polymerisation process using an anaerobic adhesive formulation as an example of this new approach toward triggering polymerisation reactions using an external magnetic field. Our strategy involves the use of a colloidal system comprising functional methacrylate ester monomers, peroxide and Cu^II-salt as polymerisation initiators and magnetic nanoparticles coated with an oxidising shell. This unique combination prevents reduction of the reactive transition metal (Cu^II) ion by the metal substrates (steel or aluminium) to be joined – hence inhibiting the redox radical initiated cationic polymerisation reaction and efficiently preventing adhesion. The polymerisation and corresponding adhesion process can be triggered by removal of the functional magnetic particles using a permanent external magnet either prior to formulation application or at the joint to be adhered, enabling the polymerisation to proceed through Cu^II-mediated reduction. This new approach enables on-demand magnetically-triggered reaction initiation and holds potential for a range of useful applications in chemistry, materials science and relevant industrial manufacturing.

Self-assembled photoadditives in polyester films allow stop and go chemical release

Near-infrared (NIR) triggered chemical delivery allows on-demand release with the advantage of external tissue stimulation. Bioresorbable polyester poly-l-lactic acid (PLLA) was compounded with photoadditives of neat zinc oxide (ZnO) nanoparticles and 980→365nm LiYF₄:Tm³⁺, Yb³⁺ upconverting nanoparticles (UCNP). Subsequently, neat ZnO and UCNP blended PLLA films of sub-50μm thickness were knife casted with a hydrophobic small molecule drug mimic, fluorescein diacetate. The PLLA films displayed a 500 times increase in fluorescein diacetate release from the 50mW NIR irradiated PLLA/photoadditive film compared to non-irradiated PLLA control films. Larger ratios of UCNP/neat ZnO increased photocatalysis efficiency at low NIR duty cycles. The synergistic increase results from the self-assembled photoadditives of neat zinc oxide and upconverting nanoparticles (UCNPs), as seen in transmission electron microscopy. Colloidal ZnO, which does not self-assemble with UCNPs, had less than half the release kinetics of the self-assembled PLLA films under similar conditions, advocating Förster resonance energy transfer as the mechanism responsible for the synergistic increase. Alternative to intensity modulation, pulse width modulation (duty cycles from 0.1 to 1) of the low intensity 50mW NIR laser diode allowed tailorable release rates from 0.01 to 1.4% per day. With the low intensity NIR activation, tailorable release rates, and favorable biocompatibility of the constituents, implanted PLLA photoadditive thin films could allow feedback mediated chemical delivery.

STATEMENT OF SIGNIFICANCE

Upconverting nanoparticles and zinc oxide nanorods were found to spontaneously self-assemble into submicron particles in organic solvents. Exposure of the submicron particles to near-infrared light allows stop and go chemical release from biocompatible polymers. Sample preparation of thin films is done with ease through physical mixing of the photoadditives followed by air-dried knife casting. A colloidal ZnO variant that does not self-assemble with upconverting nanoparticles had slower chemical release, suggesting that synergistic chemical release is brought upon by highly efficient energy transfer mechanisms when the nanoparticles are less than 10nm apart. Never before seen composite particles of UCNP/ZnO are displayed, which shows the close interaction of the photoadditives within the polymer matrix.

Photo-responsive bio-inspired adhesives: facile control of adhesion strength via a photocleavable crosslinker

A photo-responsive bio-inspired terpolymer adhesives consisting of a zwitterionic polymer, catechol moiety, and nitrobenzyl crosslinker was synthesized for convenient control of adhesion strength under UV irradiation.