Water-curable and biodegradable prepolymers

Biodegradable and Biocompatible Adhesives for the Effective Stabilisation, Repair and Regeneration of Bone

Bone defects and complex fractures present significant challenges for orthopaedic surgeons. Current surgical procedures involve the reconstruction and mechanical stabilisation of complex fractures using metal hardware (i.e., wires, plates and screws). However, these procedures often result in poor healing. An injectable, biocompatible, biodegradable bone adhesive that could glue bone fragments back together would present a highly attractive solution. A bone adhesive that meets the many clinical requirements for such an application has yet to be developed. While synthetic and biological polymer-based adhesives (e.g., cyanoacrylates, PMMA, fibrin, etc.) have been used effectively as bone void fillers, these materials lack biomechanical integrity and demonstrate poor injectability, which limits the clinical effectiveness and potential for minimally invasive delivery. This systematic review summarises conventional approaches and recent developments in the area of bone adhesives for orthopaedic applications. The required properties for successful bone repair adhesives, which include suitable injectability, setting characteristics, mechanical properties, biocompatibility and an ability to promote new bone formation, are highlighted. Finally, the potential to achieve repair of challenging bone voids and fractures as well as the potential of new bioinspired adhesives and the future directions relating to their clinical development are discussed.

Polymeric Tissue Adhesives

Polymeric tissue adhesives provide versatile materials for wound management and are widely used in a variety of medical settings ranging from minor to life-threatening tissue injuries. Compared to the traditional methods of wound closure (i.e., suturing and stapling), they are relatively easy to use, enable rapid application, and introduce minimal tissue damage. Furthermore, they can act as hemostats to control bleeding and provide a tissue-healing environment at the wound site. Despite their numerous current applications, tissue adhesives still face several limitations and unresolved challenges (e.g., weak adhesion strength and poor mechanical properties) that limit their use, leaving ample room for future improvements. Successful development of next-generation adhesives will likely require a holistic understanding of the chemical and physical properties of the tissue-adhesive interface, fundamental mechanisms of tissue adhesion, and requirements for specific clinical applications. In this review, we discuss a set of rational guidelines for design of adhesives, recent progress in the field along with examples of commercially available adhesives and those under development, tissue-specific considerations, and finally potential functions for future adhesives. Advances in tissue adhesives will open new avenues for wound care and potentially provide potent therapeutics for various medical applications.

A Novel Resorbable Composite Material Containing Poly(ester-co-urethane) and Precipitated Calcium Carbonate Spherulites for Bone Augmentation-Development and Preclinical Pilot Trials

Polyurethanes have the potential to impart cell-relevant properties like excellent biocompatibility, high and interconnecting porosity and controlled degradability into biomaterials in a relatively simple way. In this context, a biodegradable composite material made of an isocyanate-terminated co-oligoester prepolymer and precipitated calcium carbonated spherulites (up to 60% w/w) was synthesized and investigated with regard to an application as bone substitute in dental and orthodontic application. After foaming the composite material, a predominantly interconnecting porous structure is obtained, which can be easily machined. The compressive strength of the foamed composites increases with raising calcium carbonate content and decreasing calcium carbonate particle size. When stored in an aqueous medium, there is a decrease in pressure stability of the composite, but this decrease is smaller the higher the proportion of the calcium carbonate component is. In vitro cytocompatibility studies of the foamed composites on MC3T3-E1 pre-osteoblasts revealed an excellent cytocompatibility. The in vitro degradation behaviour of foamed composite is characterised by a continuous loss of mass, which is slower with higher calcium carbonate contents. In a first pre-clinical pilot trial the foamed composite bone substitute material (fcm) was successfully evaluated in a model of vertical augmentation in an established animal model on the calvaria and on the lateral mandible of pigs.

Preparation, Physicochemical Properties, and Hemocompatibility of the Composites Based on Biodegradable Poly(Ether-Ester-Urethane) and Phosphorylcholine-Containing Copolymer

To improve the hemocompatibility of the biodegradable medical poly(ether-ester-urethane) (PEEU), containing uniform-size aliphatic hard segments that was prepared in our lab, a copolymer containing phosphorylcholine (PC) groups was blended with the PEEU. The PC-copolymer of poly(MPC-co-EHMA) (PMEH) was first obtained by copolymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) and 2-ethylhexyl methacrylate (EHMA), and then dissolved in mixed solvent of ethanol/chloroform to obtain a homogeneous solution. The composite films (PMPU) with varying PMEH content were prepared by solvent evaporation method. The physicochemical properties of the composite films with varying PMEH content were researched. The PMPU films exhibited higher thermal stability than that of the pure PEEU film. With the PMEH content increasing from 5 to 20 wt%, the PMPU films also possessed satisfied tensile properties with ultimate stress of 22.9-15.8 MPa and strain at break of 925-820%. The surface and bulk hydrophilicity of the films were improved after incorporation of PMEH. In vitro degradation studies indicated that the degradation rate increased with PMEH content, and it took 12-24 days for composite films to become fragments. The protein adsorption and platelet-rich plasma contact tests were adapted to evaluate the surface hemocompatibility of the composite films. It was found that the amount of adsorbed protein and adherent platelet on the surface decreased significantly, and almost no activated platelets were observed when PMEH content was above 5 wt%, which manifested good surface hemocompatibility. Due to the biodegradability, acceptable tensile properties and good surface hemocompatibility, the composites can be expected to be applied in blood-contacting implant materials.

The Synthesis and Biodegradability of Poly(lactide-random-depsipeptide)-PEGPoly(lactide-random-depsipeptide) ABA-type Triblock Copolymers

Amphiphilic ABA-type triblock copolymers were synthesized to develop a biodegradable anti-adhesive membrane. In this particular synthesis, poly[L-lactide(LA)- co-depsipeptide] (poly[LA- co-(Glc-Leu)]: PLGL) was used as the A segment, and the poly(ethylene glycol)s (PEG)s, Mn 10,000 and Mn 20,500 were used as the B segment. The synthesis of the triblock copolymer (PLGL-PEG-PLGL) was carried out via a ring-opening copolymerization of L-lactide and cyclo(Glc-Leu) in the presence of hydroxytelechelic poly(ethylene glycol) using tin 2-ethylhexanoate as a catalyst. To evaluate the copolymer films as candidates for biodegradable anti-adhesive membranes, physicochemical properties such as degradation on behavior under physiological conditions and water absorption were investigated. The degradation rate of the PLGLPEG-PLGL films varied with changes in the molecular architecture; specifically, the molecular weight of the hydrophilic B segment and the depsipeptide unit content in the A segment were more prominent. The biocompatibility and resorption of the PLGL-PEG-PLGL films were also evaluated. The PLGLPEG-PLGL films were degraded and depleted gradually in vivo without inflammation.

Tissue adhesives for meniscus tear repair: an overview of current advances and prospects for future clinical solutions

Menisci are crucial structures in the knee joint as they play important functions in load transfer, maintaining joint stability and in homeostasis of articular cartilage. Unfortunately, ones of the most frequently occurring knee injuries are meniscal tears. Particularly tears in the avascular zone of the meniscus usually do not heal spontaneously and lead to pain, swelling and locking of the knee joint. Eventually, after a (partial) meniscectomy, they will lead to osteoarthritis. Current treatment modalities to repair tears and by that restore the integrity of the native meniscus still carry their drawbacks and a new robust solution is desired. A strong tissue adhesive could provide such a solution and could potentially improve on sutures, which are the current gold standard. Moreover, a glue could serve as a carrier for biological compounds known to enhance tissue healing. Only few tissue adhesives, e.g., Dermabond(®) and fibrin glue, are already successfully used in clinical practice for other applications, but are not considered suitable for gluing meniscus tissue due to their sub-optimal mechanical properties or toxicity. There is a growing interest and research field focusing on the development of novel polymer-based tissue adhesives, but up to now, there is no material specially designed for the repair of meniscal tears. In this review, we discuss the current clinical gold standard treatment of meniscal tears and present an overview of new developments in this field. Moreover, we discuss the properties of different tissue adhesives for their potential use in meniscal tear repair. Finally, we formulate recommendations regarding the design criteria of material properties and adhesive strength for clinically applicable glues for meniscal tears.

Design strategies and applications of tissue bioadhesives

In the past two decades tissue adhesives and sealants have revolutionized bleeding control and wound healing. This paper focuses on existing tissue adhesive design, their structure, functioning mechanism, and their pros and cons in wound management. It also includes the latest advances in the development of new tissue adhesives as well as the emerging applications in regenerative medicine. We expect that this paper will provide insightful discussion on tissue bioadhesive design and lead to innovations for the development of the next generation of tissue bioadhesives and their related biomedical applications.

Amido-modified polylactide for potential tissue engineering applications

Poly(ester amide) copolymers based on L-lactide (2) and a new depsipeptide (1) were prepared by ring opening polymerization in the presence of Sn(Oct)2 as the catalyst. Variable monomer feed ratios up to 2.3 mol% 1 afforded copolymers containing ester and amido functional groups in the backbone. Lower glass transition temperatures and reduced crystallization kinetics and crystallinity compared to homo-polylactide (PLA) was achieved with low levels of amido incorporation. A reactivity comparison between enchainment of 2 and 1 was determined using in situ infrared spectroscopy. An increase in shear viscosity was observed with the increase of 1 content as determined by rheology studies. Cellular compatibility of the co-polymers was investigated by seeding D1 mouse stem cells onto films and characterizing cell morphology by optical microscopy. Preliminary results indicate that these novel materials exhibit reduced cell attachment compared to PLA and, pending further exploration, may have potential use in biomedical applications.

Oligomer adsorption on dry and wet collagen surfaces

The development of new biodegradable polymeric tissue adhesives has been almost stagnant for the past 10 years, primarily due to the inability to overcome the problem of inadequate adhesion properties. Efforts at the synthesis and modification of chemical structures by incorporating functional groups have proven futile. This study proposes using simulation as a preliminary move to obtain a better understanding of adsorption behavior on biological tissues. It is hoped that this understanding will subsequently serve as a guide for better polymer design and synthesis. Adsorption under both dry and wet conditions were simulated applying classical molecular mechanics and dynamics (MM/MD) because of their relevancy and efficiency. Twelve types of oligomers and a model collagen surface were constructed, followed by structural optimization and equilibration treatments. The COMPASS force field was used to describe the molecular potential energy surfaces. One strand of the oligomer was then located on top of the collagen surface and their interactions at equilibrium, in terms of van der Waals (vdW) and electrostatic energies, monitored over time. For the wet environment a thick water layer was constructed and placed on top of the oligomer and collagen surface. The results showed that the vdW component dominated physical adsorption for all oligomers, under both dry and wet conditions. This implies that interactions of polymers with tissue surfaces are inherently weak. Functional groups on oligomers could improve adhesion via electrostatic interaction. This interaction is, however, screened off in a wet environment, resulting in a reduction in the adsorption energy for all molecules studied. Of all the oligomers studied, poly(glycine) showed the strongest adsorption to collagen in both dry and wet conditions. Therefore, it is proposed to include the functional groups present in poly(glycine) in future tissue adhesive systems.

Modification of islet of langerhans surfaces with immunoprotective poly(ethylene glycol) coatings via interfacial photopolymerization

Poly(ethylene glycol) (PEG) has been used previously to alter immune interactions and systemic clearance of therapeutic proteins. We present herein chemical approaches for the conceptually similar treatment of therapeutic cells and tissues whereby immune and cell adhesive interactions may be reduced or interrupted, in the context of the transplantation of xenogeneic islets of Langerhans for the treatment of insulin-dependent diabetes mellitus. Visible-light-initiated interfacial photopolymerization of multifunctional PEG-based macromers was performed directly upon the surface of rat islets of Langerhans to produce conformal barrier hydrogel coatings with thickness of order 10 microm. The islets continued to be normal in ultrastructure and function as reflected by response to a glucose challenge in vitro.

Interaction force measurements for the design of tissue adhesives

Synthesis of tissue adhesives had been carried out in various laboratories in the past decades but the development is currently stalled. One of the key reasons, it is believed, is that researchers have not fully understood and resolved the role of the functional groups that are responsible for good adhesion to biological tissues. Further progress in synthesis is significantly hindered without this fundamental understanding. With this aim in mind, atomic force microscopy (AFM) has been exploited in this work to study the interactions between functional groups that are common to biological tissues. In this work, the AFM tip and substrates were functionalized and used to measure the non-specific interaction among these common functional groups. The ultimate aim of the study is to calculate the interaction force between a single pair of functional groups. A novel calculation method based on the AFM data and probe geometry is presented. The results provide insights into the strength of the bond between different functional groups and the could serve as a guide in selecting the appropriate functional groups in tissue adhesive synthesis. This method could be further applied to studies involving interfaces of biomedical devices where intermolecular interactions are of concern.

In vivo evaluation of a new sealant material on a rat lung air leak model

The ability of an albumin-based hydrogel sealant (ABHS) to prevent air leakage through the suture line after pulmonary surgery was evaluated by comparison with that of a fibrin glue (FG). As an air-leak model, a rat lung was used in which a standard incision was made and the burst pressure for ABHS and FG was measured. The average burst pressures at time 0 for the FG and ABHS groups were 30.8+/-15.2 and 77.5 +/-19.1 mmHg, respectively. At Day 3, the value of ABHS (76.3 +/- 15.8 mmHg) was still significantly higher (P<0.05) than that of FG (60.0 +/- 21.9 mmHg). At Day 7, no statistical difference was observed between the FG group(71.2 +/- 18.6 mmHg) and the ABHS group(88.8 +/- 11.7 mmHg). Histological examination of the incision at Day 14 revealed that neither sealant was not visible at the incision site, and there was no evidence of adverse tissue reaction. It was concluded that ABHS had good sealing properties and is an alternative to FG for air leakage treatment in pulmonary surgery.

The production of poly-(gamma-glutamic acid) from microorganisms and its various applications

This review article deals with the chemistry and biosynthesis of poly-(gamma-glutamic acid) (gamma-PGA) produced by various strains of Bacillus. Potential applications of gamma-PGA as thickener, cryoprotectant, humectant, drug carrier, biological adhesive, flocculant, or heavy metal absorbent, etc. with biodegradability in the fields of food, cosmetics, medicine and water treatments are also reviewed.

Soft tissue adhesive composed of modified gelatin and polysaccharides

Although fibrin glue has been clinically used as a surgical adhesive, hemostatic agent, and sealant, it has the risk of virus infection because its components, fibrinogen and thrombin, are obtained from human blood. To circumvent this problem, we employed bioabsorbable gelatin and polysaccharides to prepare a safer hemostatic glue. Gelatin was modified with ethylenediamine using water-soluble carbodiimide to introduce additional amino groups into the original gelatin, while dextran and hydroxyethyl-starch were oxidized by sodium periodate to convert 1,2-hydroxyl groups into dialdehyde groups. Upon mixing of the two polymer components in aqueous solution, Schiff base was formed between the amino groups in the modified gelatin and the aldehyde groups in the modified polysaccharides, which thus resulted in intermolecular cross-linking and gel formation. The fastest gel formation took place within 2 s, and its bonding strength to porcine skin was about 225 gf cm(-2) when 20 wt% of an amino-gelatin (55% amino) and 10 wt% of aldehyde-HES (>84% dialdehyde) aqueous solutions were mixed. In contrast, the gelation time and bonding strength of fibrin glue was 5 s and 120 gf cm(-2), respectively.

Solvent-free fabrication of micro-porous polyurethane amide and polyurethane-urea scaffolds for repair and replacement of the knee-joint meniscus

New porous polyurethane urea and polyurethane amide scaffolds for meniscal reconstruction have been developed in a solvent-free process. As soft segments, copolymers of 50/50 L-lactide/epsilon-caprolactone have been used. After terminating the soft segment with diisocyanates, chain extension was performed with adipic acid and water. Reaction between the isocyanate groups and adipic acid or water provides carbon dioxide and results in a porous polymer. Extra hydroxyl-terminated prepolymer was added in order to regulate the amount of carbon dioxide formed in the foaming reaction. Furthermore, salt crystals ranging in size from 150 to 355 microm were added in order to induce macroporosity. The pore size was regulated by addition of surfactant and by the use of ultrasonic waves. The resulting porous polymer scaffolds exhibit good mechanical properties like a high-compression modulus of 150 kPa. Chain extension with adipic acid results in better mechanical properties due to better defined hard segments. This results from the lower nucleophilicity of carboxylic acids compared to water and alcohols. By adjusting the reaction conditions, materials in which macropores are interconnected by micropores can be obtained. On degradation only non-toxic products will be released; importantly, the materials were obtained by a simple, reproducible and solvent-free procedure.

Rapidly curable biological glue composed of gelatin and poly(L-glutamic acid)

The tissue adhesion property of a hydrogel cross-linked with water-soluble carbodiimide (WSC) was investigated and compared with that of the conventional fibrin glue. The biodegradable hydrogel was composed of gelatin and poly(L-glutamic acid) (PLGA). This study focused on the mouse skin bonding by the WSC-formed hydrogel prepared from a low-molecular-weight (Mw) gelatin whose aqueous solution did not spontaneously set to a gel at 25 degrees C, in contrast to the conventional gelatin with high Mw. At polymer concentrations lower than the incipient gelation concentration, the bonding strength of mouse skin by the WSC-cross-linked gelatin-PLGA hydrogel increased with an increase in the concentration of gelatin and PLGA, irrespective of Mw of gelatin. When compared at the highest gelatin concentration which did not cause gelation, the bonding strength of the hydrogel composed of lower Mw gelatin and PLGA was higher than that of higher Mw gelatin hydrogel with or without PLGA or the conventional fibrin glue. The mixed aqueous solution from the gelatin with Mw of 10,000 and PLGA was gelled by use of WSC as rapidly as the fibrin glue. It was concluded that the gelatin-PLGA hydrogel is a safe biological glue with the adhesion property superior to the fibrin glue.

A new biological glue from gelatin and poly (L-glutamic acid)

This study describes the potentiality of hydrogels composed of gelatin and poly(L-glutamic acid) (PLGA) as a biological glue for soft tissues and compares its effectiveness with that of a conventional fibrin glue. Water-soluble carbodiimides (WSC) were used to crosslink the aqueous mixture of gelatin and PLGA. The mixed aqueous solution of gelatin and PLGA set to a hydrogel by use of WSC as rapidly as BOLHEAL fibrin glue. An addition of PLGA to gelatin aqueous solution reduced not only its gelation time but also the WSC concentration necessary for hydrogel formation. The cured hydrogel exhibited firm adhesion to the mouse skin and other soft tissues with a higher bonding strength than BOLHEAL fibrin glue. Cohesive failure in the hydrogel was observed when the gel-tissue bond was broken, in contrast to BOLHEAL fibrin glue. The bonding strength of the gelatin-PLGA hydrogel became higher with the increasing PLGA concentration. The inflammatory reaction around the gelatin-PLGA hydrogel subcutaneously implanted in mice was mild, and the hydrogel was gradually absorbed with time in vivo. A toxicity test demonstrated that the concentration of WSC necessary as a biological glue was low enough not to induce its toxicity.