Strong and bioactive bioinspired biomaterials, next generation of bone adhesives

The bone adhesive is a clinical requirement for complicated bone fractures always articulated by surgeons. Applying glue is a quick and easy way to fix broken bones. Adhesives, unlike conventional fixation methods such as wires and sutures, improve healing conditions and reduce postoperative pain by creating a complete connection at the fractured joint. Despite many efforts in the field of bone adhesives, the creation of a successful adhesive with robust adhesion and appropriate bioactivity for the treatment of bone fractures is still in its infancy. Because of the resemblance of the body's humid environment to the underwater environment, in the latest decades, researchers have pursued inspiration from nature to develop strong bioactive adhesives for bone tissue. The aim of this review article is to discuss the recent state of the art in bone adhesives with a specific focus on biomimetic adhesives, their action mechanisms, and upcoming perspective. Firstly, the adhesive biomaterials with specific affinity to bone tissue are introduced and their rational design is studied. Consequently, various types of synthetic and natural bioadhesives for bone tissue are comprehensively overviewed. Then, bioinspired-adhesives are described, highlighting relevant structures and examples of biomimetic adhesives mainly made of DOPA and the complex coacervates inspired by proteins secreted in mussel and sandcastle worms, respectively. Finally, this article overviews the challenges of the current bioadhesives and the future research for the improvement of the properties of biomimetic adhesives for use as bone adhesives.

Incorporation of proteins into complex coacervates

Complex coacervates have found a renewed interest in the past few decades in various fields such as food and personal care products, membraneless cellular compartments, the origin of life, and, most notably, as a mode of transport and stabilization of drugs. Here, we describe general methods for characterizing the phase behavior of complex coacervates and quantifying the incorporation of proteins into these phase separated materials.

Single injection of IL-12 coacervate as an effective therapy against B16-F10 melanoma in mice

Melanoma is the deadliest type of skin cancer with one of the fastest increasing incidence rates among solid tumors. The use of checkpoint inhibitors (e.g. αPD-1 antibody) has recently emerged as a viable alternative to conventional modes of therapy. However, increasing evidence points towards the need for a tumor priming step to improve intratumoral immune cell infiltration. IL-12 is an immune-activating cytokine with such potential and was explored in earlier clinical trials as a highly concentrated systemic infusion. This unfortunately led to severe adverse effects. From this perspective, the localization and gradual release of such a potent immunotherapeutic agent in the tumor microenvironment is desired. This manuscript reports the use of a heparin-based complex coacervate to deliver IL-12, in which heparin-binding motifs on IL-12 allow for its effective encapsulation. IL-12-encapsulated complex coacervates significantly improved the bioactivity of IL-12 and provided protection from proteolytic cleavage in-vitro. Indeed, a single injection of IL-12 coacervate significantly inhibits the in-vivo growth of treated and untreated, contralateral tumor growth in a syngeneic B16F10 mouse melanoma model. Furthermore, tumors in mice receiving IL-12 complex coacervate treatment displayed increased infiltration by natural killer (NK) cells and CD8α+ T cells, and a decreased presence of CD4+Foxp3+ regulatory T cells. This study provides proof-of-concept data supporting the use of complex coacervates for sustained delivery of immunostimulatory proteins as an effective therapeutic strategy against disseminated tumors.

Prolonged cell persistence with enhanced multipotency and rapid angiogenesis of hypoxia pre-conditioned stem cells encapsulated in marine-inspired adhesive and immiscible liquid micro-droplets

Stem cell therapies are emerging regenerative treatments for ischemic and chronic diseases. Although high cell retention and prompt angiogenesis are prerequisites to improving efficacy, advancements have not yet been developed. Here, we proposed long-term surviving and angiogenesis-inducing stem cell with high cell retention thanks to fluid immiscible liquid micro-droplets bio-inspired by a glue modality 'complex coacervate' found in the sandcastle worm. Formed by the Coulombic force between polycationic MAP and polyanionic hyaluronic acid, the exploited coacervate micro-droplets enabled the encapsulation of stem cells. The underwater adhesiveness facilitated integrating the encapsulated stem cells onto various surfaces with impressive cell retention after facile injection. Stem cells encapsulated in the coacervate platform formed cell clusters capable of pre-adjusting to hypoxia by expressing hypoxia-inducible factor 1α (HIF-1α), increasing viability and reducing apoptosis under hypoxia and ischemia as well as normoxia. Interestingly, multipotent and angiogenic factors were significantly enhanced by HIF-1α expression. In the in vivo evaluation, the coacervate platform showed impressive angiogenesis with biocompatibility and long-term cell retention capacity with sustainable release as protein factories. Therefore, the proposed MAP-based water-immiscible, injectable, sticky, and bioactive 3D coacervate micro-droplets offers a promising tool for chronic diseases in body fluid-rich environments. STATEMENT OF SIGNIFICANCE: High cell retention, long-term survival, and rapid angiogenesis are prerequisites of successful stem cell therapy. However, no previous advancements have simultaneously satisfied all of these requirements. In this work, we clearly developed a novel, revolutionary stem cell carrier platform with underwater adhesiveness from a mussel-derived glue protein and water immiscibility from a sandcastle-worm-inspired glue modality via 'complex coacervation'. To the best of our knowledge, no report has emerged employing coacervate as a stem cell therapeutic platform. This fluid-immiscible, injectable, sticky, and bioactive 3-dimensional stem cell micro-droplets demonstrated the excellent stem cell retention and viability under hypoxia environments and enhanced multipotent and angiogenic effects with minimal immune response.

Complex coacervation between lysozyme and pectin: Effect of pH, salt, and biopolymer ratio

The complexation between lysozyme and pectin was studied by acidification using zeta potential, turbidity measurements and calorimetry titration. The complexes were analyzed in various NaCl concentrations with different ratios. At ratio 1:1 with 0.01M NaCl, is worth mentioning that the insoluble complexes were formed between pH 2.0 and 7.0, which represents a great range to apply this complex to different food matrices. When the ratio was increased from 1:1 to 3:1, the pH range between the pH_φ1 and pH_φ2 increased even more. When the NaCl concentration was increased from 0.01M to 0.2M, a progressive reduction of turbidity was observed. At 0.4M NaCl, there was total suppression of complex formation at ratio ≤ 3:1. The process of complex coacervate formation occurred in two different steps, presenting favorable enthalpic as well as entropic contributions. The positive entropy change is a strong indication that water molecules have been released from the complex surface, however the positive sign of TΔS suggests that hydrophobic interactions were involved in the interaction between lysozyme and pectin. Microscopy images of the samples revealed that the complexes presented a spheroid-like appearance which may contribute to possible future applications.