Translating Regenerative Biomaterials Into Clinical Practice

Engineered biochemical cues of regenerative biomaterials to enhance endogenous stem/progenitor cells (ESPCs)-mediated articular cartilage repair

As a highly specialized shock-absorbing connective tissue, articular cartilage (AC) has very limited self-repair capacity after traumatic injuries, posing a heavy socioeconomic burden. Common clinical therapies for small- to medium-size focal AC defects are well-developed endogenous repair and cell-based strategies, including microfracture, mosaicplasty, autologous chondrocyte implantation (ACI), and matrix-induced ACI (MACI). However, these treatments frequently result in mechanically inferior fibrocartilage, low cost-effectiveness, donor site morbidity, and short-term durability. It prompts an urgent need for innovative approaches to pattern a pro-regenerative microenvironment and yield hyaline-like cartilage with similar biomechanical and biochemical properties as healthy native AC. Acellular regenerative biomaterials can create a favorable local environment for AC repair without causing relevant regulatory and scientific concerns from cell-based treatments. A deeper understanding of the mechanism of endogenous cartilage healing is furthering the (bio)design and application of these scaffolds. Currently, the utilization of regenerative biomaterials to magnify the repairing effect of joint-resident endogenous stem/progenitor cells (ESPCs) presents an evolving improvement for cartilage repair. This review starts by briefly summarizing the current understanding of endogenous AC repair and the vital roles of ESPCs and chemoattractants for cartilage regeneration. Then several intrinsic hurdles for regenerative biomaterials-based AC repair are discussed. The recent advances in novel (bio)design and application regarding regenerative biomaterials with favorable biochemical cues to provide an instructive extracellular microenvironment and to guide the ESPCs (e.g. adhesion, migration, proliferation, differentiation, matrix production, and remodeling) for cartilage repair are summarized. Finally, this review outlines the future directions of engineering the next-generation regenerative biomaterials toward ultimate clinical translation.

A microfluidic-based approach to investigate the inflammatory response of macrophages to pristine and drug-loaded nanostructured hydroxyapatite

The in vitro biological characterization of biomaterials is largely based on static cell cultures. However, for highly reactive biomaterials such as calcium-deficient hydroxyapatite (CDHA), this static environment has limitations. Drastic alterations in the ionic composition of the cell culture medium can negatively affect cell behavior, which can lead to misleading results or data that is difficult to interpret. This challenge could be addressed by a microfluidics-based approach (i.e. on-chip), which offers the opportunity to provide a continuous flow of cell culture medium and a potentially more physiologically relevant microenvironment. The aim of this work was to explore microfluidic technology for its potential to characterize CDHA, particularly in the context of inflammation. Two different CDHA substrates (chemically identical, but varying in microstructure) were integrated on-chip and subsequently evaluated. We demonstrated that the on-chip environment can avoid drastic ionic alterations and increase protein sorption, which was reflected in cell studies with RAW 264.7 macrophages. The cells grown on-chip showed a high cell viability and enhanced proliferation compared to cells maintained under static conditions. Whereas no clear differences in the secretion of tumor necrosis factor alpha (TNF-α) were found, variations in cell morphology suggested a more anti-inflammatory environment on-chip. In the second part of this study, the CDHA substrates were loaded with the drug Trolox. We showed that it is possible to characterize drug release on-chip and moreover demonstrated that Trolox affects the TNF-α secretion and morphology of RAW 264.7 ​cells. Overall, these results highlight the potential of microfluidics to evaluate (bioactive) biomaterials, both in pristine form and when drug-loaded. This is of particular interest for the latter case, as it allows the biological characterization and assessment of drug release to take place under the same dynamic in vitro environment.

Experimental Characterization and Mathematical Modeling of the Adsorption of Proteins and Cells on Biomimetic Hydroxyapatite

Biomaterial development is a long process consisting of multiple stages of design and evaluation within the context of both in vitro and in vivo testing. To streamline this process, mathematical and computational modeling displays potential as a tool for rapid biomaterial characterization, enabling the prediction of optimal physicochemical parameters. In this work, a Langmuir isotherm-based model was used to describe protein and cell adhesion on a biomimetic hydroxyapatite surface, both independently and in a one-way coupled system. The results indicated that increased protein surface coverage leads to improved cell adhesion and spread, with maximal protein coverage occurring within 48 h. In addition, the Langmuir model displayed a good fit with the experimental data. Overall, computational modeling is an exciting avenue that may lead to savings in terms of time and cost during the biomaterial development process.

New insights into the biomimetic design and biomedical applications of bioengineered bone microenvironments

The bone microenvironment is characterized by an intricate interplay between cellular and noncellular components, which controls bone remodeling and repair. Its highly hierarchical architecture and dynamic composition provide a unique microenvironment as source of inspiration for the design of a wide variety of bone tissue engineering strategies. To overcome current limitations associated with the gold standard for the treatment of bone fractures and defects, bioengineered bone microenvironments have the potential to orchestrate the process of bone regeneration in a self-regulated manner. However, successful approaches require a strategic combination of osteogenic, vasculogenic, and immunomodulatory factors through a synergic coordination between bone cells, bone-forming factors, and biomaterials. Herein, we provide an overview of (i) current three-dimensional strategies that mimic the bone microenvironment and (ii) potential applications of bioengineered microenvironments. These strategies range from simple to highly complex, aiming to recreate the architecture and spatial organization of cell-cell, cell-matrix, and cell-soluble factor interactions resembling the in vivo microenvironment. While several bone microenvironment-mimicking strategies with biophysical and biochemical cues have been proposed, approaches that exploit the ability of the cells to self-organize into microenvironments with a high regenerative capacity should become a top priority in the design of strategies toward bone regeneration. These miniaturized bone platforms may recapitulate key characteristics of the bone regenerative process and hold great promise to provide new treatment concepts for the next generation of bone implants.

Mechanical Reinforcement of Polyamide 6 by Cold Hydrostatic Extrusion

This paper presents the effect of severe plastic deformation obtained using the cold hydrostatic extrusion (HE) method on the mechanical and structural properties of polyamide 6 (PA6). As a result of the plastic strain, a significant increase in ultimate tensile strength and tensile modulus were observed. Tensile strength rose by almost 500%, up to the level of 508 MPa, whereas the tensile modulus rose by about 65%. Flexural modulus increase was also observed to 3230 MPa, i.e., by approx. 160%. As a result of high plastic deformation, the structure of the polyamide 6 changed significantly, as evidenced by its fibrous nature as presented in the results of the scanning electron microscopy inspection (SEM). The surface quality of products investigated was tested using profilometry.

Enhanced Biomechanical Properties of Polyvinyl Alcohol-Based Hybrid Scaffolds for Cartilage Tissue Engineering

Articular cartilage damage is a primary feature of osteoarthritis and other inflammatory joint diseases (i.e., rheumatoid arthritis). Repairing articular cartilage is highly challenging due to its avascular/aneural nature and low cellularity. To induce functional neocartilage formation, the tissue substitute must have mechanical properties which can adapt well to the loading conditions of the joint. Among the various biomaterials which may function as cartilage replacements, polyvinyl alcohol (PVA) hydrogels stand out for their high biocompatibility and tunable mechanical features. This review article describes and discusses the enrichment of PVA with natural materials (i.e., collagen, hyaluronic acid, hydroxyapatite, chitosan, alginate, extracellular matrix) ± synthetic additives (i.e., polyacrylic acid, poly-lactic-co-glycolic acid, poly(ethylene glycol) diacrylate, graphene oxide, bioactive glass) to produce cartilage substitutes with enhanced mechanical performance. PVA-based hybrid scaffolds have been investigated mainly by compression, tensile, friction, stress relaxation and creep tests, demonstrating increased stiffness and friction properties, and with cartilage-like viscoelastic behavior. In vitro and in vivo biocompatibility studies revealed positive outcomes but also many gaps yet to be addressed. Thus, recommendations for future research are proposed in order to prompt further progress in the fabrication of PVA-based hybrid scaffolds which increasingly match the biological and mechanical properties of native cartilage.

Functional requirements for polymeric implant materials in head and neck surgery

BACKGROUND

The pharyngeal reconstruction is a challenging aspect after pharyngeal tumor resection. The pharyngeal passage has to be restored to enable oral alimentation and speech rehabilitation. Several techniques like local transposition of skin, mucosa and/or muscle, regional flaps and free vascularized flaps have been developed to reconstruct pharyngeal defects following surgery, in order to restore function and aesthetics. The reconstruction of the pharynx by degradable, multifunctional polymeric materials would be a novel therapeutical option in head and neck surgery.

MATERIALS AND METHODS

Samples of an ethylene-oxide sterilized polymer (diameter 10 mm, 200μm thick) were implanted for the reconstruction of a standardized defect of the gastric wall in rats in a prospective study. The stomach is a model for a "worst case" application site to test the stability of the implant material under extreme chemical, enzymatical, bacterial, and mechanical load.

RESULTS

Fundamental parameters investigated in this animal model were a local tight closure between the polymer and surrounding tissues, histological findings of tissue regeneration and systemic responses to inflammation. A tight anastomosis between the polymer and the adjacent stomach wall was found in all animals after polymer implantation (n = 42). Histologically, a regeneration with glandular epithelium was found in the polymer group. No differences in the systemic responses to inflammation were found between the polymer group (n = 42) and the control group (n = 21) with primary wound closure of the defect of the gastric wall.

CONCLUSIONS

A sufficient stability of the polymeric material is a requirement for the pharyngeal reconstruction with implant materials.

Role of Biomaterials and Controlled Architecture on Tendon/Ligament Repair and Regeneration

Engineering synthetic scaffolds to repair and regenerate ruptured native tendon and ligament (T/L) tissues is a significant engineering challenge due to the need to satisfy both the unique biological and biomechanical properties of these tissues. Long-term clinical outcomes of synthetic scaffolds relying solely on high uniaxial tensile strength are poor with high rates of implant rupture and synovitis. Ideal biomaterials for T/L repair and regeneration need to possess the appropriate biological and biomechanical properties necessary for the successful repair and regeneration of ruptured tendon and ligament tissues.

Biomaterials to Mimic and Heal Connective Tissues

Connective tissue is one of the four major types of animal tissue and plays essential roles throughout the human body. Genetic factors, aging, and trauma all contribute to connective tissue dysfunction and motivate the need for strategies to promote healing and regeneration. The goal here is to link a fundamental understanding of connective tissues and their multiscale properties to better inform the design and translation of novel biomaterials to promote their regeneration. Major clinical problems in adipose tissue, cartilage, dermis, and tendon are discussed that inspire the need to replace native connective tissue with biomaterials. Then, multiscale structure-function relationships in native soft connective tissues that may be used to guide material design are detailed. Several biomaterials strategies to improve healing of these tissues that incorporate biologics and are biologic-free are reviewed. Finally, important guidance documents and standards (ASTM, FDA, and EMA) that are important to consider for translating new biomaterials into clinical practice are highligted.

[Regeneration - A New Therapeutic Dimension in Otorhinolaryngology]

Regeneration as a therapeutic priniciple and regenerative medicine in general are promising new strategies to add new therapeutic dimensions to our current treatment options. Today, reconstructive surgery, drugs and implants such as the cochlear implant can replace the functions of damaged tissues. In contrast, regenerative therapies aim at the replacement of the damaged tissues themselves while at the same time replacing their lost tissue function. In this review article new technologies such as 3D-bioprinting and the application of decellularised tissues as biomaterials are introduced and explained. A summary of current preclinical and clinical regenerative studies in otorhinolaryngology is complementing these basic aspects.

Silk scaffolds in bone tissue engineering: An overview

Bone tissue plays multiple roles in our day-to-day functionality. The frequency of accidental bone damage and disorder is increasing worldwide. Moreover, as the world population continues to grow, the percentage of the elderly population continues to grow, which results in an increased number of bone degenerative diseases. This increased elderly population pushes the need for artificial bone implants that specifically employ biocompatible materials. A vast body of literature is available on the use of silk in bone tissue engineering. The current work presents an overview of this literature from materials and fabrication perspective. As silk is an easy-to-process biopolymer; this allows silk-based biomaterials to be molded into diverse forms and architectures, which further affects the degradability. This makes silk-based scaffolds suitable for treating a variety of bone reconstruction and regeneration objectives. Silk surfaces offer active sites that aid the mineralization and/or bonding of bioactive molecules that facilitate bone regeneration. Silk has also been blended with a variety of polymers and minerals to enhance its advantageous properties or introduce new ones. Several successful works, both in vitro and in vivo, have been reported using silk-based scaffolds to regenerate bone tissues or other parts of the skeletal system such as cartilage and ligament. A growing trend is observed toward the use of mineralized and nanofibrous scaffolds along with the development of technology that allows to control scaffold architecture, its biodegradability and the sustained releasing property of scaffolds. Further development of silk-based scaffolds for bone tissue engineering, taking them up to and beyond the stage of human trials, is hoped to be achieved in the near future through a cross-disciplinary coalition of tissue engineers, material scientists and manufacturing engineers.

STATEMENT OF SIGNIFICANCE

The state-of-art of silk biomaterials in bone tissue engineering, covering their wide applications as cell scaffolding matrices to micro-nano carriers for delivering bone growth factors and therapeutic molecules to diseased or damaged sites to facilitate bone regeneration, is emphasized here. The review rationalizes that the choice of silk protein as a biomaterial is not only because of its natural polymeric nature, mechanical robustness, flexibility and wide range of cell compatibility but also because of its ability to template the growth of hydroxyapatite, the chief inorganic component of bone mineral matrix, resulting in improved osteointegration. The discussion extends to the role of inorganic ions such as Si and Ca as matrix components in combination with silk to influence bone regrowth. The effect of ions or growth factor-loaded vehicle incorporation into regenerative matrix, nanotopography is also considered.

Synthetic sutures: Clinical evaluation and future developments

Today's sutures are the result of a 4000-year innovation process with regard to their materials and manufacturing techniques, yet little has been done to enhance the therapeutic value of the suture itself. In this review, we explore the historical development, regulatory database and clinical literature of sutures to gain a fuller picture of suture advances to date. First, we examine historical shifts in suture manufacturing companies and review suture regulatory databases to understand the forces driving suture development. Second, we gather the existing clinical evidence of suture efficacy from reviewing the clinical literature and the Food and Drug Administration database in order to identify to what extent sutures have been clinically evaluated and the key clinical areas that would benefit from improved suture materials. Finally, we apply tissue engineering and regenerative medicine design hypotheses to suture materials to identify routes by which bioactive sutures can be designed and passed through regulatory hurdles, to improve surgical outcomes. Our review of the clinical literature revealed that many of the sutures currently in use have been available for decades, yet have never been clinically evaluated. Since suture design and development is industry driven, incremental modifications have allowed for a steady outflow of products while maintaining a safe regulatory position and limiting costs. Until recently, there has been little academic interest in suture development, however the rise of regenerative medicine strategies is shifting the suture paradigm from an inert material, which mechanically approximates tissue, to a bioactive material, which also actively promotes cell-directed repair and a positive healing response. These materials hold significant therapeutic potential, but could be associated with an increased regulatory burden, cost, and clinical evaluation compared with current devices.

Regenerative pharmacology: recent developments and future perspectives

This review focuses on the current status of research that utilizes the application of pharmacological sciences to accelerate, optimize and characterize the development, maturation and function of bioengineered and regenerating tissues. These regenerative pharmacologic approaches have been applied to diseases of the urogenital tract, the heart, the brain, the musculoskeletal system and diabetes. Approaches have included the use of growth factors (such as VEGF and chemokines (stromal-derived factor – CXCL12) to mobilize cell to the sights of tissue loss or damage. The promise of this approach is to bypass the lengthy and expensive processes of cell isolation and implant fabrication to stimulate the body to heal itself with its own tissue regenerative pathways.

In vitro mesenchymal stem cell response to a CO2 laser modified polymeric material

With an ageing world population it is becoming significantly apparent that there is a need to produce implants and platforms to manipulate stem cell growth on a pharmaceutical scale. This is needed to meet the socio-economic demands of many countries worldwide. This paper details one of the first ever studies in to the manipulation of stem cell growth on CO2 laser surface treated nylon 6,6 highlighting its potential as an inexpensive platform to manipulate stem cell growth on a pharmaceutical scale. Through CO2 laser surface treatment discrete changes to the surfaces were made. That is, the surface roughness of the nylon 6,6 was increased by up to 4.3μm, the contact angle was modulated by up to 5° and the surface oxygen content increased by up to 1atom %. Following mesenchymal stem cell growth on the laser treated samples, it was identified that CO2 laser surface treatment gave rise to an enhanced response with an increase in viable cell count of up to 60,000cells/ml when compared to the as-received sample. The effect of surface parameters modified by the CO2 laser surface treatment on the mesenchymal stem cell response is also discussed along with potential trends that could be identified to govern the mesenchymal stem cell response.