Subcritical CO2 sintering of microspheres of different polymeric materials to fabricate scaffolds for tissue engineering

From the microspheres to scaffolds: advances in polymer microsphere scaffolds for bone regeneration applications

The treatment and repair of bone tissue damage and loss due to infection, tumours, and trauma are major challenges in clinical practice. Artificial bone scaffolds offer a safer, simpler, and more feasible alternative to bone transplantation, serving to fill bone defects and promote bone tissue regeneration. Ideally, these scaffolds should possess osteoconductive, osteoinductive, and osseointegrative properties. However, the current first-generation implants, represented by titanium alloys, have shown poor bone-implant integration performance and cannot meet the requirements for bone tissue repair. This has led to increased research on second and third generation artificial bone scaffolds, which focus on loading bioactive molecules and cells. Polymer microspheres, known for their high specific surface areas at the micro- and nanoscale, exhibit excellent cell and drug delivery behaviours. Additionally, with their unique rigid structure, microsphere scaffolds can be constructed using methods such as thermal sintering, injection, and microsphere encapsulation. These scaffolds not only ensure the excellent cell drug loading performance of microspheres but also exhibit spatial modulation behaviour, aiding in bone repair within a three-dimensional network structure. This article provides a summary and discussion of the use of polymer microsphere scaffolds for bone repair, focusing on the mechanisms of bone tissue repair and the current status of clinical bone grafts, aimed at advancing research in bone repair.

Aerogel-Based Materials in Bone and Cartilage Tissue Engineering-A Review with Future Implications

Aerogels are fascinating solid materials known for their highly porous nanostructure and exceptional physical, chemical, and mechanical properties. They show great promise in various technological and biomedical applications, including tissue engineering, and bone and cartilage substitution. To evaluate the bioactivity of bone substitutes, researchers typically conduct in vitro tests using simulated body fluids and specific cell lines, while in vivo testing involves the study of materials in different animal species. In this context, our primary focus is to investigate the applications of different types of aerogels, considering their specific materials, microstructure, and porosity in the field of bone and cartilage tissue engineering. From clinically approved materials to experimental aerogels, we present a comprehensive list and summary of various aerogel building blocks and their biological activities. Additionally, we explore how the complexity of aerogel scaffolds influences their in vivo performance, ranging from simple single-component or hybrid aerogels to more intricate and organized structures. We also discuss commonly used formulation and drying methods in aerogel chemistry, including molding, freeze casting, supercritical foaming, freeze drying, subcritical, and supercritical drying techniques. These techniques play a crucial role in shaping aerogels for specific applications. Alongside the progress made, we acknowledge the challenges ahead and assess the near and far future of aerogel-based hard tissue engineering materials, as well as their potential connection with emerging healing techniques.

Calcium Phosphate Silicate Microspheres with Soybean Lecithin as a Sustained-Release Bone Morphogenetic Protein-Delivery System for Bone Tissue Regeneration

Bone morphogenetic protein (BMP) is a growth factor that effectively promotes osteogenesis. Microsphere-based drug-delivery systems can facilitate an increase in the local concentration of BMP, thus promoting bone formation. In this study, calcium phosphate silicate (CPS) microspheres were used as drug-loading systems for BMP. Three groups─CPS, CPS + BMP, and CPS + BMP + soy lecithin (SL)─were set up, where SL was used to prolong the osteogenic effect of the microsphere system. Bone marrow mesenchymal stem cells and femoral defects in rats were used to compare the osteogenic ability of the three groups. The results indicated that CPS microspheres were good carriers of BMP, facilitating a smoother release into the cells and tissues. SL loading improved the loading rate of BMP, which promoted the osteogenic effect of the microspheres with BMP. We propose CPS microspheres as potential drug-delivery systems that can be effectively used in the treatment of bone defects.

Polymer-coated microparticle scaffolds engineered for potential use in musculoskeletal tissue regeneration

Biomaterials constructed exclusively of sintered microspheres have great potential in tissue engineering scaffold applications, offering the ability to create shape-specific scaffolds with precise controlled release yet to be matched by traditional additive manufacturing methods. The problem is that these microsphere-based scaffolds are limited in their stiffness for applications such as bone regeneration. Our vision to solve this problem was borne from a hierarchical structure perspective, focusing on the individual unit of the structure: the microsphere itself. In a core-shell approach, we envisioned a stiff core to create a stiff microsphere unit, with a polymeric shell that would enable sintering to the other microsphere units. Therefore, the current study provided a comparison of macroscopic biomaterials built on either polymer microspheres or polymer-coated hard glass microspheres. Identical polycaprolactone (PCL) polymer solutions were used to fabricate microspheres and as a thin coating on soda lime glass microspheres (hard phase). The materials were characterized as loose particles and as scaffolds via scanning electron microscopy, thermogravimetry, differential scanning calorimetry, Raman spectroscopy, mechanical testing, and a live/dead analysis with human umbilical cord-derived Wharton's jelly cells. The elastic modulus of the scaffolds with the thinly coated hard phase was about five times higher with glass microspheres (up to about 25 MPa) than pure polymer microspheres, while retaining the structure, cell adhesion, and chemical properties of the PCL polymer. This proof-of-concept study demonstrated the ability to achieve at least a five-fold increase in macroscopic stiffness via altering the core microsphere units with a core-shell approach.

Solvent-Free Approaches for the Processing of Scaffolds in Regenerative Medicine

The regenerative medicine field is seeking novel strategies for the production of synthetic scaffolds that are able to promote the in vivo regeneration of a fully functional tissue. The choices of the scaffold formulation and the manufacturing method are crucial to determine the rate of success of the graft for the intended tissue regeneration process. On one hand, the incorporation of bioactive compounds such as growth factors and drugs in the scaffolds can efficiently guide and promote the spreading, differentiation, growth, and proliferation of cells as well as alleviate post-surgical complications such as foreign body responses and infections. On the other hand, the manufacturing method will determine the feasible morphological properties of the scaffolds and, in certain cases, it can compromise their biocompatibility. In the case of medicated scaffolds, the manufacturing method has also a key effect in the incorporation yield and retained activity of the loaded bioactive agents. In this work, solvent-free methods for scaffolds production, i.e., technological approaches leading to the processing of the porous material with no use of solvents, are presented as advantageous solutions for the processing of medicated scaffolds in terms of efficiency and versatility. The principles of these solvent-free technologies (melt molding, 3D printing by fused deposition modeling, sintering of solid microspheres, gas foaming, and compressed CO2 and supercritical CO2-assisted foaming), a critical discussion of advantages and limitations, as well as selected examples for regenerative medicine purposes are herein presented.

Modular Strategies to Build Cell-Free and Cell-Laden Scaffolds towards Bioengineered Tissues and Organs

Engineering three-dimensional (3D) scaffolds for functional tissue and organ regeneration is a major challenge of the tissue engineering (TE) community. Great progress has been made in developing scaffolds to support cells in 3D, and to date, several implantable scaffolds are available for treating damaged and dysfunctional tissues, such as bone, osteochondral, cardiac and nerve. However, recapitulating the complex extracellular matrix (ECM) functions of native tissues is far from being achieved in synthetic scaffolds. Modular TE is an intriguing approach that aims to design and fabricate ECM-mimicking scaffolds by the bottom-up assembly of building blocks with specific composition, morphology and structural properties. This review provides an overview of the main strategies to build synthetic TE scaffolds through bioactive modules assembly and classifies them into two distinct schemes based on microparticles (µPs) or patterned layers. The µPs-based processes section starts describing novel techniques for creating polymeric µPs with desired composition, morphology, size and shape. Later, the discussion focuses on µPs-based scaffolds design principles and processes. In particular, starting from random µPs assembly, we will move to advanced µPs structuring processes, focusing our attention on technological and engineering aspects related to cell-free and cell-laden strategies. The second part of this review article illustrates layer-by-layer modular scaffolds fabrication based on discontinuous, where layers' fabrication and assembly are split, and continuous processes.

Effect of Icariin on Engineered 3D-Printed Porous Scaffolds for Cartilage Repair

In recent times, cartilage defects have been the most common athletic injuries, often leading to dreadful consequences such as osteoarthritis, pain, joint deformities, and other symptoms. It is also evident that damage to articular cartilage is often difficult to recover or self-heal because of poor vascular, nervous, and lymphatic supplies. Moreover, cartilage cells have poor regeneration ability and high maturity. Inspired by these facts and the rapid advances in the field of tissue engineering (TE), we fabricated highly porous three-dimensional (3D) scaffold architectures based on cell-responsive polymeric inks, i.e., sodium alginate and gelatin (SA-Gel, 1:3 ratio), by a novel 3D printing method. Moreover, the effect of various processing parameters was systematically investigated. The printed scaffolds of polymer composites gels with excellent transparency, moderate viscosity, and excellent fluid properties showed good surface morphology, better thermal stability and swelling effect, and unique interconnected porous architectures at the optimized operating parameters. In vitro cell proliferation experiments of these cytocompatible scaffolds showed the excellent adhesion rate and growth behavior of chondrocytes. In addition, the porous architectures facilitated the efficient distribution of cells with only a few remaining on the surface, which was confirmed by confocal laser scanning microscopic (CLSM) observations. Icariin (ICA) addition at a concentration of 10 μg/mL further significantly enhanced the proliferation of chondrocytes. We envision that these cell-responsive polymeric inks in the presence of growth regulators like ICA may have potential in engineering complex tissue constructs toward diverse applications in TE.

CO2-expanded nanofiber scaffolds maintain activity of encapsulated bioactive materials and promote cellular infiltration and positive host response

Traditional electrospun nanofiber membranes were incapable of promoting cellular infiltration due to its intrinsic property (e.g., dense structure and small pore size) limiting their use in tissue regeneration. Herein, we report a simple and novel approach for expanding traditional nanofiber membranes from two-dimensional to three-dimensional (3D) with controlled thickness and porosity via depressurization of subcritical CO2 fluid. The expanded 3D nanofiber scaffolds formed layered structures and simultaneously maintained the aligned nanotopographic cues. The 3D scaffolds also retained the fluorescent intensity of encapsulated coumarin 6 and the antibacterial activity of encapsulated antimicrobial peptide LL-37. In addition, the expanded 3D nanofiber scaffolds with arrayed holes can significantly promote cellular infiltration and neotissue formation after subcutaneous implantation compared to traditional nanofiber membranes. Such scaffolds also significantly increased the blood vessel formation and the ratio of M2/M1 macrophages after subcutaneous implantation for 2 and 4 weeks compared to traditional nanofiber membranes. Together, the presented method holds great potential in the fabrication of functional 3D nanofiber scaffolds for various applications including engineering 3D in vitro tissue models, antimicrobial wound dressing, and repairing/regenerating tissues in vivo.

STATEMENT OF SIGNIFICANCE

Electrospun nanofibers have been widely used in regenerative medicine due to its biomimicry property. However, most of studies are limited to the use of 2D electrospun nanofiber membranes. To the best of our knowledge, this article is the first instance of the transformation of traditional electrospun nanofiber membranes from 2D to 3D via depressurization of subcritical CO2 fluid. This method eliminates many issues associated with previous approaches such as necessitating the use of aqueous solutions and chemical reactions, multiple-step process, loss of the activity of encapsulated biological molecules, and unable to expand electrospun nanofiber mats made of hydrophilic polymers. Results indicate that these CO2 expanded nanofiber scaffolds can maintain the activity of encapsulated biological molecules. Further, the CO2 expanded nanofiber scaffolds with arrayed holes can greatly promote cellular infiltration, neovascularization, and positive host response after subcutaneous implantation in rats. The current work is the first study elucidating such a simple and novel strategy for fabrication of 3D nanofiber scaffolds.

Microsphere-Based Scaffolds in Regenerative Engineering

Microspheres have long been used in drug delivery applications because of their controlled release capabilities. They have increasingly served as the fundamental building block for fabricating scaffolds for regenerative engineering because of their ability to provide a porous network, offer high-resolution control over spatial organization, and deliver growth factors/drugs and/or nanophase materials. Because they provide physicochemical gradients via spatiotemporal release of bioactive factors and nanophase ceramics, microspheres are a desirable tool for engineering complex tissues and biological interfaces. In this review we describe various methods for microsphere fabrication and sintering, and elucidate how these methods influence both micro- and macroscopic scaffold properties, with a special focus on the nature of sintering. Furthermore, we review key applications of microsphere-based scaffolds in regenerating various tissues. We hope to inspire researchers to join a growing community of investigators using microspheres as tissue engineering scaffolds so that their full potential in regenerative engineering may be realized.

Poly (lactic acid)-based biomaterials for orthopaedic regenerative engineering

Regenerative engineering converges tissue engineering, advanced materials science, stem cell science, and developmental biology to regenerate complex tissues such as whole limbs. Regenerative engineering scaffolds provide mechanical support and nanoscale control over architecture, topography, and biochemical cues to influence cellular outcome. In this regard, poly (lactic acid) (PLA)-based biomaterials may be considered as a gold standard for many orthopaedic regenerative engineering applications because of their versatility in fabrication, biodegradability, and compatibility with biomolecules and cells. Here we discuss recent developments in PLA-based biomaterials with respect to processability and current applications in the clinical and research settings for bone, ligament, meniscus, and cartilage regeneration.

Microsphere-based scaffolds encapsulating tricalcium phosphate and hydroxyapatite for bone regeneration

Bioceramic mixtures of tricalcium phosphate (TCP) and hydroxyapatite (HAp) are widely used for bone regeneration because of their excellent cytocompatibility, osteoconduction, and osteoinduction. Therefore, we hypothesized that incorporation of a mixture of TCP and HAp in microsphere-based scaffolds would enhance osteogenesis of rat bone marrow stromal cells (rBMSCs) compared to a positive control of scaffolds with encapsulated bone-morphogenic protein-2 (BMP-2). Poly(D,L-lactic-co-glycolic acid) (PLGA) microsphere-based scaffolds encapsulating TCP and HAp mixtures in two different ratios (7:3 and 1:1) were fabricated with the same net ceramic content (30 wt%) to evaluate how incorporation of these ceramic mixtures would affect the osteogenesis in rBMSCs. Encapsulation of TCP/HAp mixtures impacted microsphere morphologies and the compressive moduli of the scaffolds. Additionally, TCP/HAp mixtures enhanced the end-point secretion of extracellular matrix components relevant to bone tissue compared to the "blank" (PLGA-only) microsphere-based scaffolds as evidenced by the biochemical, gene expression, histology, and immunohistochemical characterization. Moreover, the TCP/HAp mixture groups even surpassed the BMP-2 positive control group in some instances in terms of matrix synthesis and gene expression. Lastly, gene expression data suggested that the rBMSCs responded differently to different TCP/HAp ratios presented to them. Altogether, it can be concluded that TCP/HAp mixtures stimulated the differentiation of rBMSCs toward an osteoblastic phenotype, and therefore may be beneficial in gradient microsphere-based scaffolds for osteochondral regeneration.

Vasculogenic potential evaluation of bottom-up, PCL scaffolds guiding early angiogenesis in tissue regeneration

Vascularization is a key factor in the successful integration of tissue engineered (TE) grafts inside the host body. Biological functions of the newly formed tissue depend, in fact, on a reliable and fast spread of the vascular network inside the scaffold. In this study, we propose a technique for evaluating vascularization in TE constructs assembled by a bottom-up approach. The rational, ordered assembly of building blocks (BBs) into a 3D scaffold can improve vessel penetration, and-unlike most current technologies-is compatible with the insertion of different elements that can be designed independently (e.g. structural units, growth factor depots etc.). Poly(ε-caprolactone) scaffolds composed of orderly and randomly assembled sintered microspheres were used to assess the degree of vascularization in a pilot in vivo study. Scaffolds were implanted in a rat subcutaneous pocket model, and retrieved after 7 days. We introduce three quantitative factors as a measure of vascularization: the total percentage of vascularization, the vessels diameter distribution and the vascular penetration depth. These parameters were derived by image analysis of microcomputed tomographic scans of biological specimens perfused with a radiopaque polymer. The outcome of this study suggests that the rational assembly of BBs helps the onset and organization of a fully functional vascular network.

Electrosprayed poly(vinylidene fluoride) microparticles for tissue engineering applications

Poly(vinylidene fluoride) microparticles with diameters between 0.80 and 5.50 μm were produced. Cell viability shows their suitability for tissue engineering.