Supercritical CO2 technology for one-pot foaming and sterilization of polymeric scaffolds for bone regeneration

Sterilization is a quite challenging step in the development of novel polymeric scaffolds for regenerative medicine since conventional sterilization techniques may significantly alter their morphological and physicochemical properties. Supercritical (sc) sterilization, i.e. the use of scCO₂ as a sterilizing agent, emerges as a promising sterilization method due to the mild operational conditions and excellent penetration capability. In this work, a scCO₂ protocol was implemented for the one-pot preparation and sterilization of poly(ε-caprolactone) (PCL)/poly(lactic-co-glycolic acid) (PLGA) scaffolds. The sterilization conditions were established after screening against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) vegetative bacteria and spores of Bacillus stearothermophilus, Bacillus pumilus and Bacillus atrophaeus. The transition from the sterilization conditions (140 bar, 39 °C) to the compressed foaming (60 bar, 26 °C) was performed through controlled depressurization (3.2 bar/min) and CO₂ liquid flow. Controlled depressurization/pressurization cycles were subsequently applied. Using this scCO₂ technology toolbox, sterile scaffolds of well-controlled pore architecture were obtained. This sterilization procedure successfully achieved not only SAL-6 against well-known resistant bacteria endospores but also improved the scaffold morphologies compared to standard gamma radiation sterilization procedures.

Growth factor loading on aliphatic polyester scaffolds

Cells, scaffolds and growth factors are three elements of tissue engineering. The success of tissue engineering methods relies on precise and dynamic interactions between cells, scaffolds and growth factors. Aliphatic polyester scaffolds are promising tissue engineering scaffolds that possess good mechanical properties, low immunogenicity, non-toxicity, and adjustable degradation rates. How growth factors can be loaded onto/into aliphatic polyester scaffolds and be constantly released with the required bioactivity to regulate cell growth and promote defect tissue repair and regeneration has become the main concern of tissue engineering researchers. In this review, the existing main methods of loading growth factors on aliphatic polyester scaffolds, the release behavior of loaded growth factors and their positive effects on cell, tissue repair and regeneration are introduced. Advantages and shortcomings of each method also are mentioned. It is still a great challenge to control the release of loaded growth factors at a certain time and at a concentration simulating the biological environment of native tissue.

Solvent-Free Processing of Drug-Loaded Poly(ε-Caprolactone) Scaffolds with Tunable Macroporosity by Combination of Supercritical Foaming and Thermal Porogen Leaching

Demand of scaffolds for hard tissue repair increases due to a higher incidence of fractures related to accidents and bone-diseases that are linked to the ageing of the population. Namely, scaffolds loaded with bioactive agents can facilitate the bone repair by favoring the bone integration and avoiding post-grafting complications. Supercritical (sc-)foaming technology emerges as a unique solvent-free approach for the processing of drug-loadenu7d scaffolds at high incorporation yields. In this work, medicated poly(ε-caprolactone) (PCL) scaffolds were prepared by sc-foaming coupled with a leaching process to overcome problems of pore size tuning of the sc-foaming technique. The removal of the solid porogen (BA, ammonium bicarbonate) was carried out by a thermal leaching taking place at 37 °C and in the absence of solvents for the first time. Macroporous scaffolds with dual porosity (50-100 µm and 200-400 µm ranges) were obtained and with a porous structure directly dependent on the porogen content used. The processing of ketoprofen-loaded scaffolds using BA porogen resulted in drug loading yields close to 100% and influenced its release profile from the PCL matrix to a relevant clinical scenario. A novel solvent-free strategy has been set to integrate the incorporation of solid porogens in the sc-foaming of medicated scaffolds.

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.

Supercritical fluid-assisted controllable fabrication of open and highly interconnected porous scaffolds for bone tissue engineering

Recently tremendous progress has been evidenced by the advancements in developing innovative three-dimensional (3D) scaffolds using various techniques for addressing the autogenous grafting of bone. In this work, we demonstrated the fabrication of porous polycaprolactone (PCL) scaffolds for osteogenic differentiation based on supercritical fluid-assisted hybrid processes of phase inversion and foaming. This eco-friendly process resulted in the highly porous biomimetic scaffolds with open and interconnected architectures. Initially, a 2³ factorial experiment was designed for investigating the relative significance of various processing parameters and achieving better control over the porosity as well as the compressive mechanical properties of the scaffold. Then, single factor experiment was carried out to understand the effects of various processing parameters on the morphology of scaffolds. On the other hand, we encapsulated a growth factor, i.e., bone morphogenic protein-2 (BMP-2), as a model protein in these porous scaffolds for evaluating their osteogenic differentiation. In vitro investigations of growth factor loaded PCL scaffolds using bone marrow stromal cells (BMSCs) have shown that these growth factor-encumbered scaffolds were capable of differentiating the cells over the control experiments. Furthermore, the osteogenic differentiation was confirmed by measuring the cell proliferation, and alkaline phosphatase (ALP) activity, which were significantly higher demonstrating the active bone growth. Together, these results have suggested that the fabrication of growth factor-loaded porous scaffolds prepared by the eco-friendly hybrid processing efficiently promoted the osteogenic differentiation and may have a significant potential in bone tissue engineering.

Antimicrobial Properties and Osteogenicity of Vancomycin-Loaded Synthetic Scaffolds Obtained by Supercritical Foaming

Advanced porous synthetic scaffolds are particularly suitable for regeneration of damaged tissues, but there is the risk of infections due to the colonization of microorganisms, forming biofilms. Supercritical foaming is an attractive processing method to prepare bone scaffolds, regulating simultaneously the porosity and loading of bioactive compounds without loss of activity. In this work, scaffolds made of poly-ε-caprolactone (50 kDa), containing chitosan and an antimicrobial agent (vancomycin), were processed by supercritical CO₂ foaming for bone regeneration purposes. The obtained scaffolds showed a suitable combination of morphological (porosity, pore size distribution, and interconnectivity), time-dependent in vitro vancomycin release behavior and biological properties (cell viability and proliferation, osteodifferentiation, and tissue-scaffold integration). The scaffolds sustained vancomycin release for more than 2 weeks. Finally, the antimicrobial activity of the scaffolds was tested against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria after 24 h of incubation with full growth inhibition for S. aureus.