Morphological study on the pore growth profile of poly(ε-caprolactone) bi-modal porous foams using a modified supercritical CO2 foaming process

Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

Production and Application of Polymer Foams Employing Supercritical Carbon Dioxide

Polymeric foams have characteristics that make them attractive for different applications. However, some foaming methods rely on chemicals that are not environmentally friendly. One of the possibilities to tackle the environmental issue is to utilize supercritical carbon dioxide ScCO2 since it is a “green” solvent, thus facilitating a sustainable method of producing foams. ScCO2 is nontoxic, chemically inert, and soluble in molten plastic. It can act as a plasticizer, decreasing the viscosity of polymers according to temperature and pressure. Most foam processes can benefit from ScCO2 since the methods rely on nucleation, growth, and expansion mechanisms. Process considerations such as pretreatment, temperature, pressure, pressure drop, and diffusion time are relevant parameters for foaming. Other variables such as additives, fillers, and chain extenders also play a role in the foaming process. This review highlights the morphology, performance, and features of the foam produced with ScCO2, considering relevant aspects of replacing or introducing a novel foam. Recent findings related to foaming assisted by ScCO2 and how processing parameters influence the foam product are addressed. In addition, we discuss possible applications where foams have significant benefits. This review shows the recent progress and possibilities of ScCO2 in processing polymer foams.

Supercritical Fluid Technologies for the Incorporation of Synthetic and Natural Active Compounds into Materials for Drug Formulation and Delivery

Various active compounds isolated from natural sources exhibit remarkable benefits, making them attractive for pharmaceutical and biomedical applications, such as antioxidant, antimicrobial, and anti-inflammatory activities, which contribute to the treatment of cardiovascular diseases, neurodegenerative disorders, various types of cancer, diabetes, and obesity. However, their major drawbacks are their reactivity, instability, relatively poor water solubility, and consequently low bioavailability. Synthetic drugs often face similar challenges associated with inadequate solubility or burst release in gastrointestinal media, despite being otherwise a safe and effective option for the treatment of numerous diseases. Therefore, drug-eluting pharmaceutical formulations have been of great importance over the years in efforts to improve the bioavailability of active compounds by increasing their solubility and achieving their controlled release in body media. This review highlights the success of the fabrication of micro- and nanoformulations using environmentally friendly supercritical fluid technologies for the processing and incorporation of active compounds. Several novel approaches, namely micronization to produce micro- and nano-sized particles, supercritical drying to produce aerogels, supercritical foaming, and supercritical solvent impregnation, are described in detail, along with the currently available drug delivery data for these formulations.

Foaming of PCL-Based Composites Using scCO2—Biocompatibility and Evaluation for Biomedical Applications

The process of foaming poly(caprolactone)-based composite materials using supercritical carbon dioxide was analyzed, especially in terms of the biocompatibility of the resultant materials. The influence of foaming process conditions and composite material properties on the functional properties of polymer solid foams, intended for artificial scaffolds for bone cell culture, was investigated. The relationship between wettability (contact angle) and water absorption rate as a result of the application of variable conditions for the production of porous structures was presented. For the evaluation of potential cytotoxicity, the MTT and PrestoBlue tests were carried out, and animal cells (mouse fibroblasts) were cultured on the materials for nine days. There was no toxic effect of composite materials made of poly(caprolactone) containing porogen particles: hydroxyapatite, crystalline nanocellulose, and graphene oxide on cells. The desired effect of the porogens used in the foaming process on the affinity of cells to the resultant material was demonstrated. The tested materials have been shown to be biocompatible and suitable for applications in biomedical engineering.

Combined sterilization and fabrication of drug-loaded scaffolds using supercritical CO2 technology

The access of biodegradable scaffolds to the clinical arena is constrained by the absence of a suitable sterilization technique for the processing of advanced polymeric materials. Sterilization with supercritical CO₂ (scCO₂) may circumvent some technological limitations (e.g., low temperature, no chemical residues on the material), although scCO₂ can plasticize the polymer depending on the processing conditions used. In this latter case, the integration of the manufacturing and sterilization processes is of particular interest to obtain sterile and customized scaffolds in a single step. In this work, scCO₂ was exploited as a concomitantly foaming and sterilizing agent for the first time, developing a one-step process for the production of vancomycin-loaded poly(ε-caprolactone) (PCL) bone scaffolds. The effect of the CO₂ contact time on the sterility levels of the procedure was investigated, and the sterilization efficiency was evaluated against dry spores (Bacillus stearothermophilus, Bacillus pumilus and Bacillus atrophaeus). Vancomycin-loaded PCL scaffolds had relevant sustained release profiles for the prophylaxis of infections at the grafted area, even those caused by methicillin-resistant Staphylococcus aureus (MRSA). The biological performance of the scaffolds was evaluated in vitro regarding human mesenchymal stem cells (hMSCs) attachment and growth. Finally, the biocompatibility and angiogenic response of the manufactured sterile scaffolds was assessed in ovo through chick chorioallantoic membrane (CAM) assays.

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.

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.

Combination of electrospinning with other techniques for the fabrication of 3D polymeric and composite nanofibrous scaffolds with improved cellular interactions

The development of three-dimensional (3D) scaffolds with physical and chemical topological cues at the macro-, micro-, and nanometer scale is urgently needed for successful tissue engineering applications. 3D scaffolds can be manufactured by a wide variety of techniques. Electrospinning technology has emerged as a powerful manufacturing technique to produce non-woven nanofibrous scaffolds with very interesting features for tissue engineering products. However, electrospun scaffolds have some inherent limitations that compromise the regeneration of thick and complex tissues. By integrating electrospinning and other fabrication technologies, multifunctional 3D fibrous assemblies with micro/nanotopographical features can be created. The proper combination of techniques leads to materials with nano and macro-structure, allowing an improvement in the biological performance of tissue-engineered constructs. In this review, we focus on the most relevant strategies to produce electrospun polymer/composite scaffolds with 3D architecture. A detailed description of procedures involving physical and chemical agents to create structures with large pores and 3D fiber assemblies is introduced. Finally, characterization and biological assays including in vitro and in vivo studies of structures intended for the regeneration of functional tissues are briefly presented and discussed.

Biomaterials for stem cell engineering and biomanufacturing

Recent years have witnessed the expansion of tissue failures and diseases. The uprising of regenerative medicine converges the sight onto stem cell-biomaterial based therapy. Tissue engineering and regenerative medicine proposes the strategy of constructing spatially, mechanically, chemically and biologically designed biomaterials for stem cells to grow and differentiate. Therefore, this paper summarized the basic properties of embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs) and adult stem cells. The properties of frequently used biomaterials were also described in terms of natural and synthetic origins. Particularly, the combination of stem cells and biomaterials for tissue repair applications was reviewed in terms of nervous, cardiovascular, pancreatic, hematopoietic and musculoskeletal system. Finally, stem-cell-related biomanufacturing was envisioned and the novel biofabrication technologies were discussed, enlightening a promising route for the future advancement of large-scale stem cell-biomaterial based therapeutic manufacturing.