Preparation of highly interconnected porous poly(ε-caprolactone)/poly(lactic acid) scaffolds via supercritical foaming

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.

Lightweight and High Impact Toughness PP/PET/POE Composite Foams Fabricated by In Situ Nanofibrillation and Microcellular Injection Molding

Polypropylene (PP) has become the most promising and candidate material for fabricating lightweight products. Microcellular injection molding (MIM) is a cost-effective technology for manufacturing porous plastic products. However, it is still challenging to fabricate high-performance PP microcellular components. Herein, we reported an efficient strategy to produce lightweight and high impact toughness foamed PP/polyethylene terephthalate (PET)/polyolefin-based elastomer (POE) components by combining in situ fibrillation (INF) and MIM technologies. First, the INF composite was prepared by integrating twin-screw compounding with melt spinning. SEM analysis showed PET nanofibrils with a diameter of 258 nm were achieved and distributed uniformly in the PP due to the POE's inducing elaboration effect. Rheological and DSC analysis demonstrated PET nanofibrils pronouncedly improved PP's viscoelasticity and crystal nucleation rate, respectively. Compared with PP foam, INF composite foam showed more stretched cells in the skin layer and refined spherical cells in the core layer. Due to the synergistic toughening effect of PET nanofibrils and POE elastic particles, the impact strength of INF composite foams was 295.3% higher than that of PP foam and 191.2% higher than that of melt-blended PP/PET foam. The results gathered in this study reveal potential applications for PP based INF composite foams in the manufacturing of lightweight automotive products with enhanced impact properties.

ScCO2-assisted fabrication and compressive property of poly (lactic acid) foam reinforced by in-situ polytetrafluoroethylene fibrils

As an effective alternative for petrochemical-based polymers, bio-based poly (lactic acid) (PLA) foam has been anticipated to alleviate enormous environmental pollution caused by microplastics. However, some difficulties involved in PLA foaming process due to the inherently poor melt strength and crystallization properties. In this context, a small amount of polytetrafluoroethylene (PTFE) was incorporated into PLA matrix to solve the aforementioned issues. Scanning electron microscopy measurement exhibited that PTFE fibrils and their physical networks were formed in molten PLA after blending. Due to these PTFE networks, approximately 2 orders of magnitudes increment in the storage modulus and more than 20% improvement in crystallinity of PLA were obtained. Diverse PLA samples were successfully foamed by a cost-effective, green and supercritical CO₂-assisted foaming method. The PLA/PTFE foam with the PTFE content of 5 wt% (PLA/PTFE5) possessed the smallest pore size (9.51 μm) and the highest pore density (2.60 × 10⁸ pores/cm³). In addition, the average specific compressive strength of PLA/PTFE5 foam was enhanced 30% in comparison with that of pure PLA foam. Overall, this study could provide a prospective strategy for developing bioderived and biodegradable polymer foams with controllable pore structures and high compression property.

Fabrication of Highly Interconnected Poly(ε-caprolactone)/cellulose Nanofiber Composite Foams by Microcellular Foaming and Leaching Processes

In this study, microcellular polycaprolactone (PCL)/sodium bicarbonate (NaHCO₃)/cellulose nanofiber (CNF) composite foams with highly interconnected porous structures were successfully fabricated by microcellular foaming and particle leaching processes. Supercritical CO₂ (scCO₂) served as a physical foaming agent, NaHCO₃ was chosen as a chemical foaming agent and porogen, and CNF acted as a heterogeneous nucleating agent. The effect of scCO₂, NaHCO₃, and CNF on pore structures and the cofoaming mechanism were investigated. The results indicated that the addition of NaHCO₃ and CNF increased the melt strength of the PCL matrix significantly. During the foaming process, the presence of CNF can form a rigid network due to the hydrogen bonding or mechanical entanglement between individual nanofibers, improving the nucleating efficiency but slowing down the cell growth rate. Additionally, due to the interaction of "soft" PCL matrix and "hard" domains in a PCL-based composite during the foaming process, together with the NaHCO₃ leaching process, highly interconnected cell structures appeared. The obtained PCL/NaHCO₃/CNF composite foams had a cell size of 15.8 μm and cell density of 6.3 × 10⁷ cells/cm³, as well as an open-cell content of 82%. The reported strategy in this paper may provide the guidelines and data supports for the fabrication of a PCL-based porous scaffold.

Poly(lactic Acid): A Versatile Biobased Polymer for the Future with Multifunctional Properties-From Monomer Synthesis, Polymerization Techniques and Molecular Weight Increase to PLA Applications

Environmental problems, such as global warming and plastic pollution have forced researchers to investigate alternatives for conventional plastics. Poly(lactic acid) (PLA), one of the well-known eco-friendly biodegradables and biobased polyesters, has been studied extensively and is considered to be a promising substitute to petroleum-based polymers. This review gives an inclusive overview of the current research of lactic acid and lactide dimer techniques along with the production of PLA from its monomers. Melt polycondensation as well as ring opening polymerization techniques are discussed, and the effect of various catalysts and polymerization conditions is thoroughly presented. Reaction mechanisms are also reviewed. However, due to the competitive decomposition reactions, in the most cases low or medium molecular weight (MW) of PLA, not exceeding 20,000-50,000 g/mol, are prepared. For this reason, additional procedures such as solid state polycondensation (SSP) and chain extension (CE) reaching MW ranging from 80,000 up to 250,000 g/mol are extensively investigated here. Lastly, numerous practical applications of PLA in various fields of industry, technical challenges and limitations of PLA use as well as its future perspectives are also reported in this review.

Artificial small-diameter blood vessels: materials, fabrication, surface modification, mechanical properties, and bioactive functionalities

Cardiovascular diseases, especially ones involving narrowed or blocked blood vessels with diameters smaller than 6 millimeters, are the leading cause of death globally. Vascular grafts have been used in bypass surgery to replace damaged native blood vessels for treating severe cardio- and peripheral vascular diseases. However, autologous replacement grafts are not often available due to prior harvesting or the patient's health. Furthermore, autologous harvesting causes secondary injury to the patient at the harvest site. Therefore, artificial blood vessels have been widely investigated in the last several decades. In this review, the progress and potential outlook of small-diameter blood vessels (SDBVs) engineered in vitro are highlighted and summarized, including material selection and development, fabrication techniques, surface modification, mechanical properties, and bioactive functionalities. Several kinds of natural and synthetic polymers for artificial SDBVs are presented here. Commonly used fabrication techniques, such as extrusion and expansion, electrospinning, thermally induced phase separation (TIPS), braiding, 3D printing, hydrogel tubing, gas foaming, and a combination of these methods, are analyzed and compared. Different surface modification methods, such as physical immobilization, surface adsorption, plasma treatment, and chemical immobilization, are investigated and are compared here as well. Mechanical requirements of SDBVs are also reviewed for long-term service. In vitro biological functions of artificial blood vessels, including oxygen consumption, nitric oxide (NO) production, shear stress response, leukocyte adhesion, and anticoagulation, are also discussed. Finally, we draw conclusions regarding current challenges and attempts to identify future directions for the optimal combination of materials, fabrication methods, surface modifications, and biofunctionalities. We hope that this review can assist with the design, fabrication, and application of SDBVs engineered in vitro and promote future advancements in this emerging research field.

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.

Fabrication of fibrillated and interconnected porous poly(ε-caprolactone) vascular tissue engineering scaffolds by microcellular foaming and polymer leaching

This paper provides a method combining eco-friendly supercritical CO₂ microcellular foaming and polymer leaching to fabricate small-diameter vascular tissue engineering scaffolds.

Nanocellular Foaming Behaviors of Chain-Extended Poly(lactic acid) Induced by Isothermal Crystallization

Recently, the fabrication of semicrystalline polymer foams with a nanocellular structure by supercritical fluids has been becoming a newly developing research hotspot, owing to their peculiar properties and prospective applications. In this work, a facile and effective isothermal crystallization-induced method was proposed to prepare nanocellular semicrystalline poly(lactic acid) (PLA) foams using CO₂ as a physical blowing agent. Styrene-acrylonitrile-glycidyl methacrylate (SAG) as a chain extender (CE) was introduced into PLA through a melt-mixing method to improve the crystallization behavior and melt viscoelasticity of PLA. The chain extension reaction between PLA and SAG occurred successfully as well as the branching and micro cross-linking structures were generated in chain-extended PLA (CPLA) samples, which were confirmed by Fourier transform infrared spectra, gel fraction, and intrinsic viscosity measurements. Owing to the nucleation effect of branching points and the restricted movement of PLA molecular chains by the formation of branching and/or microcross-linking structures, a large number of small spherocrystals were generated in CPLA samples, which was beneficial to produce nanocells. Nanocellular CPLA foams were prepared successfully, when the foaming temperature was 125 °C. As the SAG content increased, the cell size of various PLA foams decreased from 364 ± 198 to 249 ± 100 nm and their volume expansion ratio increased from 1.15 ± 0.05 to 2.22 ± 0.01 times, gradually. When the foaming temperature increased from 125 to 127 °C, an interesting transition from nanocells to microcells could be observed in CPLA foam with the CE content of 2 wt %. Finally, the formation mechanism of nanocells in various PLA foams was proposed and clarified using a schematic diagram.