Poly-l-Lactic Acid (PLLA)-Based Biomaterials for Regenerative Medicine: A Review on Processing and Applications

Architecture-Engineered Electrospinning Cascade Regulates Spinal Microenvironment to Promote Nerve Regeneration

The inflammatory cascade after spinal cord injury (SCI) causes necrotizing apoptosis of local stem cells, which limits nerve regeneration. Therefore, coordinating the inflammatory immune response and neural stem cell (NSC) functions is key to promoting the recovery of central nervous system function. In this study, a hydrogel "perfusion" system and electrospinning technology are integrated, and a "concrete" composite support for the repair of nerve injuries is built. The hydrogel's hydrophilic properties activate macrophage integrin receptors to mediate polarization into anti-inflammatory subtypes and cause a 10% increase in polarized M2 macrophages, thus reprogramming the SCI immune microenvironment. Programmed stromal cell-derived factor-1α and brain-derived neurotrophic factor released from the composite increase recruitment and neuronal differentiation of NSCs by approximately four- and twofold, respectively. The fiber system regulates the SCI immune inflammatory microenvironment, recruits endogenous NSCs, promotes local blood vessel germination and maturation, and improves nerve function recovery in a rat SCI model. In conclusion, the engineering fiber composite improves the local inflammatory response. It promotes nerve regeneration through a hydrophilic programmed cytokine-delivery system, which further improves and supplements the immune response mechanism regulated by the inherent properties of the biomaterial. The new fiber composite may serve as a new treatment approach for SCI.

A proof of concept to define the parameters affecting poly-L-lactide-co-poly-ε-caprolactone shape memory electrospun nanofibers for biomedical applications

This study is a proof of concept performed to evaluate process parameters affecting shape memory effect of copolymer poly-L-lactide-co-poly-ε-caprolactone (PLA:PCL) 70:30 ratio based nanofibrous scaffolds. A design of experiment (DOE) statistical approach was used to define the interaction between independent material and process variables related to electrospun scaffold manufacturing, such as polymer solution concentration (w/v%), spinning time (min), and needle size (Gauge), and their influence on Rf% (ability of the scaffold to maintain the induced temporary shape) and Rr% (ability of the scaffold to recover its original shape) outputs. A mathematical model was obtained from DOE useful to predict scaffold Rf% and Rr% values. PLA-PCL 15% w/v, 22G needle, and 20-min spinning time were selected to confirm the data obtained from theoretical model. Subsequent morphological (SEM), chemical-physical (GPC and DSC), mechanical (uniaxial tensile tests), and biological (cell viability and adhesion) characterizations were performed.

Electroactive Biomaterials for Facilitating Bone Defect Repair under Pathological Conditions

Bone degeneration associated with various diseases is increasing due to rapid aging, sedentary lifestyles, and unhealthy diets. Living bone tissue has bioelectric properties critical to bone remodeling, and bone degeneration under various pathological conditions results in significant changes to these bioelectric properties. There is growing interest in utilizing biomimetic electroactive biomaterials that recapitulate the natural electrophysiological microenvironment of healthy bone tissue to promote bone repair. This review first summarizes the etiology of degenerative bone conditions associated with various diseases such as type II diabetes, osteoporosis, periodontitis, osteoarthritis, rheumatoid arthritis, osteomyelitis, and metastatic osteolysis. Next, the diverse array of natural and synthetic electroactive biomaterials with therapeutic potential are discussed. Putative mechanistic pathways by which electroactive biomaterials can mitigate bone degeneration are critically examined, including the enhancement of osteogenesis and angiogenesis, suppression of inflammation and osteoclastogenesis, as well as their anti-bacterial effects. Finally, the limited research on utilization of electroactive biomaterials in the treatment of bone degeneration associated with the aforementioned diseases are examined. Previous studies have mostly focused on using electroactive biomaterials to treat bone traumatic injuries. It is hoped that this review will encourage more research efforts on the use of electroactive biomaterials for treating degenerative bone conditions.

rhBMP-2-Conjugated Three-Dimensional-Printed Poly(L-lactide) Scaffold is an Effective Bone Substitute

BACKGROUND

Bone growth factors, particularly bone morphogenic protein-2 (BMP-2), are required for effective treatment of significant bone loss. Despite the extensive development of bone substitutes, much remains to be desired for wider application in clinical settings. The currently available bone substitutes cannot sustain prolonged BMP-2 release and are inconvenient to use. In this study, we developed a ready-to-use bone substitute by sequential conjugation of BMP to a three-dimensional (3D) poly(L-lactide) (PLLA) scaffold using novel molecular adhesive materials that reduced the operation time and sustained prolonged BMP release.

METHODS

A 3D PLLA scaffold was printed and BMP-2 was conjugated with alginate-catechol and collagen. PLLA scaffolds were conjugated with different concentrations of BMP-2 and evaluated for bone regeneration in vitro and in vivo using a mouse calvarial model. The BMP-2 release kinetics were analyzed using ELISA. Histological analysis and micro-CT image analysis were performed to evaluate new bone formation.

RESULTS

The 3D structure of the PLLA scaffold had a pore size of 400 µm and grid thickness of 187-230 µm. BMP-2 was released in an initial burst, followed by a sustained release for 14 days. Released BMP-2 maintained osteoinductivity in vitro and in vivo. Micro-computed tomography and histological findings demonstrate that the PLLA scaffold conjugated with 2 µg/ml of BMP-2 induced optimal bone regeneration.

CONCLUSION

The 3D-printed PLLA scaffold conjugated with BMP-2 enhanced bone regeneration, demonstrating its potential as a novel bone substitute.

Core/Double-Sheath Composite Fibers from Poly(ethylene oxide), Poly(L-lactide) and Beeswax by Single-Spinneret Electrospinning

The conventional approach for preparation of core-sheath fibers is coaxial electrospinning. Single-spinneret electrospinning of emulsions is a much less common method to obtain core-sheath fibers. Core-sheath structure may be generated by electrospinning of homogeneous blend solutions; however, reports on such cases are still scarce. Herein, the preparation of nanofibrous composites from poly(ethylene oxide) (PEO), poly(L-lactide) (PLA) and beeswax (BW) by single-spinneret electrospinning of their homogeneous blend solutions in chloroform is reported. The produced fibers had core/double-sheath structure with a PEO core, PLA inner sheath and BW outer sheath. This original fiber structure was evidenced by transmission electron microscopy, selective extraction of BW or PEO, and X-ray photoelectron spectroscopy. The PLA/BW double sheath led to hydrophobicity of the PEO/PLA/BW mats. The tensile tests revealed that PEO/PLA/BW mats had substantially improved mechanical behavior as compared to PEO, PLA and PEO/BW mats. PEO/PLA/BW mats can be used as drug carriers as evidenced by the one-pot incorporation of the model drug 5-nitro-8-hydroxyquinoline (NQ) into the fibrous materials. Microbiological tests showed that PEO/PLA/BW/NQ had antimicrobial activity. Therefore, the new materials are promising for wound healing applications.

Recent advances in PLLA-based biomaterial scaffolds for neural tissue engineering: Fabrication, modification, and applications

Repairing and regenerating injured neural tissue remains a worldwide challenge. Tissue engineering (TE) has been highlighted as a potential solution to provide functional substitutes for damaged organs or tissue. Among the biocompatible and biodegradable materials, poly-L-lactic-acid (PLLA) has been widely investigated in the TE field because of its tunable mechanical properties and tailorable surface functionalization. PLLA-based biomaterials can be engineered as scaffolds that mimic neural tissue extracellular matrix and modulate inflammatory responses. With technological advances, PLLA-based scaffolds can also have well-controlled three-dimensional sizes and structures to facilitate neurite extension. Furthermore, PLLA-based scaffolds have the potential to be used as drug-delivery carriers with controlled release. Moreover, owing to the good piezoelectric properties and capacity to carry conductive polymers, PLLA-based scaffolds can be combined with electrical stimulation to maintain stemness and promote axonal guidance. This mini-review summarizes and discusses the fabrication and modification techniques utilized in the PLLA-based biomaterial scaffolds for neural TE. Recent applications in peripheral nerve and spinal cord regeneration are also presented, and it is hoped that this will guide the future development of more effective and multifunctional PLLA-based nerve scaffolds.

Micro-/Nano-Structured Biodegradable Pressure Sensors for Biomedical Applications

The interest in biodegradable pressure sensors in the biomedical field is growing because of their temporary existence in wearable and implantable applications without any biocompatibility issues. In contrast to the limited sensing performance and biocompatibility of initially developed biodegradable pressure sensors, device performances and functionalities have drastically improved owing to the recent developments in micro-/nano-technologies including device structures and materials. Thus, there is greater possibility of their use in diagnosis and healthcare applications. This review article summarizes the recent advances in micro-/nano-structured biodegradable pressure sensor devices. In particular, we focus on the considerable improvement in performance and functionality at the device-level that has been achieved by adapting the geometrical design parameters in the micro- and nano-meter range. First, the material choices and sensing mechanisms available for fabricating micro-/nano-structured biodegradable pressure sensor devices are discussed. Then, this is followed by a historical development in the biodegradable pressure sensors. In particular, we highlight not only the fabrication methods and performances of the sensor device, but also their biocompatibility. Finally, we intoduce the recent examples of the micro/nano-structured biodegradable pressure sensor for biomedical applications.

Advances in Biodegradable Soft Robots

Biodegradable soft robots have been proposed for a variety of intelligent applications in soft robotics, flexible electronics, and bionics. Biodegradability offers an extraordinary functional advantage to soft robots for operations accompanying smart shape transformation in response to external stimuli such as heat, pH, and light. This review primarily surveyed the current advanced scientific and engineering strategies for integrating biodegradable materials within stimuli-responsive soft robots. It also focused on the fabrication methodologies of multiscale biodegradable soft robots, and highlighted the role of biodegradable soft robots in enhancing the multifunctional properties of drug delivery capsules, biopsy tools, smart actuators, and sensors. Lastly, the current challenges and perspectives on the future development of intelligent soft robots for operation in real environments were discussed.

The Performance of Nonwoven PLLA Scaffolds of Different Thickness for Stem Cells Seeding and Implantation

The 3D reconstruction of 100 μm- and 600 μm-thick fibrous poly-L/L-lactide scaffolds was performed by confocal laser scanning microscopy and supported by scanning electron microscopy and showed that the density of the fibers on the side adjacent to the electrode is higher, which can affect cell diffusion, while the pore size is generally the same. Bone marrow mesenchymal stem cells cultured in a 600 μm-thick scaffold formed colonies and produced conditions for cell differentiation. An in vitro study of stem cells after 7 days revealed that cell proliferation and hepatocyte growth factor release in the 600 μm-thick scaffold were higher than in the 100 μm-thick scaffold. An in vivo study of scaffolds with and without stem cells implanted subcutaneously onto the backs of recipient mice was carried out to test their biodegradation and biocompatibility over a 0-3-week period. The cells seeded onto the 600 μm-thick scaffold promoted significant neovascularization in vivo. After 3 weeks, a significant number of donor cells persisted only on the inside of the 600 μm-thick scaffold. Thus, the use of bulkier matrices allows to prolong the effect of secretion of growth factors by stem cells during implantation. These 600 μm-thick scaffolds could potentially be utilized to repair and regenerate injuries with stem cell co-culture for vascularization of implant.

Enhanced Mechanical Properties and Anti-Inflammation of Poly(L-Lactic Acid) by Stereocomplexes of PLLA/PDLA and Surface-Modified Magnesium Hydroxide Nanoparticles

Poly(L-lactic acid) (PLLA), as a biodegradable polymer, has attracted attention for use as a biomaterial. In order to apply PLLA as a cardiovascular stent, stronger mechanical properties and anti-inflammatory effects against acidic by-products are required. In this study, PLLA/PDLA stereocomplex microparticles (SC) were developed and surface-modified magnesium hydroxide (MH) nanoparticles with oligolactide were combined with these PLLA composites. The SC improved the mechanical properties of the PLLA composites through the formation of stereocomplex structures. The surface-modified MH nanoparticles showed enhanced mechanical properties due to the stereocomplex structures formed by PLLA chains and inhibited inflammatory responses by pH neutralization as a result of MH. Additionally, the MH nanoparticles containing PLLA composites had antibacterial effects and increased the viability of human vascular endothelial cells. This technology is expected to have great potential in the development of PLLA composite materials for the production of various medical devices, such as cardiovascular stents.

Recent Developments and Current Applications of Organic Nanomaterials in Cartilage Repair

Regeneration of cartilage is difficult due to the unique microstructure, unique multizone organization, and avascular nature of cartilage tissue. The development of nanomaterials and nanofabrication technologies holds great promise for the repair and regeneration of injured or degenerated cartilage tissue. Nanomaterials have structural components smaller than 100 nm in at least one dimension and exhibit unique properties due to their nanoscale structure and high specific surface area. The unique properties of nanomaterials include, but are not limited to, increased chemical reactivity, mechanical strength, degradability, and biocompatibility. As an emerging nanomaterial, organic nanocomposites can mimic natural cartilage in terms of microstructure, physicochemical, mechanical, and biological properties. The integration of organic nanomaterials is expected to develop scaffolds that better mimic the extracellular matrix (ECM) environment of cartilage to enhance scaffold-cell interactions and improve the functionality of engineered tissue constructs. Next-generation hydrogel technology and bioprinting can be used not only for healing cartilage injury areas but also for extensive osteoarthritic degenerative changes within the joint. Although more challenges need to be solved before they can be translated into full-fledged commercial products, nano-organic composites remain very promising candidates for the future development of cartilage tissue engineering.

Anti-Inflammatory and Mineralization Effects of an ASP/PLGA-ASP/ACP/PLLA-PLGA Composite Membrane as a Dental Pulp Capping Agent

Dental pulp is essential for the development and long-term preservation of teeth. Dental trauma and caries often lead to pulp inflammation. Vital pulp therapy using dental pulp-capping materials is an approach to preserving the vitality of injured dental pulp. Most pulp-capping materials used in clinics have good biocompatibility to promote mineralization, but their anti-inflammatory effect is weak. Therefore, the failure rate will increase when dental pulp inflammation is severe. The present study developed an amorphous calcium phosphate/poly (L-lactic acid)-poly (lactic-co-glycolic acid) membrane compounded with aspirin (hereafter known as ASP/PLGA-ASP/ACP/PLLA-PLGA). The composite membrane, used as a pulp-capping material, effectively achieved the rapid release of high concentrations of the anti-inflammatory drug aspirin during the early stages as well as the long-term release of low concentrations of aspirin and calcium/phosphorus ions during the later stages, which could repair inflamed dental pulp and promote mineralization. Meanwhile, the composite membrane promoted the proliferation of inflamed dental pulp stem cells, downregulated the expression of inflammatory markers, upregulated the expression of mineralization-related markers, and induced the formation of stronger reparative dentin in the rat pulpitis model. These findings indicate that this material may be suitable for use as a pulp-capping material in clinical applications.

Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications

The three-dimensional printing (3DP) also known as the additive manufacturing (AM), a novel and futuristic technology that facilitates the printing of multiscale, biomimetic, intricate cytoarchitecture, function-structure hierarchy, multi-cellular tissues in the complicated micro-environment, patient-specific scaffolds, and medical devices. There is an increasing demand for developing 3D-printed products that can be utilized for organ transplantations due to the organ shortage. Nowadays, the 3DP has gained considerable interest in the tissue engineering (TE) field. Polylactide (PLA) and polycaprolactone (PCL) are exemplary biomaterials with excellent physicochemical properties and biocompatibility, which have drawn notable attraction in tissue regeneration. Herein, the recent advancements in the PLA and PCL biodegradable polymer-based composites as well as their reinforcement with hydrogels and bio-ceramics scaffolds manufactured through 3DP are systematically summarized and the applications of bone, cardiac, neural, vascularized and skin tissue regeneration are thoroughly elucidated. The interaction between implanted biodegradable polymers, in-vivo and in-vitro testing models for possible evaluation of degradation and biological properties are also illustrated. The final section of this review incorporates the current challenges and future opportunities in the 3DP of PCL- and PLA-based composites that will prove helpful for biomedical engineers to fulfill the demands of the clinical field.

Effect of Polyhydroxyalkanoate (PHA) Concentration on Polymeric Scaffolds Based on Blends of Poly-L-Lactic Acid (PLLA) and PHA Prepared via Thermally Induced Phase Separation (TIPS)

Hybrid porous scaffolds composed of both natural and synthetic biopolymers have demonstrated significant improvements in the tissue engineering field. This study investigates for the first time the fabrication route and characterization of poly-L-lactic acid scaffolds blended with polyhydroxyalkanoate up to 30 wt%. The hybrid scaffolds were prepared by a thermally induced phase separation method starting from ternary solutions. The microstructure of the hybrid porous structures was analyzed by scanning electron microscopy and related to the blend composition. The porosity and the wettability of the scaffolds were evaluated through gravimetric and water contact angle measurements, respectively. The scaffolds were also characterized in terms of the surface chemical properties via Fourier transform infrared spectroscopy in attenuated total reflectance. The mechanical properties were analyzed through tensile tests, while the crystallinity of the PLLA/PHA scaffolds was investigated by differential scanning calorimetry and X-ray diffraction.

Biological Response to Bioinspired Microporous 3D-Printed Scaffolds for Bone Tissue Engineering

The scaffold is a key element in the field of tissue engineering, especially when large defects or substitutions of pathological tissues or organs need to be clinically addressed. The expected outcome is strongly dependent on the cell–scaffold interaction and the integration with the surrounding biological tissue. Indeed, mimicking the natural extracellular matrix (ECM) of the tissue to be healed represents a further optimization that can limit a possible morphological mismatch between the scaffold and the tissue itself. For this aim, and referring to bone tissue engineering, polylactic acid (PLA) scaffolds were 3D printed with a microstructure inspired by the trabecular architecture and biologically evaluated by means of human osteosarcoma SAOS-2 cells. The cells were seeded on two types of scaffolds differing for the designed pore size (i.e., 400 and 600 µm), showing the same growth exponential trend found in the control and no significant alterations in the actin distribution. The microporous structure of the two tested samples enhanced the protein adsorption capability and mRNA expression of markers related to protein synthesis, proliferation, and osteoblast differentiation. Our findings demonstrate that 3D-printed scaffolds support the adhesion, growth, and differentiation of osteoblast-like cells and the microporous architecture, mimicking the natural bone hierarchical structure, and favoring greater bioactivity. These bioinspired scaffolds represent an interesting new tool for bone tissue engineering and regenerative medicine applications.