In vitro degradation kinetics of pure PLA and Mg/PLA composite: Effects of immersion temperature and compression stress

Structure-function integrated biodegradable Mg/polymer composites: Design, manufacturing, properties, and biomedical applications

Mg is a typical biodegradable metal widely used for biomedical applications due to its considerable mechanical properties and bioactivity. Biodegradable polymers have attracted great interest owing to their favorable processability and inclusiveness. However, it is challenging for the degradation rates of Mg or polymers to precisely match tissue repair processes, and the significant changes in local pH during degradation hinder tissue repair. The concept of combining Mg with polymers is proposed to overcome the shortcomings of materials, aiming to meet repair needs from various aspects such as mechanics and biology. Therefore, it is essential to systematically understand the behavior of biodegradable Mg/polymer composite (BMPC) from the design, manufacturing, mechanical properties, degradation, and biological effects. In this review, we elaborate on the design concepts and manufacturing strategies of high-strength BMPC, the "structure-function" relationship between the microstructures and mechanical properties of composites, the variation in the degradation rate due to endogenous and exogenous factors, and the establishment of advanced degradation research platform. Additionally, the interplay among composite components during degradation and the biological function of composites under non-responsive/stimuli-responsive platforms are also discussed. Finally, we hope that this review will benefit future clinical applications of "structure-function" integrated biomaterials.

Cytocompatibility, cell-material interaction, and osteogenic differentiation of MC3T3-E1 pre-osteoblasts in contact with engineered Mg/PLA composites

Bioabsorbable Mg wire-reinforced poly-lactic acid (PLA) matrix composites are potential candidate for load-bearing orthopedic implants offering tailorable mechanical and degradation properties by stacking sequence, volume fraction and surface modification of Mg wires. In this study, we investigated the cytocompatibility, cell-material interaction, and bone differentiation behavior of MC3T3-E1 pre-osteoblast cells for medical-grade PLA, Mg/PLA, and PEO-Mg/PLA (having PEO surface modification on Mg wires) composites. MTT and live/dead assay showed excellent biocompatibility of both composites while cell-material interaction analysis revealed that cells were able to adhere and proliferate on the surface of composites. Cells on the longitudinal surface of composites showed a high and uniform cell density while those on transversal surfaces initially avoided Mg regions but later migrated back after the formation of the passivation layer. Bone differentiation tests showed that cells in extracts of PLA and composites were able to initiate the differentiation process as osteogenesis-related gene expressions, alkaline phosphatase protein quantity, and calcium mineralization increased after 7 and 14 days of culture. Interestingly, the bone differentiation response of PEO-Mg/PLA composite was found to be similar to medical-grade PLA, proving its superiority over Mg/PLA composite.

Recent developments in ureteral stent: Substrate material, coating polymer and technology, therapeutic function

The ureteral stent is an effective treatment for clinical ureteral stricture following urological surgery, and the functional coating of the stent could effectively inhibit bacterial colonization and other complications. The present review provides an analysis and description of the materials used in ureteral stents and their coatings. Emphasis is placed on the technological advancements of functional coatings, taking into consideration the characteristics of these materials and the properties of their active substances. Furthermore, recent advances in enhancing the therapeutic efficacy of functional coatings are also reviewed. It is anticipated that this article will serve as a valuable reference providing insights for future research development on new drug-loaded ureteral stents.

In Silico Evaluation of In Vivo Degradation Kinetics of Poly(Lactic Acid) Vascular Stent Devices

Biodegradable vascular stents (BVS) are deemed as great potential alternatives for overcoming the inherent limitations of permanent metallic stents in the treatment of coronary artery diseases. The current study aimed to comprehensively compare the mechanical behaviors of four poly(lactic acid) (PLA) BVS designs with varying geometries via numerical methods and to clarify the optimal BVS selection. Four PLA BVS (i.e., Absorb, DESolve, Igaki-Tamai, and Fantom) were first constructed. A degradation model was refined by simply including the fatigue effect induced by pulsatile blood pressures, and an explicit solver was employed to simulate the crimping and degradation behaviors of the four PLA BVS. The degradation dynamics here were characterized by four indices. The results indicated that the stent designs affected crimping and degradation behaviors. Compared to the other three stents, the DESolve stent had the greatest radial stiffness in the crimping simulation and the best diameter maintenance ability despite its faster degradation; moreover, the stent was considered to perform better according to a pilot scoring system. The current work provides a theoretical method for studying and understanding the degradation dynamics of the PLA BVS, and it could be helpful for the design of next-generation BVS.

A phenomenological model of pulsatile blood pressure-affected degradation of polylactic acid (PLA) vascular stent

The stent implantation may alter the post-operative patient's blood pressure, and bioresorbable vascular stents (BVS) as a candidate to treat vascular diseases, its degradation is affected by mechanical stress, thus, the altered pressure representing varying stress level will result in different degradation behaviors of the BVS. This paper first proposed a novel stress-regulated PLA degradation model that included swelling factor, and then the degradation evolutions of a PLA BVS within 180 days under normal and high blood pressures were simulated by finite element method, and more four degradation indexes were defined to study the effects of the two blood pressures on the degradation of the PLA BVS. The results showed that the high pressure weakly accelerated the degradation of the PLA BVS with respect to the normal pressure by examining the four indexes, e.g., the residual stent volumev r ( t ) decreased to 0.72 and 0.69, respectively for the normal and high pressures at day 180. The current finding provided a theoretical understanding of the PLA BVS degradation, and hinted that the PLA BVS may not need to be elaborately selected in clinical practices for treating hypertensive patients.

Additively-manufactured Mg wire-reinforced PLDL-matrix composites for biomedical applications

Coupons of a medical grade PLDL polymer matrix uniaxially reinforced with a 15% volume fraction of Mg wires have been manufactured by fused filament fabrication for the first time. Two different types of Mg wires, without and with a surface treatment by plasma electrolytic oxidation were used. Both composite materials were subjected to degradation in phosphate buffer solution over a 3-week period, and their degradation and deformation micromechanisms were analysed in detail. Additionally, the materials were subjected to extensive mechanical testing under various loading conditions, and the interface strength was also analysed. It was found that the presence of the Mg wires improves the mechanical behaviour and accelerates the corrosion rate of the composite with respect that of the polymer matrix and these properties can be further tailored through the surface-modification of Mg wires by plasma electrolytic oxidation. The additive manufacturing strategy presented opens the path to fabricate multimaterial implants and scaffolds with complex shape and tailored properties provided by biodegradable polymers reinforced with either Mg and Zn particles and/or wires.

Bioabsorbable Composites Based on Polymeric Matrix (PLA and PCL) Reinforced with Magnesium (Mg) for Use in Bone Regeneration Therapy: Physicochemical Properties and Biological Evaluation

Improvements in Tissue Engineering and Regenerative Medicine (TERM)-type technologies have allowed the development of specific materials that, together with a better understanding of bone tissue structure, have provided new pathways to obtain biomaterials for bone tissue regeneration. In this manuscript, bioabsorbable materials are presented as emerging materials in tissue engineering therapies related to bone lesions because of their ability to degrade in physiological environments while the regeneration process is completed. This comprehensive review aims to explore the studies, published since its inception (2010s) to the present, on bioabsorbable composite materials based on PLA and PCL polymeric matrix reinforced with Mg, which is also bioabsorbable and has recognized osteoinductive capacity. The research collected in the literature reveals studies based on different manufacturing and dispersion processes of the reinforcement as well as the physicochemical analysis and corresponding biological evaluation to know the osteoinductive capacity of the proposed PLA/Mg and PCL/Mg composites. In short, this review shows the potential of these composite materials and serves as a guide for those interested in bioabsorbable materials applied in bone tissue engineering.

Metastable FeMg particles for controlling degradation rate, mechanical properties, and biocompatibility of Poly(l-lactic) acid (PLLA) for orthopedic applications

Poly(l-lactic) acid (PLLA) is commonly used in bioabsorbable medical implants, but it suffers from slow degradation rate and rapid decline in mechanical properties for orthopedic applications. To address this drawback, recent research has explored the use of Mg as a filler for PLLA, resulting in composites with improved degradation rate and cytocompatibility compared to neat PLLA. In this study, FeMg powder particles were proposed as fillers for PLLA to investigate the potential of PLLA/FeMg composites for bioabsorbable implants. Cylinder specimens of PLLA, PLLA/Fe, PLLA/Mg and PLLA/FeMg were prepared using solvent casting followed by thermo-molding. The microstructure, thermal behavior, in vitro degradation behavior in simulated body fluid, mechanical properties and cytocompatibility of these composites were examined. The results indicate that the presence of FeMg particles prevents the deterioration of the composite mechanical properties, at least up to 14 days. Once a certain amount of degradation of the composite is reached, the degradation is faster than that of PLLA. Direct cytotoxicity assays revealed that pre-osteoblast MC3T3-E1 cells successfully adhered to and proliferated on the PLLA/FeMg surface. The inclusion of a low percentage of Mg into the Fe lattice not only accelerated the degradation rate of Fe but also improved its cytocompatibility. The enhanced degradation rate, mechanical properties, and osteoconductive properties of this composite make it a promising option for temporary orthopedic biomedical devices.

Accumulative Rolling Mg/PLLA Composite Membrane with Lamellar Heterostructure for Enhanced Bacteria Inhibition and Rapid Bone Regeneration

Developing composite materials with optimized mechanics, degradation, and bioactivity for bone regeneration has long been a crucial mission. Herein, a multifunctional Mg/Poly-l-lactic acid (Mg/PLLA) composite membrane based on the "materials plain" concept through the accumulative rolling (AR) method is proposed. Results show that at a rolling ratio of 75%, the comprehensive mechanical properties of the membrane in the rolling direction are self-reinforced significantly (elongation at break ≈53.2%, tensile strength ≈104.0 MPa, Young's modulus ≈2.13 GPa). This enhancement is attributed to the directional arrangement and increased crystallization of PLLA molecular chains, as demonstrated by SAXS and DSC results. Furthermore, the AR composite membrane presents a lamellar heterostructure, which not only avoids the accumulation of Mg microparticles (MgMPs) but also regulates the degradation rate. Through the contribution of bioactive MgMPs and their photothermal effect synergistically, the membrane effectively eliminates bacterial infection and accelerates vascularized bone regeneration both in vitro and in vivo. Notably, the membrane exhibits outstanding rat skull bone regeneration performance in only 4 weeks, surpassing most literature reports. In short, this work develops a composite membrane with a "one stone, four birds" effect, opening an efficient avenue toward high-performance orthopedic materials.

Thermal Properties and In Vitro Biodegradation of PLA-Mg Filaments for Fused Deposition Modeling

Additive manufacturing, in particular the fused deposition method, is a quite new interesting technique used to obtain specific 3D objects by depositing layer after layer of material. Generally, commercial filaments can be used in 3D printing. However, the obtention of functional filaments is not so easy to reach. In this work, we obtain filaments based on poly(lactic acid), PLA, reinforced with different amounts of magnesium, Mg, microparticles, using a two-step extrusion process, in order to study how processing can affect the thermal degradation of the filaments; we additionally study their in vitro degradation, with a complete release of Mg microparticles after 84 days in phosphate buffer saline media. Therefore, considering that we want to obtain a functional filament for further 3D printing, the simpler the processing, the better the result in terms of a scalable approach. In our case, we obtain micro-composites via the double-extrusion process without degrading the materials, with good dispersion of the microparticles into the PLA matrix without any chemical or physical modification of the microparticles.

Mechanism and characterization of microplastic aging process: A review

With the increasing production of petroleum-based plastics, the problem of environmental pollution caused by plastics has aroused widespread concern. Microplastics, which are formed by the fragmentation of macro plastics, are bio-accumulate easily due to their small size and slow degradation under natural conditions. The aging of plastics is an inevitable process for their degradation and enhancement of adsorption performance toward pollutants due to a series of changes in their physiochemical properties, which significantly increase the toxicity and harm of plastics. Therefore, studies should focus on the aging process of microplastics through reasonable characterization methods to promote the aging process and prevent white pollution. This review summarizes the latest progress in natural aging process and characterization methods to determine the natural aging mechanism of microplastics. In addition, recent advances in the artificial aging of microplastic pollutants are reviewed. The degradation status and by-products of biodegradable plastics in the natural environment and whether they can truly solve the plastic pollution problem have been discussed. Findings from the literature pointed out that the aging process of microplastics lacks professional and exclusive characterization methods, which include qualitative and quantitative analyses. To lessen the toxicity of microplastics in the environment, future research directions have been suggested based on existing problems in the current research. This review could provide a systematic reference for in-depth exploration of the aging mechanism and behavior of microplastics in natural and artificial systems.

Additive manufacturing of PLA-Mg composite scaffolds for hard tissue engineering applications

Polylactic acid (PLA) is considered as a great option to be employed as 3D porous scaffold in hard tissue engineering applications owing to its excellent biocompatibility and processability. However, relatively weak mechanical properties and inappropriate biodegradability limit its extensive usage. In order to overcome the mentioned challenges, micrometric magnesium particles were incorporated into the PLA matrix by the fused deposition modeling (FDM) technique. The effects of various Mg contents (i.e., 2, 4, 6, 8 and 10 wt%) on the structural, thermal, rheological, mechanical, wettability, degradability characteristics and cellular behavior of the 3D porous PLA-Mg composite scaffolds were examined. The developed PLA-Mg composites exhibit an interconnected porous structure with a mostly uniform distribution of Mg particles in the PLA matrix. It was found that incorporation of Mg particles into the PLA matrix enhances the mechanical, physical, chemical and biological characteristics of PLA. The cell studies demonstrate that the PLA-6Mg composite scaffold provides the best cellular response in terms of cell atachment and viability. The obtained results in this investigation greatly suggest that the 3D-printed PLA-Mg composite scaffold is a promising candidate for hard tissue engineering applications.

Graphene Nanoplatelets-Based Textured Polymeric Fibrous Fabrics for the Next-Generation Devices

Graphene is a 2D crystal composed of carbon atoms in a hexagonal arrangement. From their isolation, graphene nanoplatelets (nCD) have revolutionized material science due to their unique properties, and, nowadays, there are countless applications, including drug delivery, biosensors, energy storage, and tissue engineering. Within this work, nCD were combined with PLA, a widely used and clinically relevant thermoplastic polymer, to produce advanced composite texturized electrospun fabric for the next-generation devices. The electrospinning manufacturing process was set-up by virtue of a proper characterization of the composite raw material and its solution. From the morphological point of view, the nCD addition permitted the reduction of the fiber diameter while the texture allowed more aligned fibers. After that, mechanical features of fabrics were tested at RT and upon heating (40 °C, 69 °C), showing the reinforcement action of nCD mainly in the texturized mats at 40 °C. Finally, mats' degradation in simulated physiological fluid was minimal up to 30 d, even if composite mats revealed excellent fluid-handling capability. Moreover, no toxic impurities and degradation products were pointed out during the incubation. This work gains insight on the effects of the combination of composite carbon-based material and texturized fibers to reach highly performing fabrics.

Performance of PEO/Polymer Coatings on the Biodegradability, Antibacterial Effect and Biocompatibility of Mg-Based Materials

Magnesium (Mg) alloys have recently attracted attention in biomedicine as biodegradable materials with non-toxic degradable products. Such compounds have become a frontier in the study of biodegradable materials because of their remarkable biomechanical compatibility and superior biocompatibility. The use of Mg-based implants reduces the negative consequences of permanent biological implants by eliminating the necessity for biomaterial surgery following the healing process. However, the quick deterioration, formation of considerable gas of hydrogen volumes and a rise in the body environment pH are obstacles in the application of Mg as an implant material. Hence, compelling advances for erosion resistance and biocompatibility of magnesium and its alloys are noteworthy. Surface modification may be a practical approach because it improves the erosion resistance compared with extensive preparation of a treated surface for progressed bone recovery and cell attachment. Coating produced by plasma electrolytic oxidation (PEO) seems a compelling method in order to enhance magnesium and the properties of its alloys. PEO-formed coatings cannot provide long-term protection in the physiological environment due to their porous nature. Thus, a polymer coating is applied on the porous PEO-formed coating, which is steadily applied on the surface. Polymer coatings improve the biocompatibility properties of Mg and its alloys and increase corrosion resistance. In this article, the most recent advancements in PEO/polymer composite coatings are reviewed, and the biocompatibility of such coatings is examined.

A biodegradable magnesium surgical staple for colonic anastomosis: In vitro and in vivo evaluation

Staplers have been widely used in the clinical treatment of gastrointestinal reconstruction. However, the current titanium (Ti) staple will remain in the human body permanently, resulting in some adverse effects. In this study, we developed a type of biodegradable staple for colonic anastomosis using 0.3 mm diameter magnesium (Mg) alloy wires. The wire surface was modified by micro-arc oxidation treatment (MAO) and then coated with poly-l-lactic acid (PLLA) to achieve a moderate degradation rate matching the tissue healing process. The results of tensile tests on isolated porcine colon tissue anastomosed by Mg and Ti staples showed that the anastomotic property of Mg staples was almost equal to that of Ti staples. The in vitro degradation tests indicated the dual-layer coating effectively enhanced the corrosion resistance and maintained the tensile force of the coated staple stable after 14-day immersion in the simulated colonic fluid (SCF). Furthermore, 24 beagle dogs were employed to conduct a comparison experiment using Mg-based and clinical Ti staples for 90-day implantation by ent-to-side anastomosis of the colon. The integrated structure of Mg-based staples was observed after 7 days and completely degraded after 90 days. All animals did not have anastomotic leakage and stenosis, and 12 dogs with Mg-based staples fully recovered after 90 days without differences in visceral ion levels and other side effects. The favorable performance makes this Mg-based anastomotic staple an ideal candidate for colon reconstruction.

Controlled magnesium ion delivery system for in situ bone tissue engineering

Magnesium cation (Mg²⁺) has been an emerging therapeutic agent for inducing vascularized bone regeneration. However, the therapeutic effects of current magnesium (Mg) -containing biomaterials are controversial due to the concentration- and stage-dependent behavior of Mg²⁺. Here, we first provide an overview of biochemical mechanism of Mg²⁺ in various concentrations and suggest that 2-10 mM Mg²⁺in vitro may be optimized. This review systematically summarizes and discusses several types of controlled Mg²⁺ delivery systems based on polymer-Mg composite scaffolds and Mg-containing hydrogels, as well as their design philosophy and several parameters that regulate Mg²⁺ release. Given that the continuous supply of Mg²⁺ may prevent biomineral deposition in the later stage of bone regeneration and maturation, we highlight the controlled delivery of Mg²⁺ based dual- or multi-ions system, especially for the hierarchical therapeutic ion release system, which shows enhanced biomineralization. Finally, the remaining challenges and perspectives of Mg-containing biomaterials for future in situ bone tissue engineering are discussed as well.

Imaging moiety-directed co-assembly for biodegradation control with synchronous four-modal biotracking

The complexity of existing methods for biodegradation control limits the multi-functionality of biomedical materials. It is urgent to develop simple and straightforward strategies to control the biodegradation rate with precise tracking of various parameters in real-time. Here, we show an imaging moiety-directed co-assembly strategy, in which different imaging moieties bearing non-covalent interaction sites are covalently introduced into the poly (D,l-lactic acid) (PDLLA) chain as end groups, followed by alternate non-covalent interactions with polymer chains upon compression molding. This strategy takes advantage of a variety of bonding types (including CH-π, CH-F, etc.) to firmly integrate the PDLLA chains and strongly control the biodegradation rate, making the amorphous prototype degraded much slower than higher-molecular-weight counterparts, and the local inflammatory response is insignificant. On this basis, a synchronous four-modal (X-ray computed tomography + fluorescence + photoacoustics + ultrasound) imaging was achieved on the single entity in vivo, even within a millimeter-scale thick-skin tissue. These imaging signals can precisely correlate the multi parameter variation trend of material mass, volume and molecular weight, signifying that co-assembly can be utilized to develop advanced theranostic systems. SINGLE SENTENCE SUMMARY: We developed an imaging moiety-directed co-assembly strategy to control the biodegradation rate and achieve the synchronization of real-time four-modal imaging in vivo. These imaging signals can precisely correlate the multi-parameter variation trend of material mass, volume and molecular weight, which provided comprehensive biomedical information accessing both qualitatively and quantitatively.

Structure-controlled lignin complex for PLA composites with outstanding antibacterial, fluorescent and photothermal conversion properties

Polylactic acid (PLA) is increasingly being considered as an ideal biodegradable candidate to replace petroleum-based polymers. However, its practical applications are often hampered by the poor mechanical robustness and solo functionality. Herein, based on the mechanical property improvement of PLA we proposed a simple process of assembling lignin-hybridized modifier and PLA matrix, as opposed to the traditional trade-off between mechanical strength and functionality, while anchoring a biofluorescent moiety onto lignin surfaces. Specifically, the fluorophore group could act as interfacial compatibilizer of complex and facilitate the shape-tailored hybrids for promoting functionality flexibility. With the bimetallic hetero-particles, the preferable lignin-assembled complex could be controllably configured as an antibacterial, fluorophore and photothermal agent. Thus, mechanical enhancement, fluorescence introduction and favorable photothermal ability of the resulting PLA composites were successfully achieved for integrated unification of structural robustness, geometric integrity and functional multiplicity, which was never seen in the other reports. The results showed that PLA composites containing 5 wt% modified lignin, 10 wt% zinc oxide, and 5 wt% silver presented excellent mechanical, fluorescent, photothermal conversion properties. By controlling the ZnO content and morphology, strong inhibition of Escherichia coli (Gram-negative) than that of Staphylococcus Aureus (Gram-positive) was also observed. The flake-shaped ZnO /Ag hybrids contributed to better overall performance of PLA composites than the rod-shaped ZnO/Ag. In this work we developed a facile strategy to assemble a bioderived fluorophore with lignin particles for constructing a structure-controlled complex as a multitasking modifier, featuring mechanical unity and functional adaptability. Specifically, the lignin reinforcement and bimetallic hybrids with different morphologies were explored as an effective fluorophore, antibacterial and photothermal agent. Through multiple dehydration reactions, a conjugating fluorophore was successfully grafted on lignin surfaces to serve as an interface modifier without physical changing its structural robustness. And morphology-tailored hybrid was advantageously immobilized on predefined hetero-particle carrier of fluorescent lignin and endowed composites with desirable antimicrobial properties. The developed strategy would expand the functional applications of PLA materials in food packaging, biopharmaceuticals and simple fluorescent anti-counterfeiting.

Degradation of 3D-Printed Porous Polylactic Acid Scaffolds Under Mechanical Stimulus

Polylactic acid (PLA) is a biodegradable polymer commonly used as a scaffold material to repair tissue defects, and its degradation is associated with mechanical stimulus. In this study, the effect of mechanical stimulus on the degradation of 3D-printed PLA scaffolds was investigated by in vitro experiments and an author-developed numerical model. Forty-five samples with porosity 64.8% were printed to carry out the degradation experiment within 90 days. Statistical analyses of the mass, volume fraction, Young’s modulus, and number average molecular weight were made, and the in vitro experiments were further used to verify the proposed numerical model of the scaffold degradation. The results indicated that the mechanical stimulus accelerated the degradation of the PLA scaffold, and the higher mechanical stimulus led to a faster degradation of the scaffolds at the late stage of the degradation process. In addition, the Young’s modulus and the normalized number average molecular weight of the PLA scaffolds between the experiments and the numerical simulations were comparable, especially for the number average molecular weight. The present study could be helpful in the design of the biodegradable PLA scaffolds.

In vitro corrosion study of PLA/Mg composites for cardiovascular stent applications

The present investigation explores the impact of Mg volume fraction (V_Mg) as a controlling parameter of degradation rate in designing patient-specific cardiovascular stents made of PLA/Mg composites. For the purpose of this research, PLA/Mg composite plates containing 1, 3, 5, and 10% V_Mg are produced by melt blending and hot press molding. Characterization techniques such as scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) are employed to study the microstructure of PLA/Mg composites. For in vitro corrosion tests, stent prototypes and composite samples are immersed in baths of simulated body fluid (SBF). According to in vitro corrosion tests, increasing V_Mg increases the corrosion rate of the composites by accelerating the corrosion of the particles and the crystalline zones surrounding them. In addition, a 2% raise in the Mg content (from 1% to 3%), increases the overall Mg weight loss by more than 4 times. Composite samples and prototype stents containing more than 5% V_Mg exhibit cracking and brittleness after 7 days of immersion in SBF. In light of the compression tests results and also the failures and cracks observed during immersions, the upper limit of Mg content for PLA/Mg stent fabrication purposes is found to be below 3%.

Strength, corrosion resistance and cellular response of interfaces in bioresorbable poly-lactic acid/Mg fiber composites for orthopedic applications

The shear strength and the corrosion resistance of the fiber/matrix interface after immersion in simulated body fluid was studied in poly-lactic acid/Mg fiber composites. The shear strength of the interface was measured by means of push-out tests in thin slices of the composite perpendicular to the fibers. It was found that the interface strength dropped from 15.2 ± 1.4 MPa to 7.8 ± 3.7 MPa after the composite was immersed in simulated body fluid for 148 h. The reduction of the interface strength was associated to the fast corrosion of the fibers as water diffused to the interface through the polymer. The expansion of the fibers due to the formation of corrosion products was enough to promote radial cracks in the polymer matrix which facilitate the ingress of water and the development of corrosion pitting in the fibers. Moreover, cell culture testing on the material showed that early degradation of the Mg fibers affected the proliferation of pre-osteoblasts near the Mg fibers due to the local changes in the environment produced by the fiber corrosion. Thus, surface modification of Mg fibers to delay degradation seems to be a critical point for further development of Mg/PLA composites for biomedical applications.

The influence of alternating cyclic dynamic loads with different low frequencies on the bio-corrosion behaviors of AZ31B magnesium alloy in vitro

The real physiological environment of human body is complicated with different degrees and forms of dynamic loads applied to implanted medical devices due to the daily activities of the patients, which would have impacts on the degradation behaviors of magnesium alloy implants. In this work, the bio-corrosion behaviors of AZ31B magnesium alloy under alternating cyclic dynamic loads with different low frequencies (0.1–2.5 Hz) were specially investigated. It was found that the bio-degradation performances under external dynamic stressed conditions were much severer than those under unstressed conditions and static loads. The corrosion rates were generally accelerated as the rise of cyclic frequency. Hereby a numerical model for the degradation process of Mg alloy was established. The corrosion current density i_corr of Mg alloy and the applied loading frequency f matches a linear relationship of ln i_corr∝ f, which is the result of interactions between the cyclic alternating load and corrosive environment. This work could provide a theoretical reference and an experimental basis for further researches on the biodegradation behaviors of biomedical materials under dynamic conditions.

(1)

Biocorrosion of Mg alloy under dynamic load with different frequencies were studied.

(2)

A stress apparatus was designed to achieve in-situ evaluation under dynamic loads.

(3)

The rising dynamic loading frequency resulted in an increase in the corrosion rate.

(4)

Theoretical corrosion mechanism under dynamic load was systematically discussed.

(5)

A numerical relationship between corrosion rate and load frequency was determined.

Antimicrobial Polymeric Composites with Embedded Nanotextured Magnesium Oxide

Nanotextured magnesium oxide (MgO) can exhibit both antibacterial and tissue regeneration activity, which makes it very useful for implant protection. To successfully combine these two properties, MgO needs to be processed within an appropriate carrier system that can keep MgO surface available for interactions with cells, slow down the conversion of MgO to the less active hydroxide and control MgO solubility. Here we present new composites with nanotextured MgO microrods embedded in different biodegradable polymer matrixes: poly-lactide-co-glycolide (PLGA), poly-lactide (PLA) and polycaprolactone (PCL). Relative to their hydrophilicity, polarity and degradability, the matrices were able to affect and control the structural and functional properties of the resulting composites in different manners. We found PLGA matrix the most effective in performing this task. The application of the nanotextured 1D morphology and the appropriate balancing of MgO/PLGA interphase interactions with optimal polymer degradation kinetics resulted in superior bactericidal activity of the composites against either planktonic E. coli or sessile S. epidermidis, S. aureus (multidrug resistant-MRSA) and three clinical strains isolated from implant-associated infections (S. aureus, E. coli and P. aeruginosa), while ensuring controllable release of magnesium ions and showing no harmful effects on red blood cells.

Controlled SrR Delivery by the Incorporation of Mg Particles on Biodegradable PLA-Based Composites

Among several ions playing a vital role in the body, Sr²⁺ and Mg²⁺ are involved in the mechanism of bone formation, making them especially useful for bone tissue engineering applications. Recently, polylactic acid (PLA)/Mg composites have emerged as a promising family of biomaterials due to their inherent biocompatibility and biodegradability properties. In these composites, polymer and bio-metal have a synergetic effect—while the PLA inhibits the Mg fast reactivity, Mg provides bioactivity to the inert polymer buffering the medium pH during degradation. Meanwhile, the typical form of administrating Sr²⁺ to patients is through the medication strontium ranelate (SrR), which increases the bone mineral density. Following this interesting research line, a new group of composites, which integrates Mg particles and SrR charged onto halloysite nanotubes (HNT) in a polymeric matrix, was proposed. PLA/Mg/SrR–HNT composites have been processed following a colloidal route, obtaining homogenous composites granulated and film-shaped. The drug delivery profile was evaluated in terms of in vitro lixiviation/dissolution paying special attention to the synergism of both ions release. The combination of two of the most reported ions involved in bone regeneration in the composite biomaterial may generate extra interest in bone healing applications.

Programmable immune activating electrospun fibers for skin regeneration

Immune cells play a crucial regulatory role in inflammatory phase and proliferative phase during skin healing. How to programmatically activate sequential immune responses is the key for scarless skin regeneration. In this study, an "Inner-Outer" IL-10-loaded electrospun fiber with cascade release behavior was constructed. During the inflammatory phase, the electrospun fiber released a lower concentration of IL-10 within the wound, inhibiting excessive recruitment of inflammatory cells and polarizing macrophages into anti-inflammatory phenotype "M2c" to suppress excessive inflammation response. During the proliferative phase, a higher concentration of IL-10 released by the fiber and the anti-fibrotic cytokines secreted by polarized "M2c" directly acted on dermal fibroblasts to simultaneously inhibit extracellular matrix overdeposition and promote fibroblast migration. The "Inner-Outer" IL-10-loaded electrospun fiber programmatically activated the sequential immune responses during wound healing and led to scarless skin regeneration, which is a promising immunomodulatory biomaterial with great potential for promoting complete tissue regeneration.

Biodegradable Poly(Lactic Acid) Nanocomposites for Fused Deposition Modeling 3D Printing

3D printing by fused deposition modelling (FDM) enables rapid prototyping and fabrication of parts with complex geometries. Unfortunately, most materials suitable for FDM 3D printing are non-degradable, petroleum-based polymers. The current ecological crisis caused by plastic waste has produced great interest in biodegradable materials for many applications, including 3D printing. Poly(lactic acid) (PLA), in particular, has been extensively investigated for FDM applications. However, most biodegradable polymers, including PLA, have insufficient mechanical properties for many applications. One approach to overcoming this challenge is to introduce additives that enhance the mechanical properties of PLA while maintaining FDM 3D printability. This review focuses on PLA-based nanocomposites with cellulose, metal-based nanoparticles, continuous fibers, carbon-based nanoparticles, or other additives. These additives impact both the physical properties and printability of the resulting nanocomposites. We also detail the optimal conditions for using these materials in FDM 3D printing. These approaches demonstrate the promise of developing nanocomposites that are both biodegradable and mechanically robust.

A biodegradable multifunctional nanofibrous membrane for periodontal tissue regeneration

Biomaterial-based membranes represent a promising therapeutic option for periodontal diseases. Although conventional periodontal membranes function greatly in preventing the ingrowth of both fibroblasts and epithelial cells as well as connective tissues, they are not capable of promoting periodontal tissue regeneration. Here, we report a multifunctional periodontal membrane prepared by electrospinning biodegradable polymers with magnesium oxide nanoparticles (nMgO). nMgO is a light metal-based nanoparticle with high antibacterial capacity and can be fully resorbed in the body. Our results showed that incorporating nMgO into poly(L-lactic acid) (PLA)/gelatin significantly improved the overall properties of membranes, including elevated tensile strength to maintain structural stability and adjusted degradation rate to fit the time window of periodontal regeneration. Acidic degradation products of PLA were neutralized by alkaline ions from nMgO hydrolysis, ameliorating pH microenvironment beneficial for cell proliferation. In vitro studies demonstrated considerable antibacterial and osteogenic properties of nMgO-incorporated membranes that are highly valuable for periodontal regeneration. Further investigations in a rat periodontal defect model revealed that nMgO-incorporated membranes effectively guided periodontal tissue regeneration. Taken together, our data indicate that nMgO-incorporated membranes might be a promising therapeutic option for periodontal regeneration. STATEMENT OF SIGNIFICANCE: Traditional clinical treatments of periodontal diseases largely focus on the management of the pathologic processes, which cannot effectively regenerate the lost periodontal tissue. GTR, a classic method for periodontal regeneration, has shown promise in clinical practice. However, the current membranes might not fully fulfill the criteria of ideal membranes. Here, we report bioabsorbable nMgO-incorporated nanofibrous membranes prepared by electrospinning to provide an alternative for the clinical practice of GTR. The membranes not only function greatly as physical barriers but also exhibit high antibacterial and osteoinductive properties. We therefore believe that this study will inspire more practice work on the development of effective GTR membranes for periodontal regeneration.

Malic Enzyme Facilitates d-Lactate Production through Increased Pyruvate Supply during Anoxic Dark Fermentation in Synechocystis sp. PCC 6803

d-Lactate is one of the most valuable compounds for manufacturing biobased polymers. Here, we have investigated the significance of endogenous malate dehydrogenase (decarboxylating) (malic enzyme, ME), which catalyzes the oxidative decarboxylation of malate to pyruvate, in d-lactate biosynthesis in the cyanobacterium Synechocystis sp. PCC6803. d-Lactate levels were increased by 2-fold in ME-overexpressing strains, while levels in ME-deficient strains were almost equivalent to those in the host strain. Dynamic metabolomics revealed that overexpression of ME led to increased turnover rates in malate and pyruvate metabolism; in contrast, deletion of ME resulted in increased pool sizes of glycolytic intermediates, probably due to sequential feedback inhibition, initially triggered by malate accumulation. Finally, both the loss of the acetate kinase gene and overexpression of endogenous d-lactate dehydrogenase, concurrent with ME overexpression, resulted in the highest production of d-lactate (26.6 g/L) with an initial cell concentration of 75 g-DCW/L after 72 h fermentation.

Development of biocompatible and fully bioabsorbable PLA/Mg films for tissue regeneration applications

During recent years, Mg reinforced polylactic acid (PLA) composites have emerged as potential biocompatible and bioabsorbable materials for biomedical applications. It has been shown that Mg particles added to a matrix based on a biodegradable polymer can address the lack of bioactivity and the low mechanical properties of the polymers and, furthermore, it can counteract the detrimental effects associated to the high degradation rate of Mg, as alkalinization and elevated H₂ release. Additionally, the polymer can protect the Mg particles, by tailoring their degradation rate. Former processing of these composites performed by extrusion, compression and injection molding employed Mg contents up to 10 wt%. Higher amounts of Mg resulted in heterogeneous materials and thermally degraded matrices, with the corresponding higher degradation rate. In the present work, Mg reinforced PLA films with Mg content as high as 50 wt% were obtained without compromising the thermal stability of the polymer. Firstly, a successful dispersion of Mg microparticles was achieved by a breakthrough in processing introducing a colloidal step where organic additives were added to modify the Mg particle surface and promote a chemically stable suspension. The resulting colloidal suspension was then used as feedstock to obtain composite films by tape casting. The films show advantageous in vitro behaviour in terms of degradation, hydrogen release and oxygen permeability. In addition, the viability with fibroblast cells (MEF) opens a window of opportunity for these composite films as bioabsorbable material for tissue engineering and wound dressing applications. STATEMENT OF SIGNIFICANCE: Magnesium materials have extraordinary biodegradable properties and bioactive behavior due to release of Mg²⁺ ions, which offer a promising opportunity for their applicability as biomaterials for tissue regeneration. However, Mg is one of the most reactive metals with a high degradation rate. In contact with water produces H₂, associated with a risk of failure of the implant. One alternative to minimize this drawback is the use of Mg particles surrounded by a biodegradable biocompatible polymer such as polylactic acid (PLA) to obtain PLA/Mg composites. In this work we processed Mg reinforced PLA in the shape of films that would be suitable for tissue regeneration. In vitro behavior of PLA/Mg films demonstrated that Mg²⁺ ions increase the fibroblast cells growth.

In vitro degradation behavior of Mg wire/poly(lactic acid) composite rods prepared by hot pressing and hot drawing

In this study, we prepared Mg wire/poly(lactic acid) composite rods by hot pressing (HP) and hot drawing (HD) processes, which show desirable application potential as a biodegradable implant in orthopedics. The influences of the volume content of Mg wires and processing steps on the degradation behavior of composite rods as well as the mutual influence between Mg and poly(lactic acid) (PLA) during degradation were investigated. During degradation, the improved interface bonding between Mg and PLA by micro-arc oxidation can effectively inhibit the diffusion of the medium from the ends to the central part, supported by XR-CT results, which show the degradation cracks in PLA initiated and propagated along the radial direction of the rod rather than the axial direction. When compared with the hot pressed rod, the three passes hot drawn rod had lower degradation rate and better strength retention during long-time immersion. This is ascribed to the higher crystallinity of the PLA matrix by HD. For both hot pressed and hot drawn rods, more Mg content of the wires will result in an increase in the degradation rate of PLA due to alkaline catalytic hydrolysis. Therefore, we can not only predict the degradation period by the present degradation mathematical model but also further control degradation rate by adjusting processing steps and wire volume content. This will be helpful for future material design and indication selection. STATEMENT OF SIGNIFICANCE: We investigated the influence of interface microstructure, self-reinforced PLA matrix, and wire volume content on the degradation behavior and clarified the differences in the degradation mechanism of composite rods. The 3D morphology of cracks in composite rods enlightened us a clear understanding about the emergence and evolution of cracks, which revealed the reason for the failure of composite rods. The degradation model can be used to predict the degradation period and provide valuable information for their future application.

Effect of stress on corrosion of high-purity magnesium in vitro and in vivo

Magnesium-based implants are subjected to complicated stresses during implantation in the human body. The stress effects on corrosion of magnesium (Mg) in vitro were investigated in previous studies, whereas in this study, the corrosion behaviors of high-purity (HP) Mg under stress were comparatively studied in vitro in Hank's solution and in vivo in the subcutaneous environment of rats. Loading devices were designed to apply compressive stress (15.1 ± 0.5 MPa) and tensile stress (13.2 ± 0.2 MPa) on HP Mg specimens both in vitro and in vivo. Corrosion rates of HP Mg were characterized by mass and volume losses. It was shown that the applied compressive stress had no effect on in vitro corrosion behaviors and the applied tensile stress accelerated the in vitro corrosion, thereby causing severe pitting corrosions and stress corrosion cracking (SCC). However, there was no significant change for corrosion behaviors in vivo under neither compressive stress nor tensile stress. Severe pitting corrosion and SCC did not occur in vivo. Histological evaluation revealed that a fibrotic capsule induced by foreign body reaction was formed on the corrosion surfaces of HP Mg in the subcutaneous environment. It was proposed that the fibrotic capsule suppressed the effects of stress in vivo by protecting the corrosion surfaces. These results provided new insights into understanding the stress effects on the corrosion of Mg both in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Mg and its alloys have shown potential as biodegradable metallic materials. During implantation, Mg is subjected to various mechanical environments in the human body. It is necessary to have a clear understanding of different effects of stress on Mg corrosion. However, few studies were performed in vivo. It is important to analyze the effect of quantitative stress on Mg corrosion in vivo. Therefore, in this study, quantitative stresses were applied on Mg both in vitro and in vivo. The effects of stress on in vitro and in vivo corrosions of Mg were investigated and compared.

Advances in functionalized polymer coatings on biodegradable magnesium alloys - A review

Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and excellent biomechanical compatibility. However, their high degradation rate in the physiological environment should be well tackled prior to clinical applications. This review summarizes the latest progress in the development of polymeric coatings on biodegradable Mg alloys over the last decade, regarding preparation strategies for polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), polydopamine (PDA), chitosan (CS), collagen (Col) and their composite, and their performance in terms of corrosion resistance and biocompatibility. Feasible perspectives and developing directions of next generation of polymeric coatings with respect to biomedical Mg alloys are briefly discussed.

STATEMENT OF SIGNIFICANCE

Magnesium (Mg) and its alloys have become a research frontier in biodegradable materials owing to their superior biocompatibility and suitable biomechanical compatibility. However, the principal drawback of Mg-based implants is their poor corrosion resistance in physiological environments. Hence, it is vital to mitigate the degradation/corrosion behavior of Mg alloys for safe biomedical deployments. This review summarizes the latest progress in development of polymeric coatings on biomedical Mg alloys regarding preparation strategy, corrosion resistance and biocompatibility, including polylactic acid (PLA), poly (latic-co-glycolic) acid (PLGA), polycaprolactone (PCL), chitosan (CS), polydopamine (PDA), collagen (Col) and their composite. In addition, functionalized polymer coatings with Mg alloys exhibits a promising prospect owing to their ability of degradation along with biocompatibility, self-healing, drug-delivery and osteoinduction.

The Work Softening Behavior of Pure Mg Wire during Cold Drawing

We performed multiple-pass cold drawing for pure Mg wire which showed excellent formability (~138% accumulative true strain) at room temperature. Different from the continuous work hardening occurring during cold drawing of Mg alloy wires, for pure Mg, an initially rapid increase in hardness and strength was followed by significant work softening and finally reached a steady-state level, approximately 40~45 HV. The work softening can be attributed to the dynamic recovery and recrystallization of pure Mg at room temperature. Meanwhile, an abrupt change in texture component also was detected with the transition from work hardening to softening in the strain range of 28~34%. During the whole drawing, the strongest texture component gradually transformed from as-extruded basal to <101¯0> fiber (~28% accumulative true strain), and then rapidly returned to the weak basal texture.

Influence of dynamic compressive loading on the in vitro degradation behavior of pure PLA and Mg/PLA composite

The effects of dynamic compressive loading on the in vitro degradation behavior of pure poly-lactic acid (PLA) and PLA-based composite unidirectionally reinforced with micro-arc oxidized magnesium alloy wires (Mg/PLA) are investigated. Dynamic compressive loading is shown to accelerate degradation of pure PLA and Mg/PLA. As the applied stress is increased from 0.1MPa to 0.9MPa or frequency from 0.5Hz to 2.5Hz, the overall degradation rate goes up. After immersion for 21days at 0.9MPa and 2.5Hz, the bending strength retention of the composite and pure PLA is 60.1% and 50%, respectively. Dynamic loading enhances diffusion of small acidic molecules resulting in significant pH decrease in the immersion solution. The synergistic reaction between magnesium alloy wires and PLA in the composite is further clarified by electrochemical tests. The degradation behavior of the pure PLA and PLA matrix in the composite under dynamic conditions obey the first order degradation kinetics and a numerical model is postulated to elucidate the relationship of the bending strength, stress, frequency, and immersion time under dynamic conditions.

STATEMENT OF SIGNIFICANCE

We systematically study the influence of dynamic loading on the degradation behavior of pure PLA and Mg/PLA. Dynamic compressive loading is shown to accelerate degradation of pure PLA and Mg/PLA. The synergistic reaction between magnesium alloy wires and PLA in the composite is firstly clarified by electrochemical tests. The degradation behavior of the pure PLA and PLA matrix in the composite under dynamic conditions obey the first order degradation kinetics. Then, a numerical model is postulated to elucidate the relationship of the bending strength, stress, frequency, and immersion time under dynamic conditions.

Substrate Specificity and Allosteric Regulation of a D-Lactate Dehydrogenase from a Unicellular Cyanobacterium are Altered by an Amino Acid Substitution

Lactate/lactic acid is an important chemical compound for the manufacturing of bioplastics. The unicellular cyanobacterium Synechocystis sp. PCC 6803 can produce lactate from carbon dioxide and possesses d-lactate dehydrogenase (Ddh). Here, we performed a biochemical analysis of the Ddh from this cyanobacterium (SyDdh) using recombinant proteins. SyDdh was classified into a cyanobacterial clade similar to those from Gram-negative bacteria, although it was distinct from them. SyDdh can use both pyruvate and oxaloacetate as a substrate and is activated by fructose-1,6-bisphosphate and repressed by divalent cations. An amino acid substitution based on multiple sequence alignment data revealed that the glutamine at position 14 and serine at position 234 are important for the allosteric regulation by Mg²⁺ and substrate specificity of SyDdh, respectively. These results reveal the characteristic biochemical properties of Ddh in a unicellular cyanobacterium, which are different from those of other bacterial Ddhs.