Pea pod-mimicking hydroxyapatite nanowhisker-reinforced poly(lactic acid) composites with bone-like strength

Fabrication and Characterization of PLA/PBAT Blends, Blend-Based Nanocomposites, and Their Supercritical Carbon Dioxide-Induced Foams

In this study, a twin-screw extruder was used to fabricate poly(lactic acid) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) blends and blend-based nanocomposites with carbon nanotube (CNT) or nanocarbon black (CB) as nanofillers. The fabricated samples were subsequently treated with supercritical carbon dioxide (scCO2) to fabricate the corresponding foams. Bi-phasic morphology and selective distribution of CNTs or CBs in the PBAT phase were observed in the blends/composites through scanning electron microscopy. After the scCO2 treatment, the selective foaming of the PBAT phase in the prepared blends/composites was confirmed. The cellular structure of PBAT phase in scCO2-treated blends is similar to the size/shape of PBAT domains in untreated blends or treated neat PBAT foam. The addition of CNTs or CBs in the blends led to a slight reduction in cell size of the foamed PBAT phase, demonstrating CNT/CB-induced cell nucleation. Differential scanning calorimetry (DSC) results showed that CNTs and CBs played as nucleating agents and increased the initial crystallization temperature up to 14 °C compared with neat PBAT for PBAT in different composites during cooling. The scCO2 treatment induced the bimodal stability of PBAT crystals in different samples, which melted mainly in two temperature regions in DSC studies. Thermogravimetric analyses revealed that compared with parent blends, the addition of CNTs or CBs increased the temperature at 80 wt.% loss (degradation of PBAT portion) up to 6 °C. The electrical resistivity decreased by more than six orders of magnitude for certain CNT- or CB-added composites compared with the parent blends. The hardness of the blends slightly increased after forming the corresponding composites and then declined after the scCO2 treatment.

Tetracalcium phosphate/porous iron synergistically improved the mechanical, degradation and biological properties of polylactic acid scaffolds

Synergistically improving the mechanical and degradable properties of polylactic acid (PLA) scaffolds and endowing them with bioactivity are urgent problems to be solved in deepening their application in tissue engineering. In this work, tetracalcium phosphate (TTCP) and porous iron (pFe) were compounded by stirring and vacuum negative pressure, and then they were blended with polylactic acid and a porous scaffold (named TTCP@pFe/PLA) was prepared by selective laser sintering. On the one hand, molten polylactic acid penetrates the pores of porous iron to form an interlocking network, thereby achieving mechanical strengthening. On the other hand, the alkaline environment generated by the dissolution of tetracalcium phosphate can effectively catalyze the hydrolysis of polylactic acid to accelerate the degradation. Meanwhile, the dissolution of tetracalcium phosphate forms a local calcium-rich microenvironment, which rapidly induces apatite formation, that is, confers bioactivity on scaffolds. As a result, the TTCP@pFe/PLA scaffold exhibited a notable enhancement in mechanical strength, being 2.2 times stronger compared to the polylactic acid scaffold. More importantly, MC3T3E1 cells exhibit good adhesion, stretching, and proliferation on the composite scaffold, demonstrating good cytocompatibility. All these good properties of the TTCP@pFe/PLA scaffold indicate that it has potential applications as a novel alternative in bone tissue regeneration.

Nanopatterning of beaded poly(lactic acid) nanofibers for highly electroactive, breathable, UV-shielding and antibacterial protective membranes

Despite great potential in fabrication of biodegradable protective membranes by electrospinning of poly(lactic acid) (PLA) nanofibers, it is still thwarted by smooth surfaces and poor electroactivity that challenge the promotion of electret properties and long-term air filtration performance. Here, a microwave-assisted synthetic method was used to customize dielectric TiO2 nanocrystals of ultrasmall and uniform dimensions (∼30 nm), which were homogeneously embedded at beaded PLA nanofibers (PLA@TiO2, diameter of around 280 nm) by the combined "electrospinning-electrospray" approach. With small amounts of TiO2 (2, 4 and 6 wt%), the nanopatterned PLA@TiO2 nanofibrous membranes (NFMs) were characterized by largely increased dielectric constants (nearly 1.9), surface potential (up to 1.63 kV) and triboelectric properties (output voltage of 12.2 V). Arising from the improved electroactivity and self-charging mechanisms, the nanopatterned PLA@TiO2 NFMs exhibited remarkable PM0.3 filtration properties (97.9 %, 254.6 Pa) even at the highest airflow rate of 85 L/min, surpassing those of pure PLA membranes (86.2 %, 483.7 Pa). This was moreover accompanied by inhibition rates of 100 % against both E. coli and S. aureus, as well as excellent UV-blocking properties (UPF as high as 3.8, TUVA of 50.9 % and TUVB of 20.1 %). The breathable and electroactive nanopatterned PLA NFMs permit promising applications in multifunctional protective membranes toward excellent UV shielding and high-efficiency removal of both PMs and pathogens.

Stable nanoscale sea-island structure of biobased polyamide 56/poly (butylene adipate-co-terephthalate) blends compatibilized by interfacial hyperbranched structure: Toward biobased polymer blends with ultrahigh toughness

Developing biobased materials is a considerably effective approach to save fossil resources and reduce emissions. Biobased polyamide 56 (PA56) is an excellent engineering material, but it has low toughness. Herein, to enhance the toughness of PA56, an ultra-tough biodegradable material, i.e., poly (butylene adipate-co-terephthalate) (PBAT) was introduced into PA56. Moreover, a self-synthesized epoxy-terminated hyperbranched polyester (EHBP) was used to improve the compatibility of the blended materials. The results of differential scanning calorimetry and Fourier-transform infrared spectroscopy indicated that the epoxide group of EHBP could react with PA56 and PBAT to form a block-like polymer structure and limit the crystallization behavior of the blends. The scanning electron microscopy results show that the addition of EHBP considerably reduced the dispersed-phase size in the blends, forming a nanoscale island structure. Moreover, the hydrogen bonds formed between EHBP and PA56/PBAT enhanced the intermolecular interaction between the two materials. Thus, PA56 blends with ultrahigh toughness were successfully prepared. The prepared PA56/PBAT/EHBP blend exhibited a notch impact strength of 20.71 kJ/m2 and a breaking elongation of 38.3 %, which represent increases of 427.3 % and 252.8 %, respectively, compared with those of pure PA56. Thus, the proposed method is suitable for toughening PA56 and broadening its applications.

Improvement of the mechanical and shape memory properties in polylactide/polyethylene glycol blends by reactive graphene oxide

The widespread application of biodegradable polylactide (PLA) is hindered by its brittleness. Polyethylene glycol (PEG) is commonly utilized as a plasticizer because of its favorable compatibility with PLA. However, the incorporation of PEG considerably diminishes the tensile strength of PLA. To address this issue, reactive isocyanate-modified graphene oxide (mGO) was synthesized and used as an enhancer in PLA/PEG blends. By virtue of the reaction between the isocyanate group in mGO and the terminal hydroxyl groups of PLA and PEG, graphene-based polyurethane (PU) in-situ formed and enhanced the interface between GO and the matrix. Consequently, the PLA/PEG/mGO composites exhibit simultaneously improved tensile and impact strengths, achieving an increase of 20.6% and 29.4%, respectively, compared to PLA/PEG blends. Moreover, the in situ formed PU reduces the relaxation time of the molecule motion and improved the entanglement density, thereby improving the shape-memory recovery rate and final recovery degree of the composites. This work provides a facile method to simultaneously improve the dispersion of GO and enhance its interface with polymer, thereby supplying well comprehensive properties of PLA and extending the applications of biodegradable polymers.

Strontium-Substituted Nanohydroxyapatite-Incorporated Poly(lactic acid) Composites for Orthopedic Applications: Bioactive, Machinable, and High-Strength Properties

Traditional metal-alloy bone fixation devices provide structural support for bone repair but have limitations in actively promoting bone healing and often require additional surgeries for implant removal. In this study, we focused on addressing these challenges by fabricating biodegradable composites using poly(lactic acid) (PLA) and strontium-substituted nanohydroxyapatite (SrHAP) via melt compounding and injection molding. Various percentages of SrHAP (5, 10, 20, and 30% w/w) were incorporated into the PLA matrix. We systematically investigated the structural, morphological, thermal, mechanical, rheological, and dynamic mechanical properties of the prepared composites. Notably, the tensile modulus, a critical parameter for orthopedic implants, significantly improved from 2.77 GPa in pristine PLA to 3.73 GPa in the composite containing 10% w/w SrHAP. The incorporation of SrHAP (10% w/w) into the PLA matrix led to an increased storage modulus, indicating a uniform dispersion of SrHAP within the PLA and good compatibility between the polymer and nanoparticles. Moreover, we successfully fabricated screws using PLA composites with 10% (w/w) SrHAP, demonstrating their formability at room temperature and radiopacity when observed under X-ray microtomography (micro-CT). Furthermore, the water contact angle decreased from 93 ± 2° for pristine PLA to 75 ± 3° for the composite containing SrHAP, indicating better surface wettability. To assess the biological behavior of the composites, we conducted in vitro cell-material tests, which confirmed their osteoconductive and osteoinductive properties. These findings highlight the potential of our developed PLA/SrHAP10 (10% w/w) composites as machinable implant materials for orthopedic applications. In conclusion, our study presents the fabrication and comprehensive characterization of biodegradable composites comprising PLA and strontium-substituted nanohydroxyapatite (SrHAP). These composites exhibit improved mechanical properties, formability, and radiopacity while also demonstrating desirable biological behavior. Our results suggest that these PLA/SrHAP10 composites hold promise as machinable implant materials for orthopedic applications.

Highly efficient metal-organic framework based intumescent poly(L-lactic acid) towards fire safety, ignition delay and UV resistance

Developing multifunctional biodegradable PLA with ignition delay, high efficient fire retardancy, and UV resistance properties is a challenging task owing to its high flammability, and mutually exclusive phenomenon between the latter two properties. In this work, we report a superior efficient synergistic action combining piperazine pyrophosphate (PAPP) and a Co based metal-organic framework (ZIF-67). Results illustrated that with merely 0.06 wt% ZIF-67, intumescent PLA containing 4.96 wt% PAPP reached UL-94 V0 rating. The PLA/4.9PAPP/0.1MOF sample possessed a limiting oxygen index (LOI) value at 33 %, exhibited a 28 % reduction in peak heat release rate (pHRR) and a 67 % increase in fire propagation index (FPI). Moreover, the presence MOF delayed the ignition time of PLA by 12 s due to the highly porous structure of MOF and its chemical heat-sink performance. Insightful reaction to fire mechanism in the condensed phase via TG-FTIR and Raman revealed that a crack free protective intumescent char layer with higher graphitization degree was formed, which effectively enhanced the barrier effect and minimize the heat and fuel transfer. In addition, the UV resistance of PLA composites is enhanced, remaining at and below 5 % transmittance in the UVA and UVB areas. This work provides a green production way of multifunctional degradable materials and broadens their application fields.

Preparation and characterization of novel poly (lactic acid)/calcium oxide nanocomposites by electrospinning as a potential bone tissue scaffold

Calcium oxide nanoparticles (n-CaO) ca. 22 nm were obtained from eggshell waste. The n-CaO was incorporated into the PLA matrix in 10 and 20 wt% of filler content by electrospinning process to get PLA/n-CaO fibers with homogenous morphology and diameter as a potential use in scaffold for bone tissue regeneration. The incorporation of n-CaO into PLA modifies the mechanical properties, having a reinforcement effect on the matrix. The Young modulus for PLA/n-CaO nanocomposites increased between 122 and 138 % concerning neat PLA fibers, showing a more rigid behavior. The PLA/n-CaO nanocomposite fibers showed in vitro bioactivity, capable of inducing the precipitation of hydroxyapatite (HA) layer in the fiber surface after seven days in SBF solution. The biocidal and biological properties of PLA/n-Cao with 20 wt% showed a 30 % reduction in bacterial viability against S. aureus and 11 % against E. coli after 6 h of bacterial exposure. Furthermore, the fibers did not show a cytotoxic effect on the bone marrow ST-2 cell line, allowing cell adhesion and proliferation in the RPMI medium. The PLA/n-CaO with 20 wt% of nanoparticles showed a higher capacity to promote osteogenic differentiation, significantly increasing the alkaline phosphatase (ALP) expression after seven days compared to PLA and cell control. The in vivo analysis corroborated the biocompatibility of the prepared scaffolds; the presence of n-CaO in PLA reduced the formation of fibrous encapsulation of the material, improving the healing process. These results validated using n-CaO to enhance the functionality of polymer matrices as a PLA, bringing bioactive, biocide, and biocompatible properties, opening a new and interesting route to develop new biomaterials as a scaffold for bone tissue engineering.

Super toughened blends of poly(lactic acid) and poly(butylene adipate-co-terephthalate) injection-molded foams via enhancing interfacial compatibility and cellular structure

Biodegradable poly(lactic acid) (PLA) foams have drawn increasing attention due to environmental challenges and petroleum crisis. However, it still remains a challenge to prepare PLA foams with fine cellular structures and high impact property, which significantly hinders its widespread application. Herein, phase interface-enhanced PLA/ poly(butylene adipate-co-terephthalate) (PBAT) blend foam, modified by a reactive compatibilizer through a simple reactive extrusion, was produced via a core-back foam injection molding technique. The obtained PLA blend foams displayed an impact strength as high as 49.1 kJ/m², which was 9.3 and 6.4 times that of the unmodified PLA/PBAT blend and its corresponding foam, respectively. It proved that the interfacial adhesion and cell size both strongly affected the impact strength of injection-molded PLA/PBAT foams, and two major conclusions were proposed. First, enhancing interfacial adhesion could cause a brittle-tough transition of PLA/PBAT foams. Additionally, for foams with high interfacial adhesion, small cell size (<12 μm) was more favorable for the stretching of cells and extension of the whitened region in comparison with big cell size (cell size >60 μm), leading to the drastic toughening of PLA blends. This study provides a feasible, industrially scalable and practical strategy to prepare super toughened and fully biodegradable PLA materials.

Toward ultra-tough and heat-resistant biodegradable polylactide/core-shell rubber blends by regulating the distribution of rubber particles with stereocomplex crystallites

Ultra-tough and heat-resistant poly(l-lactide)/core-shell rubber (PLLA/CSR) blends were fabricated by utilizing stereocomplex (SC) crystallites to effectively regulate the CSR distribution in PLLA matrix. Linear and 3-11 armed poly(d-lactide)s (PDLAs) were synthesized and then melt-mixed with PLLA/CSR blend. Interestingly, the incorporated PDLA chains could collaborate with PLLA chains to form dense SC crystallites network in PLLA/PDLA/CSR blends, thus inducing the CSR particles to transform from uniform distribution structure to network-like structure. With increasing the PDLA arm numbers, the size of CSR clusters in the network-like structure first increased and then decreased, and the continuity of the network-like structure first remained at a high level and then decreased obviously. The formation of CSR network-like structure could remarkably improve the impact strength of PLLA/PDLA/CSR blends without deteriorating their strength and modulus (compared with PLLA/CSR blend), and the CSR network-like structure with larger-sized CSR clusters and higher continuity could help obtain higher impact strength (78.3 kJ/m²). Moreover, the heat resistance of PLLA/PDLA/CSR blends could also be significantly improved (the highest Vicat softening temperature was 131 °C) by the SC crystallites network and CSR network-like structure. This work provides an effective strategy for controlling the rubber network-like morphology and thereby preparing high-performance PLLA materials.

Structural composite based on 3D printing polylactic acid/carbon fiber laminates (PLA/CFRC) as an alternative material for femoral stem prosthesis

In recent years, surgical procedures for hip prostheses have increased. These implants are manufactured with materials with high stiffness compared to the bone, causing bone loss or aseptic loosening. This research proposes an alternative structural composite consisting of 3D-printing polylactic acid layers and carbon fiber laminates (PLA/CFRC) with potential application in prosthetic implants. Fourier-transform infrared spectroscopy (FTIR) achieved to characterize starting materials and structural composites revealed secondary chemical interactions between the carbonyl group of PLA with the hydroxyl group of epoxy resin from CFRC. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results show both components (PLA and CFRC) influence the structural composite's thermal behavior, observed in the temperatures of degradation, glass transition, and melting. Furthermore, the composite reached cell viability above 80%, a tensile modulus of 19.29 ± 0.48 GPa and tensile strength of 238.91 ± 25.95 MPa, with mechanical properties very similar to the bone. The results of this study demonstrated that the proposed PLA/CFRC composite can be used as candidate base material for the manufacturing of a hip femoral stem prostheses.

Enhancing the osteogenic differentiation of aligned electrospun poly(L-lactic acid) nanofiber scaffolds by incorporation of bioactive calcium silicate nanowires

Bone defects cause serious psychological and economic burden to patients. Artificially bone repairing materials bring hope to the treatment of bone defects. Electrospun technique has attracted great attention since it can fabricate fibers from nano- to micro- scale continuously. Scaffolds fabricated by electrospun can mimic the structure of extracellular matrix which is beneficial to cell adhesion and migration. Researches have showed that bioactive ions (such as silicon and calcium ions) can promote bone regeneration. In addition, physical cues can affect cellular behavior such as cell adhesion and differentiation. In this study, two kinds of calcium silicate - adopted poly (L-lactic acid) (CS-PLLA) electrospun scaffolds with random/aligned structures were prepared by electrospun to promote bone regeneration. The integration of CS nanowires improved the biological property of PLLA electrospun scaffolds. Furthermore, in vitro results indicated that aligned 1 wt% CS adopted PLLA (PCA1) electrospun scaffolds with better physical properties and facilitated cell adhesion, improved alkaline phosphate (ALP) activity and the expression of osteogenic genes (Osteopontin (OPN), Collagen type 1 (Col-1) and Bone morphogenetic protein-2 (BMP-2)) compared with random 1 wt% CS adopted PLLA (PCR1) electrospun scaffolds. In conclusion, the prepared PCA1 electrospun scaffolds might be a potential candidate for bone regeneration in defect areas.

The synergistic effect of heterogeneous nucleation and stress-induced crystallization on supramolecular structure and performances of poly(lactic acid) melt-spun fibers

As a kind of bio-based polymer, poly (lactic acid) has potential application in fibers fields. Due to the weak nucleation ability, PLA crystallizes slowly and forms large spherulites during the forming process, which deteriorates the properties of PLA fibers. In this work, melt-spun method is employed for the fabrication of PLA/T composite fibers using succinate diphenyl dihydrazide (TMC-306) as the nucleating agent, and then the hot-drawing and heat setting is performed to the as-spun fibers. Compared with pure PLA fibers, PLA/T fibers show faster crystallization rate and improved performance due to the synergistic effect of heterogeneous nucleation and stress-induced crystallization. The characterization of non-isothermal crystallization behavior indicates that the peak crystallization temperature as well as crystallinity of PLA composites is increased to 121.5 °C and 36.78 % respectively by blending 0.3 wt% TMC-306. Meanwhile, the obtained PLA/0.3T composite fibers are highly crystallized and oriented at hot-drawing ratio of 2.4 folds and heat setting temperature of 100 °C, and the conformational stability is noticeably enhanced. Further, the tensile strength and storage modulus of PLA/0.3T composite fiber are 3.46 cN/dtex and 46,953 MPa respectively, which are increased by 42 % and 41 % compared with neat PLA fibers.

Biomineralization of bone-like hydroxyapatite to upgrade the mechanical and osteoblastic performances of poly(lactic acid) scaffolds

Biomimetic mineralization of high-strength apatite structure essentially relies on mimicking the inorganic building blocks of naturally occurring bones. However, conventional routes still have substantial function gaps in providing precision control over the geometrical dimensions and crystalline morphology of biomineralized apatite. Herein, we conceived the concept of microwave-assisted biomineralization (MAB) to customize 1D hydroxyapatite nanowhiskers (HANWs) at graphene templates, rendering the formation of graphene-hydroxyapatite (Gr-HA) nanohybrids. The HANWs essentially resembled bone apatite in elemental composition (Ca/P = 1.74), diameter (~20 nm), crystallinity (63 %), and rodlike geometry (aspect ratio of ~6). The Gr-HA nanohybrids were uniformly incorporated into poly(lactic acid) (PLA) microfibers (~1 μm) by electrospinning, engendering fibrous membranes with a set of Gr-HA loadings (10, 20 and 30 wt%). Intimate interactions were generated between Gr-HA and PLA matrix, contributing to significant promotion of the mechanical properties for PLA composite membranes. For example, the yield strength and elastic modulus of the PLA composite membranes loaded with 30 wt% Gr-HA achieved 5.4 and 66.4 MPa, increasing nearly 182 % and over 94 % compared to those of pure PLA, respectively. Moreover, the bone-like HANWs endowed PLA membranes with excellent cytocompatibility and good bioactivity, as demonstrated by over 38 % increase in cell viability and rapid apatite formation in mineral solution. The impressive combination of mechanical properties and biological characteristics make the PLA/Gr-HA scaffolds promising for guided tissue/bone regeneration therapy.

Evaluation of Mechanical Properties of Porous Chitosan/Gelatin/Polycaprolactone Bone Scaffold Prepared by Microwave Foaming Method

Bone tissue may suffer from bone injury and bone defects due to accidents or diseases. Since the demand for autologous bone and allograft tissue far exceeds the supply, bone scaffolds have taken the lead. The use of bone scaffolds is one of the measures to help heal or regenerate bone tissue. Therefore, a new bone scaffold was proposed in this study, which has a simpler preparation process and stronger performance. This study proposes bone scaffolds with an attempt to use polymers that are synthesized separately with three types of minerals as the filler using the microwave foaming method as follows. A 0.1 wt% of montmorillonite (MMT), zinc oxide (ZnO), or titanium dioxide (TiO₂) is added to chitosan (CS)/gelatin mixtures, respectively, after which sodium bicarbonate is added as a foaming agent, thereby forming porous gels. The polymer synthesized from three minerals was used as filler. The following microwave foaming method was adopted: 0.1 wt% MMT, ZnO, or TiO₂ was added to the CS/gelatin mixture, and then sodium bicarbonate was added as a foaming agent to form a porous gel. Next, porous gels and polycaprolactone were combined in a self-made mold in order to form bone scaffolds. A stereo microscope is used to observe the morphology of bone scaffolds, after which the pore size analysis, pore connectivity, swell property, porosity, and compressive strength are tested, examining the effects of the mineral type on bone scaffolds. The test results indicate that with MMT being the filler and sodium bicarbonate being the foaming agent, the resulting bone scaffolds yield a porous structure with a pore size between 120 μm and 370 μm. Besides, the incorporation of polycaprolactone also provides samples of 1MCG-P, 2MCG-P, and 5MCG-P with a certain compressive strength of 150–170 MPa. To sum up, the test results substantiate that a combination of the microwave foaming method and MMT generates a porous structure for bone scaffolds (1MCG-P, 2MCG-P, and 5MCG-P), involving a porosity of 38%, an inter-connected porous structure, and the compressive strength that exceeds 150 MPa.