Systematical evolution on a Zn-Mg alloy potentially developed for biodegradable cardiovascular stents.

To reduce the long-term side effects of permanent metallic stents, a new generation of cardiovascular stents called "biodegradable stents" is being extensively developed. Zinc has been considered as a promising candidate material for biodegradable cardiovascular stents due to its excellent biocompatibility and appropriate biodegradability. However, weak mechanical properties limit its further clinic application. In this study, hot extruded pure Zn and Zn-0.02 Mg alloy were prepared. Compared with pure Zn, Zn-0.02 Mg alloy showed more homogeneous microstructure, much smaller grain size and higher mechanical strength. Zn-0.02 Mg alloy presented uniform corrosion morphologies during the immersion process, and its corrosion rates was higher than that of pure Zn. Hemocompatibility results showed that the Zn-based alloy had extremely low hemolysis rate (0.74 ± 0.15%) and strong inhibitory effect on blood coagulation, platelet adhesion and aggregation. Zn-0.02 Mg alloy also exhibited excellent cytocompatibility. Its extracts could significantly promote the proliferation of endothelial cells. Moreover, the antibacterial activities of the Zn-based alloy were demonstrated by spread plate assay, live/dead viability assay and bacterial morphology observation. These results indicate that the extruded Zn-0.02 Mg alloy has a potential in biodegradable cardiovascular stents.

Fabrication, biodegradation behavior and cytotoxicity of Mg-nanodiamond composites for implant application.

Mg-based biodegradable implants offer several advantages over their non-degradable or degradable polymeric counterparts used today. However, the low corrosion resistance of Mg in physiologic environment remained as concerns. In this research, nanodiamond (ND) was uniformly dispersed in Mg matrix to induce a protective layer on Mg surface during corrosion. Compared with pure Mg, fabricated Mg-ND nanocomposites had lower corrosion rates, higher corrosion potential, and higher corrosion resistance. Specifically, the corrosion rate of Mg was reduced by 4.5 times by adding 5 wt% of ND particles. Corrosion inhibition effect of ND was thus validated. The chemical interaction and physical adsorption of the ions from simulated body fluid on ND might be the main reason for enhanced corrosion resistance. In vitro biocompatibility test results indicated that Mg-ND nanocomposites were biocompatible since cells growing in contact with corrosion products of Mg-ND maintained high cell viability and healthy morphology. Therefore, Mg-ND nanocomposites with homogenous ND dispersion, enhanced corrosion resistance, and good biocompatibility might be an excellent candidate material for biodegradable implant application.

Biomimetic design and fabrication of porous chitosan­gelatin liver scaffolds with hierarchical channel network.

The presence of a hierarchical channel network in tissue engineering scaffold is essential to construct metabolically demanding liver tissue with thick and complex structures. In this research, chitosan­gelatin (C/G) scaffolds with fine three-dimensional channels were fabricated using indirect solid freeform fabrication and freeze-drying techniques. Fabrication processes were studied to create predesigned hierarchical channel network inside C/G scaffolds and achieve desired porous structure. Static in-vitro cell culture test showed that HepG2 cells attached on both micro-pores and micro-channels in C/G scaffolds successfully. HepG2 proliferated at much higher rates on C/G scaffolds with channel network, compared with those without channels. This approach demonstrated a promising way to engineer liver scaffolds with hierarchical channel network, and may lead to the development of thick and complex liver tissue equivalent in the future.

Biocompatibility and biodegradation studies of PCL/ß-TCP bone tissue scaffold fabricated by structural porogen method.

Three-dimensional printer (3DP) (Z-Corp) is a solid freeform fabrication system capable of generating sub-millimeter physical features required for tissue engineering scaffolds. By using plaster composite materials, 3DP can fabricate a universal porogen which can be injected with a wide range of high melting temperature biomaterials. Here we report results toward the manufacture of either pure polycaprolactone (PCL) or homogeneous composites of 90/10 or 80/20 (w/w) PCL/beta-tricalcium phosphate (ß-TCP) by injection molding into plaster composite porogens fabricated by 3DP. The resolution of printed plaster porogens and produced scaffolds was studied by scanning electron microscopy. Cytotoxicity test on scaffold extracts and biocompatibility test on the scaffolds as a matrix supporting murine osteoblast (7F2) and endothelial hybridoma (EAhy 926) cells growth for up to 4 days showed that the porogens removal process had only negligible effects on cell proliferation. The biodegradation tests of pure PCL and PCL/ß-TCP composites were performed in DMEM with 10 % (v/v) FBS for up to 6 weeks. The PCL/ß-TCP composites show faster degradation rate than that of pure PCL due to the addition of ß-TCP, and the strength of 80/20 PCL/ß-TCP composite is still suitable for human cancellous bone healing support after 6 weeks degradation. Combining precisely controlled porogen fabrication structure, good biocompatibility, and suitable mechanical properties after biodegradation, PCL/ß-TCP scaffolds fabricated by 3DP porogen method provide essential capability for bone tissue engineering.

In vivo biocompatibility and degradability of a Zn-Mg-Fe alloy osteosynthesis system.

Zinc is generally considered to be one of the most promising materials to be used in biodegradable implants, and many zinc alloys have been optimized to improve implant biocompatibility, degradation, and mechanical properties. However, long-term degradation leads to the prolonged presence of degradation products, which risks foreign body reactions. Herein, we investigated the in vivo biocompatibility and degradation of a biodegradable Zn-Mg-Fe alloy osteosynthesis system in the frontal bone, mandible, and femur in beagles for 1 year. Results of the routine blood, biochemical, trace element, and histological analyses of multiple organs, peripheral blood CD4/CD8a levels, and serum interleukin 2 and 4 levels showed good biocompatibility of the Zn-Mg-Fe alloy. Zinc content analysis revealed zinc accumulation in adjacent bone tissue, but not in the liver, kidney, and spleen, which was related to the degradation of the Zn-Mg-Fe alloy. The alloy demonstrated a uniform slowing degradation rate in vivo. No degradation differences in the frontal bone, mandible, and femur were observed. The degradation products included zinc oxide [ZnO], zinc hydroxide [Zn(OH)2], hydrozincite [Zn5(OH)6(CO3)2], and hopeite [Zn3(PO4)2·4H2O]. The good biocompatibility and degradation properties of the Zn-Mg-Fe alloy render it a very attractive osteosynthesis system for clinical applications.

Controllable growth of gradient porous structures.

Cocontinuous phase structures of immiscible polymers can be developed under appropriate melt-blending conditions. Because of the presence of interfacial tension, such cocontinuous structures start to coarsen when heated to a temperature higher than the melting/softening temperature of both phases. In this study, a method for controllable growth of gradient porous structures utilizing variable coarsening rates in a gradient temperature field was investigated. The phase structure coarsens at a higher rate in higher temperature regions but at a slower rate in lower temperature regions, resulting in the generation of a gradient phase morphology. Subsequent dissolution of one phase in the binary blend yields a gradient porous structure made of the remaining polymer component. A polystyrene/poly(lactic acid) (PLA) blend was used as a model system. By designing proper thermal boundary conditions and introducing different thermal gradients during annealing, different types of gradient porous structures of PLA were created.

In vivo study of the efficacy, biosafety, and degradation of a zinc alloy osteosynthesis system.

In this study, a comprehensive analysis of a novel zinc alloy osteosynthesis system in a canine mandibular fracture model is presented. The efficacy of the system was compared for PLLA (poly-l-lactic acid) and titanium materials using X-ray radiography, micro-CT tomography, undecalcified bone histomorphometry, and a three-point bending test. Histology, blood normal, blood biochemical, and serum zinc concentration tests were also performed to assess the biosafety of the zinc alloy osteosynthesis system. The degradability of the zinc alloy was evaluated using a micro-CT and scanning electron microscope during the 24-week post operation period. The results showed that zinc alloy possesses good mechanical properties that support fracture healing. Its uniform and slow corrosion leads to adequate degradation behavior in 24 weeks. Additionally, the zinc alloy proved to be biocompatible, indicating that this novel osteosynthesis system is safe for use in the body. The results of the study demonstrate that this zinc alloy-based osteosynthesis system is a promising candidate for a new generation of osteosynthesis systems, with further improvements required in the future.

Corrosion behavior and biocompatibility evaluation of a novel zinc-based alloy stent in rabbit carotid artery model.

Zinc (Zn) and its alloys have been proved to be promising candidate materials for biodegradable cardiovascular stents. In this study, a novel extruded Zn-0.02 Mg-0.02Cu alloy was prepared. Compared with pure Zn, the Zn-based alloy showed higher mechanical properties, and the Zn-based alloy could significantly accelerate Zn2+ release, reaching 0.61 ± 0.11 µg/mL at 15 days of immersion. In vitro biocompatibility studies demonstrated that the Zn-based alloy had excellent cytocompatibility and hemocompatibility, including low hemolysis rate (0.63 ± 0.12%) and strong inhibitory effect on platelet adhesion. Subsequently, the Zn-based alloy stent was implanted into the left carotid arteries of New Zealand white rabbits for 12 months. All the rabbits survived without any adverse clinical events, and all the stented arteries were patent during the study period. Rapid endothelialization at 1 week of implantation was observed, suggesting a low cytotoxicity and thrombosis risk. The stent corroded slowly and no obvious intimal hyperplasia was observed for 6 months, after which corrosion accelerated at 12 months. In addition, no obvious thrombosis and systemic toxicity during implantation period were observed, indicating its potential as the backbone of biodegradable cardiovascular stents. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1814-1823, 2019.

A study on in vitro degradation behavior of a poly(glycolide-co-L-lactide) monofilament.

The in vitro degradation behaviors of a poly(glycolide-co-l-lactide) 90/10 monofilament were investigated in phosphate buffer solution at pH 7.4 over a temperature range of 27.5-47.5 degrees C. The property changes of the monofilament with time at different temperatures were evaluated by tensile mechanical test, gel permeation chromatography analysis, and image techniques (optical microscopy, scanning electron microscopy and atomic force microscopy). The interrelationships among material properties, in vitro time and experimental conditions were explored. The results showed that the polymer monofilament gradually lost its tensile strength and molecular weight with increasing in vitro time. The hydrolytic degradation of the monofilaments followed a first order behavior. Higher temperatures accelerated the degradation process significantly. It was found that for a given tensile breaking strength retention (BSR), the dependence of degradation time on temperature could be illustrated by an Arrhenius-type equation, from which the activation energy was derived. Further analysis indicated that there are well-defined relationships between molecular weight and tensile strength, which could be illustrated mathematically. Finally, the microscopic evaluation of the monofilament samples revealed visible changes in morphology on the surface and cross-section area during degradation process. The results from atomic force microscopy showed that the surface roughness of the monofilament tended to increase with the in vitro time.

A bioabsorbable microcatheter for the continuous and direct treatment of surgical sites.

A bioabsorbable microcatheter comprising of a polyglycolideco-lactide microtube and an absorbable braid is developed for the continuous delivery of therapeutic fluids directly to a surgical site. Studies conducted with tissue staining dye and radio-opaque solution using porcine models indicate that infusion of fluids in the wound site is rapid and complete. The tissue reaction to the microtube and braid is generally characterized by minimal to mild chronic inflammation or foreign body reaction that decreases in severity as the material is absorbed. The microtube is considered to be absorbed at 56 days postimplantation while the bulk of the braid is considered to be absorbed by 70 days postimplantation.

Porogen-based solid freeform fabrication of polycaprolactone-calcium phosphate scaffolds for tissue engineering.

Drop on demand printing (DDP) is a solid freeform fabrication (SFF) technique capable of generating microscale physical features required for tissue engineering scaffolds. Here, we report results toward the development of a reproducible manufacturing process for tissue engineering scaffolds based on injectable porogens fabricated by DDP. Thermoplastic porogens were designed using Pro/Engineer and fabricated with a commercially available DDP machine. Scaffolds composed of either pure polycaprolactone (PCL) or homogeneous composites of PCL and calcium phosphate (CaP, 10% or 20% w/w) were subsequently fabricated by injection molding of molten polymer-ceramic composites, followed by porogen dissolution with ethanol. Scaffold pore sizes, as small as 200 microm, were attainable using the indirect (porogen-based) method. Scaffold structure and porosity were analyzed by scanning electron microscopy (SEM) and microcomputed tomography, respectively. We characterized the compressive strength of 90:10 and 80:20 PCL-CaP composite materials (19.5+/-1.4 and 24.8+/-1.3 Mpa, respectively) according to ASTM standards, as well as pure PCL scaffolds (2.77+/-0.26 MPa) fabricated using our process. Human embryonic palatal mesenchymal (HEPM) cells attached and proliferated on all scaffolds, as evidenced by fluorescent nuclear staining with Hoechst 33258 and the Alamar Blue assay, with increased proliferation observed on 80:20 PCL-CaP scaffolds. SEM revealed multilayer assembly of HEPM cells on 80:20 PCL-CaP composite, but not pure PCL, scaffolds. In summary, we have developed an SFF-based injection molding process for the fabrication of PCL and PCL-CaP scaffolds that display in vitro cytocompatibility and suitable mechanical properties for hard tissue repair.

In vitro biodegradation behavior, mechanical properties, and cytotoxicity of biodegradable Zn-Mg alloy.

Zinc-Magnesium (Zn-Mg) alloy as a novel biodegradable metal holds great potential in biodegradable implant applications as it is more corrosion resistant than Magnesium (Mg). However, the mechanical properties, biodegradation uniformity, and cytotoxicity of Zn-Mg alloy remained as concerns. In this study, hot extrusion process was applied to Zn-1 wt % Mg (Zn-1Mg) to refine its microstructure. Effects of hot extrusion on biodegradation behavior and mechanical properties of Zn-1Mg were investigated in comparison with Mg rare earth element alloy WE43. Metallurgical analysis revealed significant grain size reduction, and immersion test found that corrosion rates of WE43 and Zn-1Mg were reduced by 35% and 57%, respectively after extrusion. Moreover, hot extrusion resulted in a much more uniform biodegradation in extruded Zn-1Mg alloy and WE43. In vitro cytotoxicity test results indicated that Zn-1Mg alloy was biocompatible. Therefore, hot extruded Zn-1Mg with homogenous microstructure, uniform as well as slow degradation, improved mechanical properties, and good biocompatibility was believed to be an excellent candidate material for load-bearing biodegradable implant application.

Mechanical study of polycaprolactone-hydroxyapatite porous scaffolds created by porogen-based solid freeform fabrication method.

MATERIALS AND METHODS: Polycaprolactone (PCL) and polycaprolactone-hydroxyapatite (PCL-HA) scaffolds with 600-µm pore size were fabricated by drop-on-demand printing (DDP) structured porogen method followed with injection molding. Specimens with special dimensions of 4.2×4.2×5.4 mm3 and 6.6×6.6×13.8 mm3 were designed and fabricated for compression and tensile tests, respectively. The mechanical study was performed on both solid and porous PCL and PCL-HA samples. The effect on mechanical properties of the HA content ratio in PCL-HA composites was investigated. RESULTS: Porous scaffold made of 80/20 PCL-HA composite had an ultimate compressive strength of 3.7±0.2 MPa and compression modulus of 61.4±3.4 MPa, which is in the range of reported trabecular bone's compressive strength. Increasing the concentration of HA in the composites raised compressive properties and stiffness significantly (P<0.05), which demonstrates that PCL-HA composites have the potential for application in bone regeneration. Tensile test of solid PCL and PCL-HA composites showed that the ultimate tensile strength and tensile modulus increased with increases of the concentration of HA in the composites. The tensile test was also conducted on PCL porous scaffold; the result indicated that the scaffold was slightly softer and weaker in tension compared with compression. CONCLUSIONS: Combining compression and tensile test results, our study may guide the possible application of these biomaterials in bone tissue engineering and support further development of microstructure-based models of scaffold mechanical properties.

Design, fabrication, and characterization of archaeal tetraether free-standing planar membranes in a PDMS- and PCB-based fluidic platform.

The polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius contains exclusively bipolar tetraether lipids, which are able to form extraordinarily stable vesicular membranes against a number of chemical, physical, and mechanical stressors. PLFE liposomes have thus been considered appealing biomaterials holding great promise for biotechnology applications such as drug delivery and biosensing. Here we demonstrated that PLFE can also form free-standing "planar" membranes on micropores (∼100 µm) of polydimethylsiloxane (PDMS) thin films embedded in printed circuit board (PCB)-based fluidics. To build this device, two novel approaches were employed: (i) an S1813 sacrificial layer was used to facilitate the fabrication of the PDMS thin film, and (ii) oxygen plasma treatment was utilized to conveniently bond the PDMS thin film to the PCB board and the PDMS fluidic chamber. Using electrochemical impedance spectroscopy, we found that the dielectric properties of PLFE planar membranes suspended on the PDMS films are distinctly different from those obtained from diester lipid and triblock copolymer membranes. In addition to resistance (R) and capacitance (C) that were commonly seen in all the membranes examined, PLFE planar membranes showed an inductance (L) component. Furthermore, PLFE planar membranes displayed a relatively large membrane resistance, suggesting that, among the membranes examined, PLFE planar membrane would be a better matrix for studying channel proteins and transmembrane events. PLFE planar membranes also exhibited a sharp decrease in phase angle with the frequency of the input AC signal at ∼1 MHz, which could be utilized to develop sensors for monitoring PLFE membrane integrity in fluidics. Since the stability of free-standing planar lipid membranes increases with increasing membrane packing tightness and PLFE lipid membranes are more tightly packed than those made of diester lipids, PLFE free-standing planar membranes are expected to be considerably stable. All these salient features make PLFE planar membranes particularly attractive for model studies of channel proteins and transmembrane events and for high-throughput drug screening and artificial photosynthesis. This work can be extended to nanopores of PDMS thin films in microfluidics and eventually aid in membrane-based new lab-on-a-chip applications.

Mechanical properties and biomineralization of multifunctional nanodiamond-PLLA composites for bone tissue engineering.

Multifunctional bone scaffold materials have been produced from a biodegradable polymer, poly(L-lactic acid) (PLLA), and 1-10% wt of octadecylamine-functionalized nanodiamond (ND-ODA) via solution casting followed by compression molding. By comparison to pure PLLA, the addition of 10% wt of ND-ODA resulted in a significant improvement of the mechanical properties of the composite matrix, including a 280% increase in the strain at failure and a 310% increase in fracture energy in tensile tests. The biomimetic process of bonelike apatite growth on the ND-ODA/PLLA scaffolds was studied using microscopic and spectroscopic techniques. The enhanced mechanical properties and the increased mineralization capability with higher ND-ODA concentration suggest that these biodegradable composites may potentially be useful for a variety of biomedical applications, including scaffolds for orthopedic regenerative engineering.

Fluorescent PLLA-nanodiamond composites for bone tissue engineering.

Superior mechanical properties, rich surface chemistry, and good biocompatibility of diamond nanoparticles make them attractive in biomaterial applications. A multifunctional fluorescent composite bone scaffold material has been produced utilizing a biodegradable polymer, poly(l-lactic acid) (PLLA), and octadecylamine-functionalized nanodiamond (ND-ODA). The uniform dispersion of nanoparticles in the polymer led to significant increase in hardness and Young's modulus of the composites. Addition of 10%wt of ND-ODA resulted in more than 200% increase in Young's modulus and 800% increase in hardness, bringing the nanocomposite properties close to that of the human cortical bone. Testing of ND-ODA/PLLA as a matrix supporting murine osteoblast (7F2) cell growth for up to 1 week showed that the addition of ND-ODA had no negative effects on cell proliferation. ND-ODA serves as a multifunctional additive providing improved mechanical properties, bright fluorescence, and options for drug loading and delivery via surface modification. Thus ND-ODA/PLLA composites open up numerous avenues for their use as components of bone scaffolds and smart surgical tools such as fixation devices in musculoskeletal tissue engineering and regenerative medicine. Intense fluorescence of ND-ODA/PLLA scaffolds can be used to monitor bone re-growth replacing the implant in vivo.

Fabrication of interconnected microporous biomaterials with high hydroxyapatite nanoparticle loading.

Hydroxyapatite (HA) is known to promote osteogenicity and enhance the mechanical properties of biopolymers. However, incorporating a large amount of HA into a porous biopolymer still remains a challenge. In the present work, a new method was developed to produce interconnected microporous poly(glycolic-co-lactic acid) (PLGA) with high HA nanoparticle loading. First, a ternary blend comprising PLGA/PS (polystyrene)/HA (40/40/20 wt%) was prepared by melt blending under conditions for formation of a co-continuous phase structure. Next, a dynamic annealing stage under small-strain oscillation was applied to the blend to facilitate nanoparticle redistribution. Finally, the PS phase was sacrificially extracted, leaving a porous matrix. The results from different characterizations suggested that the applied small-strain oscillation substantially accelerated the migration of HA nanoparticles during annealing from the PS phase to the PLGA phase; nearly all HA particles were uniformly presented in the PLGA phase after a short period of annealing. After dissolution of the PS phase, a PLGA material with interconnected microporous structure was successfully produced, with a high HA loading above 30 wt%. The mechanisms beneath the experimental observations, particularly on the enhanced particle migration process, were discussed, and strategies for producing highly particle loaded biopolymers with interconnected microporous structures were proposed.

Thermal stability and melt rheology of poly(p-dioxanone).

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO's thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples' viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (Mw = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox-Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.

Effects of temperature and strain level on stress relaxation behaviors of polypropylene sutures.

An investigation has been conducted on stress relaxation behaviors of polypropylene sutures under different temperatures and strain levels in a temperature-controlled water bath. The study showed that the temperature and strain level significantly affected the stress relaxation behaviors of the sutures. High temperature resulted in fast stress relaxation. The stress relaxation data could be well described by two empirical formulas. For most of the test conditions, the stress relaxation tests caused limited permanent deformation in sutures. Effects of temperature on the permanent deformation may be illustrated by a power law. The tensile properties of the sutures were not affected adversely by the stress relaxation tests, indicating good properties of polypropylene sutures.