Nanocomposite Foams with Balanced Mechanical Properties and Energy Return from EVA and CNT for the Midsole of Sports Footwear Application.

Polymer foam that provides good support with high energy return (low energy loss) is desirable for sport footwear to improve running performance. Ethylene-vinyl acetate copolymer (EVA) foam is commonly used in the midsole of running shoes. However, EVA foam exhibits low mechanical properties. Conventional mineral fillers are usually employed to improve EVA's mechanical performance, but the energy return is sacrificed. Here, we produced nanocomposite foams from EVA and multi-walled carbon nanotubes (CNT) using a chemical foaming process. Two kinds of CNT derived from the upcycling of commodity plastics were prepared through a catalytic chemical vapor deposition process and used as reinforcing and nucleating agents. Our results show that EVA foam incorporated with oxygenated CNT (O-CNT) demonstrated a more pronounced improvement of physical, mechanical, and dynamic impact response properties than acid-purified CNT (A-CNT). When CNT with weight percentage as low as 0.5 wt% was added to the nanocomposites, the physical properties, abrasion resistance, compressive strength, dynamic stiffness, and rebound performance of the EVA foams were improved significantly. Unlike the conventional EVA formulation filled with talc mineral fillers, the incorporation of CNT does not compromise the energy return of the EVA foam. From the long-cycle dynamic fatigue test, the CNT/EVA foam displays greater properties retention as compared to the talc/EVA foam. This work demonstrates a good balanced of mechanical-energy return properties of EVA nanocomposite foam with very low CNT content, which presents promising opportunities for lightweight-high rebound midsoles for running shoes.

On the Mode I and Mode II Delamination Characteristics and Surface Morphological Aspects of Composites with Carbon-Thermoplastic Hybrid Fabrics and Innovative Liquid Thermoplastic Resin.

In the current research, the delamination behavior under Mode I and Mode II loading for the hybrid carbon-thermoplastic fabrics in conjunction with novel liquid thermoplastic acrylic Elium® resin processable at ambient conditions was studied. The experimentation by incorporating doublers methodology, studying the performance under Mode I and Mode II loading, and understanding failure mechanisms using surface morphological fractography is deliberated. Hybrid Carbon-Ultra-high molecular weight polyethylene (UHMWPP)/Elium® composite has shown a 22.81% higher GIC and a 22.2% higher GIIC than Carbon-UHMWPP/Epoxy composite. On the contrary, the Carbon_Ultra-high molecular weight polypropylene (UHMWPE)/Elium® has shown an 11.11% higher Mode I critical energy release rate (GIC) and a 7.58% higher Mode II critical energy release rate (GIIC) than Carbon_UHMWPE/Epoxy composite. Hybrid fiber reinforced thermoplastic composites have shown severe plastic deformation of the matrix, rough fracture surface, and micro-cracks on the de-bonding surface, extensive fiber bridging, and crack branching which contributed to the improvement in the delamination behavior. Hybrid fiber architecture is also found to be detrimental by inducing crack arresting mechanisms including the tortuous crack path and the resin-rich pockets path due to the mismatch of the size of the fiber yarns.

Effect of PMMA Coupling Layer in Enhancing the Ultrasonic Weld Strength of Novel Room Temperature Curable Acrylic Thermoplastic to Epoxy Based Composites.

The joining of composites can be performed in an extremely short time with more energy-efficient ultrasonic welding techniques. The current research investigated the performance optimization of ultrasonic welding of carbon/Elium® composite to carbon/epoxy composite using a polymethyl methacrylate (PMMA) coupling interlayer. The weld strength was quantified by static lap shear strength (LSS) testing. A new methodology was used by creating a PMMA coupling layer on the epoxy composite adherend to achieve an improved interphase and thus enhance the weld properties. The LSS of Elium (EL)-Epoxy (EP) _0.25_0.25 was found to be 190% higher compared to that of EL-EP, confirming the effectiveness of the strategy used for creating an interlayer thermoplastic coupling layer. The time required for welding was optimized to be 2s as compared to 10 min required for adhesive bonding. Scanning electron microscopic images of epoxy and PMMA/Elium matrix interphase were observed to have a rough surface and remained largely unaffected by welding. There was an interphase change further away from the interphase to a rougher texture. There was little to no effect on the penultimate layer on the weld strength, as no interphase change could be observed after welding. Fractography investigation revealed shear cusps, matrix plastic deformation, fiber imprints, fiber pull-out, and good adhesion between matrix and fiber, features seen for configuration with maximum LSS. The current research findings present a way to join Elium® with epoxy composites that could be used in applications that require a selective strengthening, such as in sporting goods and consumer products. Furthermore, a detailed investigation is ongoing to use different filler particles and coupling layers to reach the maximum welding performance.

Behaviour of Rectangular Hollow Thin Ply Carbon Thermoset and Thermoplastic Composite Tubes Subjected to Bending.

Tubular composites are widely used in many industrial applications, and there is need to use new material and reliable manufacturing processes to improve the performance and process aspects. The current research presents a detailed study to understand the flexure response of rectangular tubular composites based on thin ply carbon fibres and Elium® resin. Another aim was to understand the failure mechanisms of novel tubular thermoplastic composite systems and carry out a baseline comparison with Epoxy-based tubular systems. In the current research, a bladder-assisted resin transfer moulding process was used to manufacture hollow thermoplastic composite tubes, and the bending behaviour of thin ply carbon (TPC) composite parts with novel Elium® (EL) and Epoxy (EP) resin as the matrix material was studied using a detailed experimental study. A testing method with optimized support span and a saddle was used to carry out three-point bending tests on the tubular composite structures. The TPC/EL composite tubes have shown 10% higher bending strength, with a noticeable increase in deformation due the presence of extended plasticity attributes for acrylic Elium resin. Failure mechanisms studied with the detailed microscopic investigation have shown severe catastrophic failure for epoxy-based composite tubes; however, acrylic Elium®-based composite tubes have shown different damage modes such as fibre splitting, resin infragmentation, and fibre resin-interfacial cracking.

Optimizing Bladder Resin Transfer Molding Process to Manufacture Complex, Thin-Ply Thermoplastic Tubular Composite Structures: An Experimental Case Study.

The bladder molding process is primarily used in sporting applications but mostly with prepregs. Bladder-Assisted Resin Transfer Molding (B-RTM) presents the tremendous potential to automate and mass produce the complex hollow-composite profiles. Thin-ply, non-crimp fabrics (NCFs) provide excellent mechanical, fracture toughness, and vibration damping properties on top of the weight saving it offers to a final product. However, these fiber architectures are difficult to inject due to the resistance they provide for the polymer flow using the liquid injection process. Therefore, it is mandatory to optimize the process parameters to reduce the time for injection and simultaneously achieve better consolidation. This work presents a first, detailed, experimental case study to successfully inject a low-permeability, thin-ply, complex, thermoplastic tubular structure, and the effect of process parameters, boundary conditions, the associated manufacturing challenges, and proposed solutions are deliberated in this paper.

An exploratory study of the use of ultrasound in the measurement of anterior tibial translation under gastrocnemius muscle stimulation.

Gastrocnemius' role as an agonist or antagonist of the anterior cruciate ligament (ACL) is not well understood. This study explored the use of ultrasound imaging to investigate how gastrocnemius stimulation levels influenced anterior tibial translation. The gastrocnemii of 10 participants were stimulated to four different levels using electrical muscle stimulation. The quadriceps were co-activated at a fixed level. Anterior tibial translation was determined using ultrasound imaging. Intraclass correlation coefficient [ICC (2,1)] was used to assess the intra-rater reliability over two sessions. Intra-rater reliability was good at rest and under most muscle stimulation levels (ICC = 0.84 to 0.92), and moderate with the lowest (ICC = 0.71) and highest stimulation (ICC = 0.61). While anterior tibial translation was not significantly different across simulation levels, ultrasound imaging recorded the anterior movement of the tibia as the gastrocnemius was activated, thus supporting gastrocnemius' role as an antagonist of the ACL.

Ultrasonic Welding of Novel Carbon/ Elium® Thermoplastic Composites with Flat and Integrated Energy Directors: Lap Shear Characterisation and Fractographic Investigation.

The current research work presents a first attempt to investigate the welding attributes of Elium® thermoplastic resin and the fusion bonding using ultrafast ultrasonic welding technique. The integrated energy director (ED) polymer-matrix composites (PMCs) panel manufacturing was carried out using the Resin Transfer Moulding (RTM) technique and the scheme is deduced to manufacture a bubble-free panel. Integrated ED configurations and flat specimens with Elium® film of different thickness at the interface were investigated for ultrasonic welding optimization. Optimised weld time for integrated ED and flat Elium® panels with film (0.5 mm thick) configuration was found to be 1 s and 5.5 s, respectively. The ED integrated configuration showed the best welding results with a lap shear strength of 18.68 MPa. The morphological assessment has shown significant plastic deformation of Elium® resin and the shear cusps formation, which enhances the welding strength. This research has the potential to open up an excellent and automated way of joining Elium® composite parts in automotive, wind turbines, sports, and many other industrial applications.

Advances in Ultrasonic Welding of Thermoplastic Composites: A Review.

The ultrasonic welding (UW) technique is an ultra-fast joining process, and it is used to join thermoplastic composite structures, and provides an excellent bonding strength. It is more cost-efficient as opposed to the conventional adhesive, mechanical and other joining methods. This review paper presents the detailed progress made by the scientific and research community to date in the direction of the UW of thermoplastic composites. The focus of this paper is to review the recent development of the ultrasonic welding technique for thermoplastic composites to thermoplastic composites, and to dissimilar materials. Different ultrasonic welding modes and their processing parameters, namely, weld time, weld pressure, amplitude, type of energy directors (EDs) affecting the welding quality and the advantages and disadvantages of UW over other bonding techniques, are summarized. The current state of the ultrasonic welding of thermoplastic composites and their future perspectives are also deliberated.

Influence of Foot-Landing Positions at Initial Contact on Knee Flexion Angles for Single-Leg Drop Landings.

Purpose: Small knee flexion angles are associated with increased non-contact anterior cruciate ligament (ACL) injury risks. The purpose of this study was to provide insights into how ankle plantar flexion angles influenced knee flexion angles at initial contact during single-leg drop landings. Method: Thirteen male recreational basketball players performed single-leg drop landings from a 30-cm high platform using three randomized foot-landing positions (natural, fore-foot, and flat-foot). A motion capture system and a force plate were used to measure lower extremity kinematics and vertical ground reaction force (VGRF). A one-way repeated measures Analysis of Variance and the Friedman test were conducted (α = .05). Results: Foot-landing position had a significant effect on knee flexion angles at initial contact (p < .001). As compared to flat-foot landings (18° (SD 5°), significantly smaller knee flexion angles were found for natural (mean 12° (SD 5°), p = .004) and fore-foot landings (mean 12° (SD 3°), p = .001). There was however significantly smaller knee flexion range of motion in flat-foot landings (mean 42° (SD 3°), as compared to natural landings (mean 48° (SD 4°) (p < .001). Flat-foot landings also resulted in a 1.4 times higher maximum VGRF than fore-foot landings (p < .001) and 1.3 times higher than natural landings (p = .005). Conclusions: Natural and fore-foot landings are better for force absorption but are associated with smaller knee flexion angles at initial contact. These findings have important implications for non-contact ACL injuries.

3D Bioprinting of Highly Thixotropic Alginate/Methylcellulose Hydrogel with Strong Interface Bonding.

A robust alginate/methylcellulose (Alg/MC) blend hydrogel, with a strategy to improve adhesion between printed layers, has been fabricated for the first time for three-dimensional (3D) bioprinting. The optimized Alg/MC blend hydrogel exhibits a highly thixotropic property, great extrudability, and stackability. With treatment by a trisodium citrate (TSC) solution, the interfacial bonding between the printed layers is significantly improved. The TSC solution acts as a chelating agent to remove the superficial calcium ions at each layer. Post-cross-linking in a CaCl2 bath after 3D printing further enhances the adhesion strength between the layers. The key parameters affecting the interfacial strength of the Alg/MC hydrogel are found to be the concentration of TSC, the volume of TSC, and the concentration of CaCl2 in the bath. The Alg/MC hydrogel with the aid of TSC demonstrates superior printability, high stackability (150 layers can be printed), and high shape fidelity. A good cell viability of >95% is obtained for a freshly 3D-bioprinted Alg/MC construct. The novel Alg/MC hydrogel with the aid of TSC has been shown to have a great potential as an advanced 3D bioprinting material.

Revealing martensitic transformation and α/ß interface evolution in electron beam melting three-dimensional-printed Ti-6Al-4V.

As an important metal three-dimensional printing technology, electron beam melting (EBM) is gaining increasing attention due to its huge potential applications in aerospace and biomedical fields. EBM processing of Ti-6Al-4V as well as its microstructure and mechanical properties were extensively investigated. However, it is still lack of quantitative studies regarding its microstructural evolution, indicative of EBM thermal process. Here, we report α' martensitic transformation and α/ß interface evolution in varied printing thicknesses of EBM-printed Ti-6Al-4V block samples by means of atom probe tomography. Quantitative chemical composition analysis suggests a general phase transformation sequence. By increasing in-fill hatched thickness, elemental partitioning ratios arise and ß volume fraction is increased. Furthermore, we observe kinetic vanadium segregation and aluminum depletion at interface front and the resultant α/ß interface widening phenomenon. It may give rise to an increased α/ß lattice mismatch and weakened α/ß interfaces, which could account for the degraded strength as printing thickness increases.

A Solvent-Free Surface Suspension Melt Technique for Making Biodegradable PCL Membrane Scaffolds for Tissue Engineering Applications.

In tissue engineering, there is limited availability of a simple, fast and solvent-free process for fabricating micro-porous thin membrane scaffolds. This paper presents the first report of a novel surface suspension melt technique to fabricate a micro-porous thin membrane scaffolds without using any organic solvent. Briefly, a layer of polycaprolactone (PCL) particles is directly spread on top of water in the form of a suspension. After that, with the use of heat, the powder layer is transformed into a melted layer, and following cooling, a thin membrane is obtained. Two different sizes of PCL powder particles (100 µm and 500 µm) are used. Results show that membranes made from 100 µm powders have lower thickness, smaller pore size, smoother surface, higher value of stiffness but lower ultimate tensile load compared to membranes made from 500 µm powder. C2C12 cell culture results indicate that the membrane supports cell growth and differentiation. Thus, this novel membrane generation method holds great promise for tissue engineering.

Characterization, mechanical behavior and in vitro evaluation of a melt-drawn scaffold for esophageal tissue engineering.

Tubular esophageal scaffolds with fiber diameter ranging from 13.9±1.7µm to 65.7±6.2µm were fabricated from the highly elastic poly(l-lactide-co-ε-caprolactone) (PLC) via a melt-drawing method. The morphology, crystallinity, thermal and mechanical properties of the PLC fibers were investigated. They were highly aligned and have a uniform diameter. PLC is found to be semicrystalline consisting of α- and ß- lactide (LA) crystals. The crystallinity increases up to 16.8% with increasing melt-drawing speeds due to strain-induced crystallization. Modulus and strength increases while ductility decreases with an increase in crystallinity of the PLC samples. Moisture will not degrade the overall tensile properties but affect its tangent modulus at the low strain. L929 cells are able to attach and proliferate on the scaffolds very well. The cells seeded on the scaffolds show normal morphology with >90% cell viability after 6 days of culture. These results demonstrate that the PLC fibrous scaffold has good potential for use in esophageal tissue engineering application.

Clothing polymer fibers with well-aligned and high-aspect ratio carbon nanotubes.

It is believed that the crucial step towards preparation of electrical conductive polymer-carbon nanotube (CNT) composites is dispersing CNTs with a high length-to-diameter aspect ratio in a well-aligned manner. However, this process is extremely challenging when dealing with long and entangled CNTs. Here in this study, a new approach is demonstrated to fabricate conductive polymer-CNT composite fibers without involving any dispersion process. Well-aligned CNT films were firstly drawn from CNT arrays, and then directly coated on polycaprolactone fibers to form polymer-CNT composite fibers. The conductivity of these composite fibers can be as high as 285 S m(-1) with only 2.5 wt% CNT loading, and reach 1549 S m(-1) when CNT loading is 13.4 wt%. As-prepared composite fibers also exhibit 82% retention of conductivity at a strain of 7%, and have improved mechanical properties.

Impact of short-term perfusion on cell retention for 3D bioconstruct development.

Perfusion culture is a commonly used dynamic culture technique in tissue engineering for promoting higher cell number growth. Under perfusion culture, the cell number could be co-influenced by cell detachment and cell proliferation. However, previous studies have mainly focused on the perfusion effects on cell proliferation but largely ignored the aspect of cell detachment. This work demonstrates that perfusion as small as 2.3 mL/min have induced cell loss compared with static culture even for a culture period of 12 minutes Both perfusion rate and direction have influenced the cell retention in the construct. In general, higher perfusion rates have caused more cell loss, largely due to the induced higher flow shear. The influence of perfusion directions on cell retention has been closely related to cell distribution in the construct. The study shows that cell retention is an important aspect that is worth more research attention and should be considered for cell number analysis in 3D construct development.

A mathematical model for fluid shear-sensitive 3D tissue construct development.

This research studies dynamic culture for 3D tissue construct development with computational fluid dynamics. It proposes a mathematical model to evaluate the impact of flow rates and flow shear stress on cell growth in 3D constructs under perfusion. The modeling results show that dynamic flow, even at flow rate as low as 0.002 cm/s, can support much better mass exchange, higher cell number, and more even cell and nutrient distribution compared to static culture. Higher flow rate can further improve nutrient supply and mass exchange in the construct, promoting better nutritious environment and cell proliferation compared to lower flow rate. In addition, consideration of flow shear stress predicts much higher cell number in the construct compared to that without shear consideration. While the nutrient can dominate shear stress in influencing cell proliferation, the shear effect increases with flow rate. The proposed model helps tissue engineers better understand the cell-flow relationship at the molecular level during dynamic culture.

The development of computer-aided system for tissue scaffolds (CASTS) system for functionally graded tissue-engineering scaffolds.

Computer-aided system for tissue scaffolds (CASTS) is an in-house parametric library of polyhedral units that can be assembled into customized tissue scaffolds. Thirteen polyhedral configurations are available to select, depending on the biological and mechanical requirements of the target tissue/organ. Input parameters include the individual polyhedral units and overall scaffold block as well as the scaffold strut diameter. Taking advantage of its repeatability and reproducibility, the scaffold file is then converted into .STL file and fabricated using selective laser sintering, a rapid prototyping system. CASTS seeks to fulfill anatomical, biological, and mechanical requirements of the target tissue/organ. Customized anatomical scaffold shape is achieved through a Boolean operation between the scaffold block and the tissue defect image. Biological requirements, such as scaffold pore size and porosity, are unique for different type of cells. Matching mechanical properties, such as stiffness and strength, between the scaffold and target organ is very important, particularly in the regeneration of load-bearing organ, i.e., bone. This includes mimicking the compressive stiffness variation across the bone to prevent stress shielding and ensuring that the scaffold can withstand the load normally borne by the bone. The stiffness variation is tailored by adjusting the scaffold porosity based on the porosity-stiffness relationship of the CASTS scaffolds. Two types of functional gradients based on the gradient direction include radial and axial/linear gradient. Radial gradient is useful in the case of regenerating a section of long bones while the gradient in linear direction can be used in short or irregular bones. Stiffness gradient in the radial direction is achieved by using cylindrical unit cells arranged in a concentric manner, in which the porosity decreases from the center of the structure toward the outside radius, making the scaffold stiffer at the outer radius and more porous at the center of the scaffold. On the other hand, the linear gradient is accomplished by varying the strut diameter along the gradient direction. The parameters to vary in both gradient types are the strut diameter, the unit cell dimension, and the boundaries between two scaffold regions with different stiffness.

Solvent-free fabrication of three dimensionally aligned polycaprolactone microfibers for engineering of anisotropic tissues.

Fabrication of aligned microfiber scaffolds is critical in successful engineering of anisotropic tissues such as tendon, ligaments and nerves. Conventionally, aligned microfiber scaffolds are two dimensional and predominantly fabricated by electrospinning which is solvent dependent. In this paper, we report a novel technique, named microfiber melt drawing, to fabricate a bundle of three dimensionally aligned polycaprolactone microfibers without using any organic solvent. This technique is simple yet effective. It has been demonstrated that polycaprolactone microfibers of 10 µm fiber diameter can be directly drawn from a 2 mm orifice. Orifice diameter, temperature and take-up speed significantly influence the final linear density and fiber diameter of the microfibers. Mechanical test suggests that mechanical properties such as stiffness and breaking force of microfiber bundles can be easily adjusted by the number of fibers. In vitro study shows that these microfibers are able to support the proliferation of human dermal fibroblasts over 7 days. In vivo result of Achilles tendon repair in a rabbit model shows that the microfibers were highly infiltrated by tendon tissue as early as in 1 month, besides, the repaired tendon have a well-aligned tissue structure under the guidance of aligned microfibers. However whether these three dimensionally aligned microfibers can induce three dimensionally aligned cells remains inconclusive.

Cryogenic prototyping of chitosan scaffolds with controlled micro and macro architecture and their effect on in vivo neo-vascularization and cellular infiltration.

A major challenge in tissue engineering has been to develop scaffolds with controlled complex geometries, on both the macro- and micro-scale. One group of techniques, using rapid prototyping (RP) processes, has the capability to produce complex three-dimensional structures with good control over the size, geometry, and connectivity of the pores. In this article, a novel technique based on RP technology, that is, cryogenic prototyping (CP), that has the capability to fabricate scaffolds with controlled macro- and micro-structures, is presented. Our in vivo studies showed that the micro architecture (i.e., both pore size and pore orientation) and macro structures of the CP scaffolds affect both cellular infiltration and neo-vascularization. Full cellular infiltration and neo-vascularization were observed after 28 days in scaffolds with micropore sizes of 90 microm. In addition, it was observed that channels (300 microm) created in scaffolds were effective at enhancing cellular infiltration and vascularization. Our results have demonstrated that CP is a viable method for fabricating scaffolds for a wide range of tissue engineering applications.

Fibroblast response to interstitial flow: A state-of-the-art review.

Interstitial flow (IF) modulates both the biochemical and biophysical cues surrounding cells. It represents a very important regulating mechanism for cell/tissue function and has been commonly utilized in tissue engineering (TE). This article discusses the possible regulating mechanisms of IF on fibroblasts, the various fibroblast responses to IF, the current challenges in understanding the IF-fibroblast relationship and the application of IF for fibroblast involved TE. In particular, IF can affect fibroblast growth at both intracellular (e.g., calcium signaling, protein/proteinase secretion) and cellular (e.g., autocrine/paracrine signaling, proliferation, differentiation, alignment, adhesion, migration) levels. One major challenge for understanding IF-fibroblast interaction has been the determination of the flow and cell growth condition at microlevel especially in a three-dimensional environment. To utilize IF and optimize the fluidic environment for TE, several influencing factors in the system including perfusate composition, flow profile, nutrient supply, signaling molecule effect, scaffold property, and fibroblast type should be considered.