Fixation strength of swelling copolymeric anchors in artificial bone.

Fixation with suture anchors and metallic hardware for osteosynthesis is common in orthopedic surgeries. Most metallic commercial bone anchors achieve their fixation to bone through shear of the bone located between the threads. They have several deficiencies, including stress-shielding due to mechanical properties mismatch, generation of acidic by-products, poor osteointegration, low mechanical strength and catastrophic failure often associated with large bone defects that may be difficult to repair. To overcome these deficiencies, a swelling porous copolymeric material, to be used as bone anchors with osteointegration potential, was introduced. The purpose of this study was to investigate the fixation strength of these porous, swelling copolymeric bone anchors in artificial bone of various densities. The pull-out and subsidence studies indicate an effective fixation mechanism based on friction including re-fixation capabilities, and minimization of damage following complete failure. The study suggests that this swelling porous structure may provide an effective alternative to conventional bone anchors, particularly in low-density bone.

Engineering Toughness in a Brittle Vinyl Ester Resin Using Urethane Acrylate for Additive Manufacturing.

Thermosetting polymers tend to have a stiffness-toughness trade-off due to the opposing relationship of stiffness and toughness on crosslink density. We hypothesize that engineering the polymer network, e.g., by incorporating urethane oligomers, we can improve the toughness by introducing variations in crosslink density. In this work, we show that a brittle methacrylated Bis-GMA resin (known as DA2) is toughened by adding a commercial urethane acrylate resin (known as Tenacious) in different proportions. The formulations are 3D printed using a vat photopolymerization technique, and their mechanical, thermal, and fracture properties are investigated. Our results show that a significant amount of Tenacious 60% w/w is required to produce parts with improved toughness. However, mechanical properties drop when the Tenacious amount is higher than 60% w/w. Overall, our results show that optimizing the amount of urethane acrylate can improve toughness without significantly sacrificing mechanical properties. In fact, the results show that synergistic effects in modulus and strength exist at specific blend concentrations.

Engineered Interleaved Random Glass Fiber Composites Using Additive Manufacturing: Effect of Mat Properties, Resin Chemistry, and Resin-Rich Layer Thickness.

Standard lay-up fabrication of fiber-reinforced composites (FRCs) suffer from poor out-of-plane properties and delamination resistance. While advanced manufacturing techniques (e.g., interleaving, braiding, and z-pinning) increase delamination resistance in FRCs, they typically result in significant fabrication complexity and limitations, increased manufacturing costs, and/or overall stiffness reduction. In this work, we demonstrate the use of facile digital light processing (DLP) technique to additively manufacture (AM) random glass FRCs with engineered interleaves. This work demonstrates how vat photo-polymerization techniques can be used to build composites layer-by-layer with controlled interleaf material, thickness, and placement. Note that this engineering control is almost impossible to achieve with traditional manufacturing techniques. A range of specimens were printed to measure the effect of interleaf thickness and material on tensile/flexural properties as well as fracture toughness. One important observation was the ≈60% increase in interlaminar fracture toughness achieved by using a tough resin material in the interleaf. The comparison between AM and traditionally manufactured specimens via vacuum-assisted resin transfer molding (VARTM) highlighted the limitation of AM techniques in achieving high mat consolidation. In other words, the volume fraction of AM parts is limited by the wet fiber mat process, and engineering solutions are discussed. Overall, this technique offers engineering control of FRC design and fabrication that is not available with traditional methods.

Predicting the Dynamics and Steady-State Shape of Cylindrical Newtonian Filaments on Solid Substrates.

The spreading of liquid filaments on solid surfaces is of paramount importance to a wide range of applications including ink-jet printing, coating, and direct ink writing (DIW). However, there is a considerable lack of experimental, numerical, and theoretical studies on the spreading of filaments on solid substrates. In this work, we studied the dynamics of spreading of Newtonian filaments via experiment, numerical simulations, and theoretical analysis. More specifically, we used a novel experimental setup to validate a 2D moving mesh computational fluid dynamics (CFD) model. The CFD model is used to determine the effect of processing and fluid parameters on the dynamics of filament spreading. We experimentally showed that for a Newtonian filament, the same spreading dynamics and final shape are obtained when the initial radius is constant, independent of the magnitude in printing parameters. In other words, the only important parameter on the spreading of filaments is the initial filament radius. Using a numerical model, we showed that the initial filament radius manifests itself in two important dimensionless parameters, Bond number, Bo, and viscous timescale, τµ. Furthermore, the results clearly show that the dynamics of spreading are governed by the static advancing contact angle, θs. These three parameters determine a master spreading curve that can be used to predict the spreading of cylindrical filaments on flat substrates. Finally, we developed a theoretical model that was parameterized using experimental data to correlate the steady-state shape of filaments with Bo and θs. These results are particularly applicable for predicting and controlling the dynamics of filaments in DIW and other extrusion-based processes.

High Tg, Bio-Based Isosorbide Methacrylate Resin Systems for Vat Photopolymerization.

The use of isosorbide-derived polymers has garnered significant attention in recent decades as a high-performance, renewable material sourced from biomass. Of particular interest is isosorbide methacrylate, which possesses low viscosity (<500 cps), high thermal properties (Tg ≈ 220 °C), and high modulus (>4 GPa). These characteristics present a promising opportunity to replace BPA-derived methacrylate compounds in various applications. This investigation aims to synthesize and characterize isosorbide-based low-viscosity resin systems for 3D printing. The resin blends are composed of isosorbide methacrylate and two bio-renewable methacrylates, furfuryl methacrylate (FM) and bis-hydroxymethyl-furan methacrylate (BHMF-M), polymerized through a digital light processing (DLP) technique. The addition of the bio-based co-monomers serves to enhance the fracture toughness of the brittle isosorbide methacrylate crosslinked homopolymer (GIc = 37 J/m2). The resulting polymers exhibit Tg values greater than 200 °C and GIc around 100 J/m2. These resin systems hold potential for imparting high bio-based content to polymers used in additive manufacturing for high-performance applications.

A generalized scaling theory for spontaneous spreading of Newtonian fluids on solid substrates.

HYPOTHESIS: There exists a generalized solution for the spontaneous spreading dynamics of droplets taking into account the influence of interfacial tension and gravity. EXPERIMENTS: This work presents a generalized scaling theory for the problem of spontaneous dynamic spreading of Newtonian fluids on a flat substrate using experimental analysis and numerical simulations. More specifically, we first validate and modify a dynamic contact angle model to accurately describe the dependency of contact angle on the contact line velocity, which is generalized by the capillary number. The dynamic contact model is implemented into a two-phase moving mesh computational fluid dynamics (CFD) model, which is validated using experimental results. FINDINGS: We show that the spreading process is governed by three important parameters: the Bo number, viscous timescale τviscous, and static advancing contact angle, θs. More specifically, there exists a master spreading curve for a specific Bo and θs by scaling the spreading time with the τviscous. Moreover, we developed a correlation for prediction of the equilibrium shape of the droplets as a function of both Bo and θs. The results of this study can be used in a wide range of applications to predict both dynamic and equilibrium shape of droplets, such as in droplet-based additive manufacturing.

Effect of Microcapsule Content on Diels-Alder Room Temperature Self-Healing Thermosets.

A furan functionalized epoxy-amine thermoset with an embedded microcapsule healing system that utilizes reversible Diels-Alder healing chemistry was used to investigate the influence of microcapsule loading on healing efficiency. A urea-formaldehyde encapsulation technique was used to create capsules with an average diameter of 150 µm that were filled with a reactive solution of bismaleimide in phenyl acetate. It was found that optimum healing of the thermoset occurred at 10 wt% microcapsule content for the compositions investigated. The diffusion of solvent through the crack interface and within fractured samples was investigated using analytical diffusion models. The decrease in healing efficiency at higher microcapsule loading was attributed partially to solvent-induced plasticization at the interface. The diffusion analysis also showed that the 10% optimum microcapsule concentration occurs for systems with the same interfacial solvent concentration. This suggests that additional physical and chemical phenomena are also responsible for the observed optimum. Such phenomena could include a reduction in surface area available for healing and the saturation of interfacial furan moieties by reaction with increasing amounts of maleimide. Both would result from increased microcapsule loading.

Synthesis and Swelling Behavior of Highly Porous Epoxy Polymers.

Many advantageous properties of cross-linked polymers relate to their network structures. In this study, network structures of three DGEBA-based epoxy systems at various DGEBA monomer sizes were investigated via equilibrium swelling and glass transition behavior. Each system was cured with a tetra-functional diamine, 4,4'-methylenebiscyclohexanamine, in the presence of a nonreactive solvent, i.e., THF at a solvent-to-monomer volume fraction ranging from 0 to 92%. Experimental results revealed that the conventional swelling model (the Dusek model) accurately calculates M c values of the cured gels prepared in moderate dilute environments, up to approximately 60% by volume of THF. For gels cured in extreme dilute environments, i.e., in the presence of above 60% by volume of THF, the calculated M c values using the Dusek model were found to increase sharply as a function of the initial solvent content. The observed dramatic increase in M c values was not supported by the dry T g of the identical polymer systems. In fact, the dry T g values of the polymer systems were found to be relatively insensitive to the initial solvent content. A modification was proposed to the Dusek model that incorporates an additional term, which accounts for the probability of finding elastic chains in a polymer network. Using the modified equation, M c values were varied as expected with the molecular weight of DGEBA and insensitive to the amount of the solvent initially used during cure. Furthermore, the modified M c values were shown to be consistent with the dry T g values in view of the Fox and Loshaek model.

Epoxidation of Cardanol's Terminal Double Bond.

In this investigation, the terminal double bonds of the side chain epoxidized cardanol glycidyl ether (SCECGE) molecule were further epoxidized in the presence of Oxone® (potassium peroxomonosulfate) and fluorinated acetone. Regular methods for the double bond epoxidation are not effective on the terminal double bonds because of their reduced electronegativity with respect to internal double bonds. The terminal double bond functionality of the SCECGE was epoxidized to nearly 70%, increasing the epoxy functionality of SCECGE from 2.45 to 2.65 epoxies/molecule as measured using proton magnetic nuclear resonance (1H-NMR). This modified material-side chain epoxidized cardanol glycidyl ether with terminal epoxies (TE-SCECGE)-was thermally cured with cycloaliphatic curing agent 4-4'-methylenebis(cyclohexanamine) (PACM) at stoichiometry, and the cured polymer properties, such as glass transition temperature (Tg) and tensile modulus, were compared with SCECGE resin cured with PACM. The Tg of the material was increased from 52 to 69 °C as obtained via a dynamic mechanical analysis (DMA) while the tensile modulus of the material increased from 0.88 to 1.24 GPa as a result of terminal double bond epoxidation. In addition to highlighting the effects of dangling side groups in an epoxy network, this modest increase in Tg and modulus could be sufficient to significantly expand the potential uses of amine-cured cardanol-based epoxies for fiber reinforced composite applications.

Formulation of a Model Resin System for Benchmarking Processing-Property Relationships in High-Performance Photo 3D Printing Applications.

A well-defined resin system is needed to serve as a benchmark for 3D printing of high-performance composites. This work describes the design and characterization of such a system that takes into account processability and performance considerations. The Grunberg-Nissan model for resin viscosity and the Fox equation for polymer Tg were used to determine proper monomer ratios. The target viscosity of the resin was below 500 cP, and the target final Tg of the cured polymer was 150 °C based on tan-δ peak from dynamic mechanical analysis. A tri-component model resin system, termed DA-2 resin, was determined and fully characterized. The printed polymer post-cure exhibited good thermal properties and high mechanical strength, but has a comparatively low fracture toughness. The model resin will be used in additive manufacturing of fiber reinforced composite materials as well as for understanding the fundamental processing-property relationships in light-based 3D printing.

Influence of Epoxidized Cardanol Functionality and Reactivity on Network Formation and Properties.

Cardanol is a renewable resource based on cashew nut shell liquid (CNSL), which consists of a phenol ring with a C15 long aliphatic side chain in the meta position with varying degrees of unsaturation. Cardanol glycidyl ether was chemically modified to form side-chain epoxidized cardanol glycidyl ether (SCECGE) with an average epoxy functionality of 2.45 per molecule and was cured with petroleum-based epoxy hardeners, 4-4'-methylenebis(cyclohexanamine) and diethylenetriamine, and a cardanol-based amine hardener. For comparison, cardanol-based diphenol diepoxy resin, NC514 (Cardolite), and a petroleum-based epoxy resin, diglycidyl ether of bisphenol-A (DGEBA) were also evaluated. Chemical and thermomechanical analyses showed that for SCECGE resins, incomplete cure of the secondary epoxides led to reduced cross-link density, reduced thermal stability, and reduced elongation at break when compared with difunctional resins containing only primary epoxides. However, because of functionality greater than two, amine-cured SCECGE produced a Tg very similar to that of NC514 and thus could be useful in formulating epoxy with renewable cardanol content.

Water Transport and Thermomechanical Properties of Ti3C2Tz MXene Epoxy Nanocomposites.

Herein, we present the fabrication of dispersed, 5.0 wt % (1.74 vol %) Ti3C2Tz MXene epoxy nanocomposites (NCs), and report on their water transport and mechanical properties. To make the composites, Li+ ions between Ti3C2Tz MXene multilayers, MLs, present after the etching step were exchanged with either 12-aminolauric acid, ALA, or di(hydrogenated tallow)benzyl methyl ammonium chloride, DHT. After drying, the resulting ML powders were added at room temperature to the epoxy resin (diglycidyl ether of bisphenol A), followed by the curing agent, triethylenetetramine. The NCs were characterized by X-ray diffraction, thermogravimetric analysis, dynamic vapor sorption, dynamic mechanical analysis, scanning and transmission electron microscopies, and infrared spectroscopy. From XRD, the lack of signature MXene basal peaks, as well as evidence of exfoliation supported by TEM micrographs, we conclude that the MXene ML had indeed been intercalated by the epoxy. The distribution of the exfoliated multilayers, MLs, however, was not uniform. Nevertheless, our relative permeabilities, with a 1.74 vol % loading, are 5 times lower than results obtained in the carbon- or clay-reinforced epoxy NC literature. The lower permeabilities are due to reductions in both solubilities and diffusivities relative to the neat polymer. In the case of DHT, the water solubility at all temperatures was almost halved. The mechanical properties and thermal stability are found to be slightly improved with the addition of DHT-MXene. As far as we are aware, this is the first report of exfoliation of MXene in an epoxy matrix. Additionally, this study is the first to measure the diffusion of water in MXene epoxy NCs. More work on better dispersion of the MLs is indicated and ongoing.

The evolution of crystalline structures during gel spinning of ultra-high molecular weight polyethylene fibers.

Most studies are focused on the final mechanical properties of the fiber and the processing window required to achieve high moduli and tensile strength. Several studies have alluded to the fact that the crystalline morphologies developed during gel spinning and post-drawing are very important in determining the final mechanical properties. However, it is surprising to know that no clear correlation exists between the crystalline structure and initial, evolving, and final mechanical properties. In an attempt to define structure-property relationships, we have developed novel tools to quantify the effect of processing on crystalline structure evolution. We examine through controlled gel-spinning and SAXS analysis the effect of flow kinematics on the development of crystalline structures. Direct correlations are made between polymer solution relaxation time, extension rates, crystallization time and gel-spun crystalline morphologies. We report direct evidence of flow induced crystallization, which approaches an asymptotic crystallization rate at high Weissenberg numbers. For Wi < 1, the crystalline structure is only slightly affected by equilibrium. For Wi > 1, the crystalline structure is highly anisotropic due to chain orientation/stretch during spinning. Fibers spun at different Weissenberg numbers are drawn to low draw ratios at constant temperature to measure the initial structure evolution. A qualitative SAXS analysis clearly shows similar evolution of different starting structures with the formation of more straight chain crystals upon drawing. However, there remain quantitative differences between the length of straight chain crystals and the size and distribution of lamellar domains depending on the starting structure.

Toughening Anhydride-Cured Epoxy Resins Using Fatty Alkyl-Anhydride-Grafted Epoxidized Soybean Oil.

The aim of this work is to develop a series of advanced biobased tougheners for thermosetting epoxy resins suitable for high-performance applications. These bio-rubber (BR) tougheners were prepared via a one-step chemical modification of epoxidized soybean oil using biobased hexanoic anhydride. To investigate their toughening performance, these BR tougheners were blended with diglycidyl ether of bisphenol A epoxy monomers at various weight fractions and cured with anhydride hardeners. Significant improvements in fracture toughness properties, as well as minimal reductions in glass transition temperature (Tg), were observed. When 20 wt % of a BR toughener was utilized, the critical stress intensity factor and critical strain energy release rate of a thermosetting matrix were enhanced by >200 and >500%, respectively, whereas the Tg was reduced by only 20 °C. The phase-separated domains were evenly dispersed across the fracture surfaces as observed through scanning electron microscopy and atomic force microscopy. Moreover, domain sizes were demonstrated to be tunable within the micrometer range by altering the toughener molecular structure and weight fractions. These BR tougheners demonstrate the possibility of achieving toughness while having the thermal properties of standard bisphenol epoxy thermosetting resins.

Experimental Data Extraction and in Silico Prediction of the Estrogenic Activity of Renewable Replacements for Bisphenol A.

Bisphenol A (BPA) is a ubiquitous compound used in polymer manufacturing for a wide array of applications; however, increasing evidence has shown that BPA causes significant endocrine disruption and this has raised public concerns over safety and exposure limits. The use of renewable materials as polymer feedstocks provides an opportunity to develop replacement compounds for BPA that are sustainable and exhibit unique properties due to their diverse structures. As new bio-based materials are developed and tested, it is important to consider the impacts of both monomers and polymers on human health. Molecular docking simulations using the Estrogenic Activity Database in conjunction with the decision forest were performed as part of a two-tier in silico model to predict the activity of 29 bio-based platform chemicals in the estrogen receptor-α (ERα). Fifteen of the candidates were predicted as ER binders and fifteen as non-binders. Gaining insight into the estrogenic activity of the bio-based BPA replacements aids in the sustainable development of new polymeric materials.

Interfacial optimization of fiber-reinforced hydrogel composites for soft fibrous tissue applications.

Meniscal tears are the most common orthopedic injuries to the human body, yet the current treatment of choice is a partial meniscectomy, which is known to lead to joint degeneration and osteoarthritis. As a result, there is a significant clinical need to develop materials capable of restoring function to the meniscus following an injury. Fiber-reinforced hydrogel composites are particularly suited for replicating the mechanical function of native fibrous tissues due to their ability to mimic the native anisotropic property distribution present. A critical issue with these materials, however, is the potential for the fiber-matrix interfacial properties to severely limit composite performance. In this work, the interfacial properties of an ultra-high-molecular-weight polyethylene (UHMWPE) fiber-reinforced poly(vinyl alcohol) (PVA) hydrogel are studied. A novel chemical grafting technique, confirmed using X-ray photoelectron spectroscopy, is used to improve UHMWPE-PVA interfacial adhesion. Interfacial shear strength is quantified using fiber pull-out tests. Results indicate significantly improved fiber-hydrogel interfacial adhesion after chemical grafting, where chemically grafted samples have an interfacial shear strength of 256.4±64.3kPa compared to 11.5±2.9kPa for untreated samples. Additionally, scanning electron microscopy of fiber surfaces after fiber pull-out reveal cohesive failure within the hydrogel matrix for treated fiber samples, indicating that the UHMWPE-PVA interface has been successfully optimized. Lastly, inter-fiber spacing is observed to have a significant effect on interfacial adhesion. Fibers spaced further apart have significantly higher interfacial shear strengths, which is critical to consider when optimizing composite design. The results in this study are applicable in developing similar chemical grafting techniques and optimizing fiber-matrix interfacial properties for other hydrogel-based composite systems.

Isosorbide as the structural component of bio-based unsaturated polyesters for use as thermosetting resins.

In recent years, the development of renewable bio-based resins has gained interest as potential replacements for petroleum based resins. Modified carbohydrate-based derivatives have favorable structural features such as fused bicyclic rings that offer promising candidates for the development of novel renewable polymers with improved thermomechanical properties when compared to early bio-based resins. Isosorbide is one such compound and has been utilized as the stiffness component for the synthesis of novel unsaturated polyesters (UPE) resins. Resin blends of BioUPE systems with styrene were shown to possess viscosities (120-2200 cP) amenable to a variety of liquid molding techniques, and after cure had Tgs (53-107 °C) and storage moduli (430-1650 MPa) that are in the desired range for composite materials. These investigations show that BioUPEs containing isosorbide can be tailored during synthesis of the prepolymer to meet the needs of different property profiles.

Room temperature self-healing thermoset based on the Diels-Alder reaction.

A self-healing epoxy-amine thermoset based on the compatible functionalization of the thermoset and encapsulated healing agent has been successfully developed. Healing of the thermoset resulted from the reaction of furans in the thermoset and multimaleimides (MMIs) in the healing agent solution. The healing agent, MMI dissolved in phenyl acetate, was encapsulated using a urea-formaldehyde encapsulation method. Autonomic healing of the thermoset was achieved by incorporating microcapsules filled with the healing agent solution within a furan-functionalized epoxy-amine thermoset. The resulting self-healing thermoset recovered 71% of its initial load after fracture.

Kinetic considerations for strength recovery at the fiber-matrix interface based on the Diels-Alder reaction.

The Diels-Alder reaction was used to yield thermal reversibility of the bonding between a partially furan-functionalized epoxy thermosetting matrix and a maleimide-treated glass fiber. Under ambient temperature conditions, the covalent bond forming product reaction dominates, but this reaction reverses at elevated temperatures to allow for interfacial healing. Single-fiber microdroplet pull-out testing was used to characterize the coupled effects of healing temperature and the glass transition temperature (T(g)) of the epoxy on interfacial strength recovery. In particular, the roles of mobility and reaction kinetics were independently varied to understand the individual effects of both.

Aging behavior of PVA hydrogels for soft tissue applications after in vitro swelling using osmotic pressure solutions.

The osmotic pressure of the medium used for in vitro swelling evaluation has been shown to have a significant effect on the swelling behavior of a material. In this study, the effect of osmotic pressure during swelling on poly(vinyl alcohol) hydrogel material properties was evaluated in vitro. Osmotic pressure solutions are necessary in order to mimic the swelling pressure observed in vivo for soft tissues present in load-bearing joints. Hydrogels were characterized after swelling by mechanical testing, X-ray diffraction and optical microscopy in the hydrated state. Results indicated that hydrogel mechanical properties remained tailorable with respect to initial processing parameters; however, significant aging occurred in osmotic solution. This was observed when evaluating the mechanical properties of the hydrogels, which, before swelling, ranged from 0.04 to 0.78 MPa but, after swelling in vitro using osmotic pressure solution, ranged from 0.32 to 0.93 MPa. Significant aging was also noted when evaluating crystallinity, with the relative crystallinity ranging between 0.4 and 5.0% before swelling and between 6.5 nd 8.0% after swelling. When compared to swelling in a non-osmotic pressure solution or in phosphate-buffered saline solution, the mechanical properties were more dependent upon the final swelling content. Furthermore, increases in crystallinity were not as significant after swelling. These results highlight the importance of choosing the appropriate swelling medium for in vitro characterization based on the desired application.