Compressive cyclic ratcheting and fatigue of synthetic, soft biomedical polymers in solution

Photo-Polymerizable Autologous Growth-Factor Loaded Silk-Based Biomaterial-Inks toward 3D Printing-Based Regeneration of Meniscus Tears

Meniscus tears in the avascular region undergoing partial or full meniscectomy lead to knee osteoarthritis and concurrent lifestyle hindrances in the young and aged alike. Here they reported ingenious photo-polymerizable autologous growth factor loaded 3D printed scaffolds to potentially treat meniscal defects . A shear-thinning photo-crosslinkable silk fibroin methacrylate-gelatin methacrylate-polyethylene glycol dimethacrylate biomaterial-ink is formulated and loaded with freeze-dried growth factor rich plasma (GFRP) . The biomaterial-ink exhibits optimal rheological properties and shape fidelity for 3D printing. Initial evaluation revealed that the 3D printed scaffolds mimic mechanical characteristics of meniscus, possess favourable porosity and swelling characteristics, and demonstrate sustained GFRP release. GFRP laden 3D scaffolds are screened with human neo-natal stem cells in vitro and biomaterial-ink comprising of 25 mg mL-1 of GFRP (GFRP25) is found to be amicable for meniscus tissue engineering. GFRP25 ink demonstrated rigorous rheological compliance, and printed constructs demonstrated long term degradability (>6 weeks), GFRP release (>5 weeks), and mechanical durability (3 weeks). GFRP25 scaffolds aided in proliferation of seeded human neo-natal stem cellsand their meniscus-specific fibrochondrogenic differentiation . GFRP25 constructs show amenable inflammatory response in vitro and in vivo. GFRP25 biomaterial-ink and printed GFRP25 scaffolds could be potential patient-specific treatment modalities for meniscal defects.

3D Printing Materials and Technologies for Orthopaedic Applications

SUMMARY

3D printing technologies have evolved tremendously over the last decade for uses in orthopaedic surgical applications, including being used to manufacture implants for spine, upper extremity, foot and ankle, oncologic, and traumatic reconstructions. Materials used for 3D-printed orthopaedic devices include metals, degradable and nondegradable polymers, and ceramic composites. There are 2 primary advantages for use of 3D printing technologies for orthopaedics: first, the ability to create complex porous lattices that allow for osseointegration and improved implant stability and second, the enablement of complex geometric designs allowing for patient-specific devices based on preoperative imaging. Given continually evolving technology, and the relatively early stage of the materials and 3D printers themselves, the possibilities for continued innovation in orthopaedics are great.

Use of a novel magnetically actuated compression system to study the temporal dynamics of axial and lateral strain in human osteochondral plugs

The high water content of articular cartilage allows this biphasic tissue to withstand large compressive loads through fluid pressurization. The system presented here, termed the "MagnaSquish", provides new capabilities for quantifying the effect of rehydration on cartilage behavior during cyclic loading. An imbalanced rate of fluid exudation during load and fluid re-entry during recovery can lead to the accumulation of strain during successive loading cycles - a phenomenon known as ratcheting. Typical experimental systems for cartilage biomechanics use continuous contact between the platen and sample, which may affect tissue rehydration by compressing the top layer of cartilage and slowing fluid re-entry. To address this limitation, we developed a magnetically actuated device that provides full lift-off of the platen in between loading cycles. We investigated strain accumulation in cadaveric human osteochondral plugs during 750 loading cycles, with two dimensional profiles of the cartilage captured at 30 frames per second throughout loading and 10 min of additional free swelling recovery. Axial and lateral strain measurements were extracted from the tissue profiles using a UNet-based deep learning algorithm to circumvent manual tracing. We observed increased axial strain accumulation with shorter inter-cycle recovery, with static loading serving as the extreme case of zero recovery. The loading waveform during the 750 cycles dictated the pace of the recovery during the extended free swelling period, as shorter inter-cycle recovery led to more persistent axial strain accumulation for up to five minutes. This work showcases the importance of fluid re-entry in resisting strain accumulation during cyclical compression.

Deformation and Durability of Soft Three-Dimensional-Printed Polycarbonate Urethane Porous Membranes for Potential Use in Pelvic Organ Prolapse

Pelvic organ prolapse (POP) is the herniation of the pelvic organs into the vaginal space, resulting in the feeling of a bulge and organ dysfunction. Treatment of POP often involves repositioning the organs using a polypropylene mesh, which has recently been found to have relatively high rates of complications. Complications have been shown to be related to stiffness mismatches between the vagina and polypropylene, and unstable knit patterns resulting in mesh deformations with mechanical loading. To overcome these limitations, we have three-dimensional (3D)-printed a porous, monofilament membrane composed of relatively soft polycarbonate-urethane (PCU) with a stable geometry. PCU was chosen for its tunable properties as it is comprised of both hard and soft segments. The bulk mechanical properties of PCU were first characterized by testing dogbone samples, demonstrating the dependence of PCU mechanical properties on its measurement environment and the effect of print pathing. The pore dimensions and load-relative elongation response of the 3D-printed PCU membranes under monotonic tensile loading were then characterized. Finally, a fatigue study was performed on the 3D-printed membrane to evaluate durability, showing a similar fatigue resistance with a commercial synthetic mesh and hence its potential as a replacement.

RATCHETING EVALUATION OF BIOLOGICAL TISSUES OVER ASYMMETRIC LOADING CYCLES

This study intends to evaluate the ratcheting response of biological samples prepared from bovine and porcine trabecular bone, articular cartilage, meniscus, and skin tissues and tested under asymmetric (nonzero mean stress) cycles. Meniscus and skin samples were tested with stress ratios of [Formula: see text] and [Formula: see text], respectively, while other tissues were tested at [Formula: see text]. Experimental ratcheting data and related influential parameters including stress level, stress rate, and testing frequency were discussed. A parametric ratcheting equation was further calibrated to estimate the ratcheting response of tissues. The predicted ratcheting data were found to be in close agreement with the reported experimental data.

Polycarbonate Urethane Mesh: A New Material for Pelvic Reconstruction

OBJECTIVE

Polycarbonate urethane (PCU) is a new biomaterial, and its mechanical properties can be tailored to match that of vaginal tissue. We aimed to determine whether vaginal host immune and extracellular matrix responses differ after PCU versus lightweight polypropylene (PP) mesh implantation.

METHODS

Hysterectomy and ovariectomy were performed on 24 Sprague-Dawley rats. Animals were divided into 3 groups: (1) PCU vaginal mesh, (2) PP vaginal mesh, and (3) sham controls. Vagina-mesh complexes or vaginas (controls) were excised 90 days after surgery. We quantified responses by comparing: (1) histomorphologic scoring of hematoxylin and eosin- and Masson trichrome-stained slides, (2) macrophage subsets (immunolabeling), (3) pro-inflammatory and anti-inflammatory cytokines (Luminex panel), (4) matrix metalloproteinase (MMP)-2 and -9 using an enzyme-linked immunosorbent assay, and (5) type I/III collagen using picrosirius red staining.

RESULTS

There was no difference in histomorphologic score between PCU and PP (P = 0.211). Although the histomorphologic response was low surrounding all mesh fibers, groups with PCU and PP mesh had a higher histomorphologic score than the control group (P < 0.005 and P < 0.002, respectively). There were no differences between groups in terms of macrophage subsets, pro-inflammatory cytokines, anti-inflammatory cytokines, MMP-2 and MMP-9, or collagen ratio.

CONCLUSIONS

Polycarbonate urethane, an elastomer with material properties similar to those of vaginal tissue, elicits minimal host inflammatory responses in a rat model. Because its implantation does not elicit more inflammation than currently used lightweight PP, using PCU for prolapse mesh warrants further investigation with larger animal models.

Lumbar Disk Arthroplasty for Degenerative Disk Disease: Literature Review

Low back pain is the principal cause of long-term disability worldwide. We intend to address one of its main causes, degenerative disk disease, a spinal condition involving degradation of an intervertebral disk. Following unsuccessful conservative treatment, patients may be recommended for surgery. The two main surgical treatments for lumbar degenerative disk disease are lumbar fusion: traditional standard surgical treatment and lumbar disk arthroplasty, also known as lumbar total disk replacement. Lumbar fusion aims to relieve pain by fusing vertebrae together to eliminate movement at the joint, but it has been criticized for problems involving insignificant pain relief, a reduced range of motion, and an increased risk of adjacent segment degeneration. This leads to development of the lumbar total disk replacement technique, which aims to relieve pain replacing a degenerated intervertebral disk with a moveable prosthesis, thus mimicking the functional anatomy and biomechanics of a native intervertebral disk. Over the years a large range of prosthetic disks has been developed. The efficacy and current evidence for these prostheses are discussed in this review. The results of this study are intended to guide clinical practice and future lumbar total disk replacement device choice and design.

Parametric analysis of electromechanical and fatigue performance of total knee replacement bearing with embedded piezoelectric transducers

Total knee arthroplasty (TKA) is a common procedure in the United States; it has been estimated that about 4 million people are currently living with primary knee replacement in this country. Despite huge improvements in material properties, implant design, and surgical techniques, some implants fail a few years after surgery. A lack of information about in vivo kinetics of the knee prevents the establishment of a correlated intra- and postoperative loading pattern in knee implants. In this study, a conceptual design of an ultra high molecular weight (UHMW) knee bearing with embedded piezoelectric transducers is proposed, which is able to measure the reaction forces from knee motion as well as harvest energy to power embedded electronics. A simplified geometry consisting of a disk of UHMW with a single embedded piezoelectric ceramic is used in this work to study the general parametric trends of an instrumented knee bearing. A combined finite element and electromechanical modeling framework is employed to investigate the fatigue behavior of the instrumented bearing and the electromechanical performance of the embedded piezoelectric. The model is validated through experimental testing and utilized for further parametric studies. Parametric studies consist of the investigation of the effects of several dimensional and piezoelectric material parameters on the durability of the bearing and electrical output of the transducers. Among all the parameters, it is shown that adding large fillet radii results in noticeable improvement in the fatigue life of the bearing. Additionally, the design is highly sensitive to the depth of piezoelectric pocket. Finally, using PZT-5H piezoceramics, higher voltage and slightly enhanced fatigue life is achieved.

3D porous polyurethanes featured by different mechanical properties: Characterization and interaction with skeletal muscle cells

The fabrication of biomaterials for interaction with muscle cells has attracted significant interest in the last decades. However, 3D porous scaffolds featured by a relatively low stiffness (almost matching the natural muscle one) and highly stable in response to cyclic loadings are not available at present, in this context. This work describes 3D polyurethane-based porous scaffolds featured by different mechanical properties. Biomaterial stiffness was finely tuned by varying the cross-linking degree of the starting foam. Compression tests revealed, for the softest material formulation, stiffness values close to the ones possessed by natural skeletal muscles. The materials were also characterized in terms of local nanoindenting, rheometric properties and long-term stability through cyclic compressions, in a strain range reflecting the contraction extent of natural muscles. Preliminary in vitro tests revealed a preferential adhesion of C2C12 skeletal muscle cells over the softer, rougher and more porous structures. All the material formulations showed low cytotoxicity.

Deformation and fatigue of tough 3D printed elastomer scaffolds processed by fused deposition modeling and continuous liquid interface production

Polyurethane (PU) based elastomers continue to gain popularity in a variety of biomedical applications as compliant implant materials. In parallel, advancements in additive manufacturing continue to provide new opportunities for biomedical applications by enabling the creation of more complex architectures for tissue scaffolding and patient specific implants. The purpose of this study was to examine the effects of printed architecture on the monotonic and cyclic mechanical behavior of elastomeric PUs and to compare the structure-property relationship across two different printing approaches. We examined the tensile fatigue of notched specimens, 3D crosshatch scaffolds, and two 3D spherical pore architectures in a physically crosslinked polycarbonate urethane (PCU) printed via fused deposition modeling (FDM) as well as a photo-cured, chemically-crosslinked, elastomeric PU printed via continuous liquid interface production (CLIP). Both elastomers were relatively tolerant of 3D geometrical features as compared to stiffer synthetic implant materials such as PEEK and titanium. PCU and crosslinked PU samples with 3D porous structures demonstrated a reduced tensile failure stress as expected without a significant effect on tensile failure strain. PCU crosshatch samples demonstrated similar performance in strain-based tensile fatigue as solid controls; however, when plotted against stress amplitude and adjusted by porosity, it was clear that the architecture had an impact on performance. Square shaped notches or pores in crosslinked PU appeared to have a modest effect on strain-based tensile fatigue while circular shaped notches and pores had little impact relative to smooth samples. When plotted against stress amplitude, any differences in fatigue performance were small or not statistically significant for crosslinked PU samples. Despite the slight difference in local architecture and tolerances, crosslinked PU solid samples were found to perform on par with PCU solid samples in tensile fatigue, when appropriately adjusted for material hardness. Finally, tests of samples with printed architecture localized to the gage section revealed an effect in which fatigue performance appeared to drastically improve despite the localization of strain.

Standardized static and dynamic evaluation of myocardial tissue properties

INTRODUCTION

Quantifying the mechanical behaviors of soft biological tissues is of considerable research interest. However, validity and reproducibility between different researchers and apparatus is questionable. This study aims to quantify the mechanical properties of myocardium while investigating methodologies that can standardize biological tissue testing.

MATERIALS AND METHODS

Tensile testing was performed to obtain Young's modulus and a dynamic mechanical analysis (DMA) determined the viscoelastic properties. A frequency range of 0.5 Hz (30bpm) to 3.5 Hz (210bpm) was analyzed. For tensile testing three different preconditioning settings were tested: no load, 0.05 N preload, and a cyclic preload at 2.5% strain and 10 cycles. Samples were placed in saline and tested at 37 °C. Five ovine and five porcine hearts were tested.

RESULTS AND DISCUSSION

Cyclic loading results in the most consistent moduli values. The modulus of ovine/porcine tissue was mean = 0.05/.06 MPa, SD = 0.02/0.03 MPa. The storage/loss modulus varied from = 0.02/0.003 MPa at 0.5 Hz to 0.04/0.008 MPa at 3.5 Hz; Stiffness increases linearly from 400 to 800 N m^-1 with a tan delta around 0.175.

CONCLUSIONS

Static analysis of the mechanical properties of myocardial tissue confirms that; preconditioning is necessary for reproducibility, and DMA provides a platform for reproducible testing of soft biological tissues.

Fatigue life of bovine meniscus under longitudinal and transverse tensile loading

The knee meniscus is composed of a fibrous extracellular matrix that is subjected to large and repeated loads. Consequently, the meniscus is frequently torn, and a potential mechanism for failure is fatigue. The objective of this study was to measure the fatigue life of bovine meniscus when applying cyclic tensile loads either longitudinal or transverse to the principal fiber direction. Fatigue experiments consisted of cyclic loads to 60%, 70%, 80% or 90% of the predicted ultimate tensile strength until failure occurred or 20,000 cycles was reached. The fatigue data in each group was fit with a Weibull distribution to generate plots of stress level vs. cycles to failure (S-N curve). Results showed that loading transverse to the principal fiber direction gave a two-fold increase in failure strain, a three-fold increase in creep, and a nearly four-fold increase in cycles to failure (not significant), compared to loading longitudinal to the principal fiber direction. The S-N curves had strong negative correlations between the stress level and the mean cycles to failure for both loading directions, where the slope of the transverse S-N curve was 11% less than the longitudinal S-N curve (longitudinal: S=108-5.9ln(N); transverse: S=112-5.2ln(N)). Collectively, these results suggest that the non-fibrillar matrix is more resistant to fatigue failure than the collagen fibers. Results from this study are relevant to understanding the etiology of atraumatic radial and horizontal meniscal tears, and can be utilized by research groups that are working to develop meniscus implants with fatigue properties that mimic healthy tissue.