The orthotropic viscoelastic characterisation of sub-zero 3D-printed poly(vinyl alcohol) cryogel.

ABSTRACT: Poly(vinyl alcohol) cryogel (PVA) is a versatile biomaterial used to replicate the biomechanics of tissues. Additive manufacture (AM) at sub-zero (°C) temperatures enables the manufacture of PVA with complex geometry; however, the effect of processing parameters on the mechanical properties of PVA has not been evaluated. The aim of this study is to understand the impact of print nozzle diameter and orientation on the viscoelastic mechanical properties of PVA. Samples of sub-zero AM PVA, with different filament thicknesses, were tested under tension relative to the print direction, to calculate the storage and loss moduli. As the nozzle size was decreased, AM PVA exhibited more pronounced orthotropic properties; the smallest size showed a 33% decrease in storage moduli when tested perpendicular to the print direction, as opposed to parallel. This study has demonstrated the ability of sub-zero AM to tailor the orthotropic properties of PVA. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1557/s43580-021-00086-1.

The dynamic viscoelastic characterisation and magnetic resonance imaging of poly(vinyl alcohol) cryogel: Identifying new attributes and opportunities.

Poly(vinyl alcohol) (PVA) cryogel is a biocompatible, synthetic hydrogel, compatible with magnetic resonance (MR) imaging. It is widely used as a biomaterial in tissue scaffolds and mimics to test various diagnostic techniques. The aim of this study is to characterise the effect of varying PVA concentration, molecular weight (MW) and manufacturing protocol on the viscoelastic mechanical properties and MR T2 relaxation time. Further to this MR imaging (MRI) was investigated as a method to quantify material homogeneity. Cylindrical samples of PVA, of varying MW, concentration and number of freeze thaw cycles (FTCs), were manufactured. Dynamic mechanical analysis was performed to evaluate the storage and loss moduli between frequencies of 0.5 and 10 Hz. MR T2 relaxation maps were imaged using a 7 T MRI instrument. Storage and loss moduli were shown to increase with MW, concentration, or the number of FTCs; with storage modulus ranging from 55 kPa to 912 kPa and loss modulus ranging from 6 kPa to 103 kPa. MR T2 relaxation time was shown to increase linearly with PVA concentration. The qualitative and quantitative heterogeneity of the PVA sample were identified through MR T2 relaxation time maps. Excitingly, PVA demonstrated a composition-dependent casual correlation between the viscoelastic mechanical properties and MR T2 relaxation time. In conclusion, this research thoroughly characterised the viscoelastic mechanical properties of PVA to support its extensive use as a biomaterial, and demonstrated the use of MRI to non-invasively identify sample heterogeneity and to predict the composition-dependent viscoelastic properties of PVA.

The role of subchondral bone, and its histomorphology, on the dynamic viscoelasticity of cartilage, bone and osteochondral cores.

OBJECTIVE: Viscoelastic properties of articular cartilage have been characterised at physiological frequencies. However, studies investigating the interaction between cartilage and subchondral bone and the influence of underlying bone histomorphometry on the viscoelasticity of cartilage are lacking. METHOD: Dynamic Mechanical Analysis (DMA) has been used to quantify the dynamic viscoelasticity of bovine tibial plateau osteochondral cores, over a frequency sweep from 1 to 88 Hz. Specimens (approximately aged between 18 and 30 months) were neither osteoarthritic nor otherwise compromised. A maximum nominal stress of 1.7 MPa was induced. Viscoelastic properties of cores have been compared with that of its components (cartilage and bone) in terms of the elastic and viscous components of both structural stiffness and material modulus. Micro-computed tomography scans were used to quantify the histomorphological properties of the subchondral bone. RESULTS: Opposing frequency-dependent loss stiffness, and modulus, trends were witnessed for osteochondral tissues: for cartilage it increased logarithmically (P < 0.05); for bone it decreased logarithmically (P < 0.05). The storage stiffness of osteochondral cores was logarithmically frequency-dependent (P < 0.05), however, the loss stiffness was typically frequency-independent (P > 0.05). A linear relationship between the subchondral bone plate (SBP) thickness and cartilage thickness (P < 0.001) was identified. Cartilage loss modulus was linearly correlated to bone mineral density (BMD) (P < 0.05) and bone volume (P < 0.05). CONCLUSION: The relationship between the subchondral bone histomorphometry and cartilage viscoelasticity (namely loss modulus) and thickness, have implications for the initiation and progression of osteoarthritis (OA) through an altered ability of cartilage to dissipate energy.

Effect of glutaraldehyde based cross-linking on the viscoelasticity of mitral valve basal chordae tendineae.

BACKGROUND: Mitral valve failure can require repair or replacement. Replacement bioprosthetic valves are treated with glutaraldehyde prior to implantation. The aim of this study was to determine the changes in mechanical properties following glutaraldehyde fixation of mitral valve chordae. METHODS: To investigate the impact of glutaraldehyde on mitral valve chordae, 24 basal chordae were dissected from four porcine hearts. Anterior and posterior basal (including strut) chordae were used. All 24 chordae were subjected to a sinusoidally varying load (mean level 2N, dynamic amplitude 2N) over a frequency range of 0.5-10 Hz before and after glutaraldehyde treatment. RESULTS: The storage and loss modulus of all chordal types decreased following glutaraldehyde fixation. The storage modulus ranged from: 108 to 119 MPa before fixation and 67.3-87.4 MPa following fixation for basal chordae; 52.3-58.4 MPa before fixation and 47.9-53.5 MPa following fixation for strut chordae. Similarly, the loss modulus ranged from: 5.47 to 6.25 MPa before fixation and 3.63-4.94 MPa following fixation for basal chordae; 2.60-2.97 MPa before fixation and 2.31-2.93 MPa following fixation for strut chordae. CONCLUSION: The viscoelastic properties of mitral valve chordae are affected by glutaraldehyde fixation; in particular, the reduction in storage moduli decreased with an increase in chordal diameter.

Lumbar model generator: a tool for the automated generation of a parametric scalable model of the lumbar spine.

Low back pain is a major cause of disability and requires the development of new devices to treat pathologies and improve prognosis following surgery. Understanding the effects of new devices on the biomechanics of the spine is crucial in the development of new effective and functional devices. The aim of this study was to develop a preliminary parametric, scalable and anatomically accurate finite-element model of the lumbar spine allowing for the evaluation of the performance of spinal devices. The principal anatomical surfaces of the lumbar spine were first identified, and then accurately fitted from a previous model supplied by S14 Implants (Bordeaux, France). Finally, the reconstructed model was defined according to 17 parameters which are used to scale the model according to patient dimensions. The developed model, available as a toolbox named the lumbar model generator, enables generating a population of models using subject-specific dimensions obtained from data scans or averaged dimensions evaluated from the correlation analysis. This toolbox allows patient-specific assessment, taking into account individual morphological variation. The models have applications in the design process of new devices, evaluating the biomechanics of the spine and helping clinicians when deciding on treatment strategies.

Effect of frequency on crack growth in articular cartilage.

Cracks can occur in the articular cartilage surface due to the mechanical loading of the synovial joint, trauma or wear and tear. However, the propagation of such cracks under different frequencies of loading is unknown. The objective of this study was to determine the effect of frequency of loading on the growth of a pre-existing crack in cartilage specimens subjected to cyclic tensile strain. A 2.26mm crack was introduced into cartilage specimens and crack growth was achieved by applying a sinusoidally varying tensile strain at frequencies of 1, 10 and 100Hz (i.e. corresponding to normal, above normal and up to rapid heel-strike rise times, respectively). These frequencies were applied with a strain of between 10-20% and the crack length was measured at 0, 20, 50, 100, 500, 1000, 5000 and 10,000 cycles of strain. Crack growth increased with increasing number of cycles. The maximum crack growth was 0.6 ± 0.3 (mean ± standard deviation), 0.8 ± 0.2 and 1.1 ± 0.4mm at frequencies of 1, 10 and 100Hz, respectively following 10,000 cycles. Mean crack growth were 0.3 ± 0.2 and 0.4 ± 0.2 at frequencies of 1 and 10Hz, respectively. However, this value increased up to 0.6 ± 0.4mm at a frequency of 100Hz. This study demonstrates that crack growth was greater at higher frequencies. The findings of this study may have implications in the early onset of osteoarthritis. This is because rapid heel-strike rise times have been implicated in the early onset of osteoarthritis.

Fatigue strength of bovine articular cartilage-on-bone under three-point bending: the effect of loading frequency.

BACKGROUND: The objective of this study was to determine the influence of loading frequency on the failure of articular cartilage-on-bone specimens under three-point bending. METHODS: In this study, cyclic three-point bending was used to introduce failure into cartilage-on-bone specimens at varying loading frequencies. Sinusiodally varying maximum compressive loads in the range 40-130 N were applied to beam-shaped cartilage-on-bone specimens at frequencies of 1, 10, 50 and 100 Hz. RESULTS: The number of cycles to failure decreased when loading frequency increased from normal and above gait (1 and 10 Hz) to impulsive loading frequencies (50 and 100 Hz). It was found that 67 and 27% of the specimens reached run-out at loading of 10,000 cycles at frequencies of 1 and 10 Hz, respectively. However, 0% of the specimens reached run-out at loading frequencies of 50 and 100 Hz. CONCLUSION: The results indicate that increasing the loading frequency reduces the ability of specimens to resist fracture during bending. The findings underline the importance of the loading frequency concerning the failure of articular cartilage-on-bone and it may have implications in the early onset of osteoarthritis.

Viscoelastic properties of human bladder tumours.

The urinary bladder is an organ which facilitates the storage and release of urine. The bladder can develop tumours and bladder cancer is a common malignancy throughout the world. There is a consensus that there are differences in the mechanical properties of normal and malignant tissues. However, the viscoelastic properties of human bladder tumours at the macro-scale have not been previously studied. This study investigated the viscoelastic properties of ten bladder tumours, which were tested using dynamic mechanical analysis at frequencies up to 30Hz. The storage modulus ranged between 0.052MPa and 0.085MPa while the loss modulus ranged between 0.019MPa and 0.043MPa. Both storage and loss moduli showed frequency dependent behaviour and the storage modulus was higher than the loss modulus for every frequency tested. Viscoelastic properties may be useful for the development of surgical trainers, surgical devices, computational models and diagnostic equipment.

Effect of the variation of loading frequency on surface failure of bovine articular cartilage.

BACKGROUND: Mechanical loading of synovial joints can damage the articular cartilage surface and may lead to osteoarthritis. It is unknown if, independent of load, frequency alone can cause failure in cartilage. This study investigated the variation of articular cartilage surface damage under frequencies associated with normal, above normal and traumatic loading frequencies. METHOD: Cartilage on bone, obtained from bovine shoulder joints, was tested. Damage was created on the cartilage surface through an indenter being sinusoidally loaded against it at loading frequencies of 1, 10 and 100 Hz (i.e., relevant to normal, above normal and up to rapid heel-strike rise times, respectively). The frequencies were applied with a maximum load in the range 60-160 N. Surface cracks were marked with India ink, photographed and their length measured using image analysis software. RESULTS: Surface damage increased significantly (P < 0.0001) with frequency throughout all load ranges investigated. The dependence of crack length, c, on frequency, f, could be represented by, c=A(log10(f))2+B(log10(f))+Dc=A(log10(f))2+B(log10(f))+D where A = 0.006 ± 0.23, B = 0.62 ± 0.23 and D = 0.38 ± 0.51 mm (mean ± standard deviation). CONCLUSION: The increase in crack length with loading frequency indicated that, increased loading frequency can result in cartilage becoming damaged. The results of this study have implications in the early stages of osteoarthritis.

Frequency dependent viscoelastic properties of porcine bladder.

The aim of this study was to measure the viscoelastic properties of bladder tissue. Porcine bladders were dissected into rectangular strips and loops. Dynamic Mechanical Analysis was used to measure the viscoelastic properties of the bladder tissue (storage and loss stiffness) tested in a frequency range of up to 10 Hz. Storage stiffness was found to be consistently higher than loss stiffness. Average storage stiffness was found to be 1.89 N/mm and 0.74 N/mm for looped and rectangular samples, respectively. Average loss stiffness was found to be 0.24 N/mm and 0.11 N/mm for looped and rectangular samples, respectively. The results of this study are important for computational modelling of the bladder and for ensuring that engineered bladder tissues have physiological viscoelastic properties.

Frequency and diameter dependent viscoelastic properties of mitral valve chordae tendineae.

This study aimed to characterise viscoelastic properties of different categories of chordae tendineae over a range of frequencies. Dynamic Mechanical Analysis (DMA) was performed using a materials testing machine. Chordae (n=51) were dissected from seven porcine hearts and categorised as basal, marginal, strut or commissural. Chordae were loaded under a sinusoidally varying tensile load at a range of frequencies between 0.5 and 5Hz, both at a standardised load (i.e. same mean load of 4N for all chordae) and under chordal specific loading (i.e. based on in vivo loads for different chordae). Storage modulus and stiffness were frequency-dependent. Loss modulus and stiffness were frequency-independent. Storage and loss moduli, but not stiffness, decreased with chordal diameter. Therefore, strut chordae have the lowest moduli and marginal chordae the highest moduli. The hierarchy of dynamic storage and loss moduli is: marginal, commissural, basal and strut. In conclusion, viscoelastic properties of chordae are dependent on both frequency and chordal type. Future/novel replacement chordal materials should account for frequency and diameter dependent viscoelastic properties of chordae tendineae.

Determination of the pressure required to cause mitral valve failure.

A method has been developed for applying water pressure to a closed mitral valve on the side corresponding to the heart's left ventricle. The pressure is increased until fluid flows through the valve, i.e. until it fails. A specific dissection technique has been developed to produce a specimen with two annular rings, mitral annulus and papillary muscle annulus. Since the valve is maintained intact, with its leaflets attached to papillary muscles by the chordae tendineae, this method allows the effects of ruptured chordae and their surgical repair or replacement to be assessed in vitro. The chamber that holds the valve supports both the mitral annulus and papillary muscle annulus of the specimen. The mitral annulus is sutured onto rubber sheeting held in the chamber. The papillary muscle annulus is held in place by a Perspex support. The main part of the apparatus consists of a water pump connected through flexible tubing to the chamber that holds the valve in place. The pressure at failure is measured using a pressure transducer. Preliminary experiments demonstrate that anterior leaflet marginal chordae, but not strut chordae, are vital to valve function. Posterior leaflet chordae have been found to be important for valve competence.