The design and characterisation of sinusoidal toolpaths using sub-zero bioprinting of polyvinyl alcohol

Sub-zero (°C) additive manufacturing (AM) systems present a promising solution for the fabrication of hydrogel structures with complex external geometry or a heterogeneous internal structure. Polyvinyl alcohol cryogels (PVA-C) are promising tissue-mimicking materials, with mechanical properties that can be designed to satisfy a wide variety of soft tissues. However, the design of more complex mechanical properties into additively manufactured PVA-C samples, which can be enabled using the toolpath, is a largely unstudied area. This research project will investigate the effect of toolpath variation on the elastic and viscoelastic properties of PVA-C samples fabricated using a sinusoidal toolpath. Samples were fabricated using parametric variation of a sinusoidal toolpath, whilst retaining the same overall cross-sectional area, using a sub-zero AM system. To mechanically characterise the samples, they were tested under tension in uniaxial ramp tests, and through dynamic mechanical analysis (DMA). The elastic and viscoelastic moduli of the samples are presented. No correlations between the parametric variation of the design and the Young's modulus were observed. Analysis of the data shows high intra-sample repeatability, demonstrated robust testing protocols, and variable inter-sample repeatability, indicating differences in the printability and consistency of fabrication between sample sets. DMA of the wavelength samples, show a frequency-dependent loss moduli. The storage modulus demonstrates frequency independence, and a large increase in magnitude as the sample increases to 3 wavelengths.

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 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.

Graphic abstract