Mechanics of bacterial cellulose hydrogel

Natural polymer-based hydrogels, like bacterial cellulose (BC) hydrogels, gained a growing interest in the past decade mainly thanks to their good biological properties and similar fibrous structure as real human tissues that make them good potential candidate materials for various applications in a biomedical field. BC hydrogels are produced in a process of primary metabolism of some microorganisms. They were intensively studied with regard to their biological aspects, revealing many potential applications such as a direct implant replacement of some real tissues and an excellent scaffold for in-vitro tissue regeneration; still, its mechanical behaviour under application-relevant conditions has not been well documented. [Continues.

Assessing stiffness of nanofibres in bacterial cellulose hydrogels: Numerical-experimental framework

This work presents a numerical-experimental framework for assessment of stiffness of nanofibres in a fibrous hydrogel – bacterial cellulose (BC) hydrogel – based on a combination of in-aqua mechanical testing, microstructural analysis and finite-element (FE) modelling. Fibrous hydrogels attracted growing interest as potential replacements to some tissues. To assess their applicability, a comprehensive understanding of their mechanical response under relevant conditions is desirable; a lack of such knowledge is mainly due to changes at microscale caused by deformation that are hard to evaluate in-situ because of the dimensions of nanofibres and aqueous environment. So, discontinuous FE simulations could provide a feasible solution; thus, properties of nanofibres could be characterised with a good accuracy. An alternative – direct tests with commercial testing systems – is cumbersome at best. Hence, in this work, a numerical-experimental framework with advantages of convenience and relative easiness in implementation is suggested to determine the stiffness of BC nanofibres. The obtained magnitudes of 53.7–64.9 GPa were assessed by calibrating modelling results with the original experimental data

Fracture behaviour of bacterial cellulose hydrogel: Microstructural effect

A growing interest in fibrous biomaterials, especially hydrogels, is due to a fact that they promise a good potential in biomedical applications thanks to their attractive biological properties and similar microstructure that mimics its in vivo environment. Since they are usually employed as a main load-bearing-component when introduced into body environment, a comprehensive understanding of their application-relevant mechanical behaviour, such as deformation and fracture, as well as structure-function relationships is essential. To date, deformation behaviour and mechanisms of hydrogels were well documented; still, a lack of understanding of their fracture behaviour, especially structure-function relationships, could complicate an evaluation of their applicability. Hence, this work carried out four types of test – uniaxial tension, single-notch, double-notch and central-notch fracture testing – to investigate fracture behaviour of fully-hydrated and freeze-dried bacterial cellulose (BC) hydrogel. Our results support a significant role of interstitial water – free and bonded water – played in fracture behaviour of the studied BC hydrogel

Inelastic behaviour of bacterial cellulose hydrogel: in aqua cyclic tests

Hydrogels are finding increasingly broad use, especially in biomedical applications. Their complex structure - a low-density network of microfibrils - defines their non-trivial mechanical behaviour. The focus of this work is on test-based quantification of mechanical behaviour of a bacterial cellulose (BC) hydrogel exposed to cyclic loading. Specimens for the tests were produced using Gluconacetobacter xylinus ATCC 53582 and tested in aqua under uniaxial cyclic loading conditions in a displacement-controlled regime. Substantial microstructural changes were observed in the process of deformation. A combination of qualitative microstructural observations with quantitative force-displacement relations allowed identification of main deformation mechanisms, confirming inelastic behaviour of the BC hydrogel under a loading-unloading-reloading regime. Elastic deformation was accompanied by non-elastic (viscoplastic) deformation in both tension and compression. This study also aims to establish a background for micromechanical modelling of overall properties of BC hydrogels

Through-thickness stress relaxation in bacterial cellulose hydrogel

Biological hydrogels, e.g. bacterial cellulose (BC) hydrogel, attracted increasing interest in recent decades since they show a good potential for biomedical engineering as replacements of real tissues thanks mainly to their good biocompatibility and fibrous structure. To select potential candidates for such applications, a comprehensive understanding of their performance under application-relevant conditions is needed. Most hydrogels demonstrate time-dependent behaviour due to the contribution of their liquid phase and reorientation of fibres in a process of their deformation. To quantify such time-dependent behaviour is crucial due to their exposure to complicated loading conditions in body environment. Some hydrogel-based biomaterials with a multi-layered fibrous structure demonstrate a promise as artificial skin and blood vessels. To characterise and model time-dependent behaviour of these multi-layered hydrogels along their through-thickness direction is thereby of vital importance. Hence, a holistic study combining mechanical testing and micro-morphological observations of BC hydrogel with analytical modelling of its relaxation behaviour based on fraction-exponential operators was performed. The results show a good potential to use a fraction-exponential model to describe such behaviour of multi-layered hydrogels, especially at stages of stress decay at low forces and of stress equilibrium at high forces

Effect of microstructure on anomalous strain-rate-dependent behaviour of bacterial cellulose hydrogel

This study is focused on anomalous strain-rate-dependent behaviour of bacterial cellulose (BC) hydrogel that can be strain-rate insensitive, hardening, softening, or strain-rate insensitive in various ranges of strain rate. BC hydrogel consists of randomly distributed nanofibres and a large content of free water; thanks to its ideal biocompatibility, it is suitable for biomedical applications. Motivated by its potential applications in complex loading conditions of body environment, its time-dependent behaviour was studied by means of in-aqua uniaxial tension tests at constant temperature of 37 °C at various strain rates ranging from 0.0001 s- 1 to 0.3 s- 1. Experimental results reflect anomalous strain-rate-dependent behaviour that was not documented before. Micro-morphological observations allowed identification of deformation mechanisms at low and high strain rates in relation to microstructural changes. Unlike strain-rate softening behaviours in other materials, reorientation of nanofibres and kinematics of free-water flow dominate the softening behaviour of BC hydrogel at high strain rates

Time-dependent rheological behaviour of bacterial cellulose hydrogel

© 2015 Elsevier B.V. All rights reserved. This work focuses on time-dependent rheological behaviour of bacterial cellulose (BC) hydrogel. Due to its ideal biocompatibility, BC hydrogel could be employed in biomedical applications. Considering the complexity of loading conditions in human body environment, time-dependent behaviour under relevant conditions should be understood. BC specimens are produced by Gluconacetobacter xylinus ATCC 53582 at static-culture conditions. Time-dependent behaviour of specimens at several stress levels is experimentally determined by uniaxial tensile creep tests. We use fraction-exponential operators to model the rheological behaviour. Such a representation allows combination of good accuracy in analytical description of viscoelastic behaviour of real materials and simplicity in solving boundary value problems. The obtained material parameters allow us to identify time-dependent behaviour of BC hydrogel at high stress level with sufficient accuracy

Fracture processes in cortical bone: effect of microstructure

Understanding of bone fracture can improve medical and surgical procedures. Therefore, investigation of the effect of bone’s microstructure and properties as well as loading conditions on crack initiation and propagation is of great importance. In this paper, several modelling approaches are used to study fracture of cortical bone tissue at various length scales and different types of loading. Two major problems are tackled: crack propagation under impact loading and bone cutting in surgical procedures. In the former case, a micro-scale finite-element (FE) fracture model was suggested, accounting for bone’s microstructure and using X-FEM for crack-propagation analysis [1, 2]. The cortical bone tissue was modelled as four-component heterogeneous materials. The morphology of a transverse-radial cross section captured with optical microscopy was used to generate FE models; extensive experimental studies provided necessary mechanical input data [3]. The problem of bone cutting was treated within the framework of tool-bone interaction analysis [4, 5]. A two-domain approach was used, with a process zone simulated using a smooth-particle hydrodynamics method. This zone was embedded in a continuum domain with macroscopic anisotropic properties obtained in experiments. This study is supported by analysis of damage induced by interaction between the cutting tool and the bone tissue using wedge-indentation tests and considering also the anisotropic behaviour of the bone

Properties and application of polyimide-based composites by blending surface functionalized boron nitride nanoplates

Properties and application of polyimide-based composites by blending surface functionalized boron nitride nanoplate

Effects of surface-functionalized aluminum nitride on thermal, electrical, and mechanical behaviors of polyarylene ether nitrile-based composites

Aluminum nitride (AlN) with high thermal conductivity was blended in polyarylene ether nitrile (PEN) to obtain a composite system. A ball milling process could provide AlN particles of smaller size with higher surface silylation for homogeneous particle distribution in polymeric matrix. Thermal, electrical, and mechanical behaviors of the produced composites were characterized to investigate the effects of particles on the performance of PEN-based composites with functionalized AlN. The composite exhibited thermal conductivity of 0.779 W m−1 K−1, a dielectric constant of 7.7, dielectric loss of 0.032, electrical resistivity of 1.39 GΩ.cm, and break strength of 36 N when the fraction of functionalized AlN increased to 42.3 vol%. A fitted equation based on the improved Russell's model could effectively predict a trend for thermal conductivity of the composite systems with consideration of interfacial resistance between AlN and surrounding PEN