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

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

Poly(4-vinylaniline)/polyaniline bilayer functionalized bacterial cellulose membranes as bioelectronics interfaces

Bacterial cellulose (BC) fibers are chemically functionalized with poly(4-vinylaniline) (PVAN) interlayer for further enhancement of electrical conductivity and cell viability of polyaniline (PANI) coated BC nanocomposites. PVAN is found to have promoted the formation of a uniform PANI layer with nanofiber- and nanorod-like supramolecular structures, as an overall augmentation of PANI yield. Compositional and microstructural analysis indicates a PVAN/PANI bilayer of approximately 2 μm formed on BC. The solid-state electrical conductivity of such synthesized BC nanocomposites can be as high as (4.5 ± 1.7) × 10−2 S cm−1 subject to the amounts of PVAN chemically embraced. BC/PVAN/PANI nanocomposites are confirmed to be thermally stable up to 225 °C, and no signs of cytotoxicity for SVZ neural stem cells are detected, with cell viability up to 90% on BC/PVAN/PANI membranes. We envisage these new electrically conductive BC/PVAN/PANI nanocomposites can potentially enable various biomedical applications, such as for the fabrication of bioelectronic interfaces and biosensors

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

Microstructural and mechanical characteristics of PHEMA-based nanofibre-reinforced hydrogel under compression

Natural network-structured hydrogels (e.g. bacterial cellulose (BC)) can be synthesised with specific artificial hydrogels (e.g. poly(2-hydroxyethyl methacrylate)(PHEMA)) to form a tougher and stronger nanofibre-reinforced composite hydrogel, which possesses micro- and nano-porous structure. These synthetic hydrogels exhibit a number of advantages for biomedical applications, such as good biocompatibility and better permeability for molecules to pass through. In this paper, the mechanical properties of this nanofibre-reinforced hydrogel containing BC and PHEMA have been characterised in terms of their tangent modulus and fracture stress/strain by uniaxial compressive testing. Numerical simulations based on Mooney-Rivlin hyperelastic theory are also conducted to understand the internal stress distribution and possible failure of the nanofibre-reinforced hydrogel under compression. By comparing the mechanical characteristics of BC, PHEMA, and PHEMA-based nanofibre reinforced hydrogel (BC-PHEMA) under the compression, it is possible to develop a suitable scaffold for tissue engineering on the basis of fundamental understanding of mechanical and fracture behaviours of nanofibre-reinforced hydrogels

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

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

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

Table_5_The changing relationship between health risk behaviors and depression among birth cohorts of Canadians 65+, 1994–2014.DOCX

Background and objectiveThe older adult residents of Canada form an increasingly larger proportion of the population and are becoming better educated and have more income. Depression is a common mental disorder, particularly among seniors. Several health risk behaviors–physical inactivity, tobacco use, and alcohol consumption–are linked to mental health problems. This study examines whether these health risk behaviors and their association with depression among Canadians 65+ born in eight cohorts between 1910–1914 and 1945–1949, have changed.MethodsPooled data drawn from 11 nationally representative health surveys conducted by Statistics Canada between 1994 and 2014 are analyzed–88,675 survey participants met inclusion criteria. Depression was assessed by the Composite International Diagnostic Interview–Short Form. Health risk behaviors examined were physical activity/inactivity, smoking, and alcohol use. A Cochran Armitage trend test for categorical outcomes and a log-binomial modeling for binary outcomes were used to estimate the risk ratios across cohorts.ResultsThe proportions of Canadians 65+ who are physically active, regular drinkers, and regular smokers have increased; however, depression prevalence fluctuated non-significantly. Depression increased among all health risk behaviors, particularly in recent birth cohorts. Depression among physically inactive seniors, current smokers, and non-drinkers was significantly higher than among active, non-smokers, and regular drinkers (all P ConclusionInactivity and smoking were consistently associated with a significantly increased risk of depression among Canadian residents 65+, with smoking becoming more firmly connected to depression risk in more recent birth cohorts. In contrast, moderate alcohol use was associated with a decreased risk of depression, but that protective effect ceased in most recent birth cohort. Identifying the changing relationships between health risk behaviors and depression is meaningful for developing prevention strategies for depression and other emotional and mental health problems.</p

Geographical distribution of the WTWR of sea level pressure anomalies (SLPAs) in the Eurasian continent for the starting month of February.

<p>Geographical distribution of the WTWR of sea level pressure anomalies (SLPAs) in the Eurasian continent for the starting month of February.</p