Stephen J. Russell

Handbook of nonwovens

Development of the Nonwoven Industry. Dry-Laid Web Formation. Wet-Laid Formation. Polymer-Laid Web Formation. Mechanical Bonding. Thermal Bonding. Chemical Bonding. Chemical and Mechanical Finishing. Characteristics, Testing, and Modelling of Nonwoven Fabrics.

The Thermal Insulation Properties of Spacer Fabrics with a Mechanically Integrated Wool Fiber Surface

In relation to the engineering of compression resistant thermal insulation materials, an unconventional application of hydroentangling technology is introduced in which, lightweight, wool webs are mechanically attached to one side of preformed knitted spacer fabrics to partially occlude the underlying apertures. In contrast to conventional homogeneous fabrics, the resulting hydroentangled wool fiber web-spacer fabric constructs have markedly reduced thermal conductivity while there is little change in the overall fabric density; this is attributed to increased air entrapment within the cross-section of the spacer fabric due to the occlusion of the large apertures in the surface. Basic underlying theoretical principles are reviewed in relation to heat transfer in fibrous materials.

Anisotropic liquid absorption in homogeneous two-dimensional nonwoven structures

The anisotropy of liquid absorption is influenced by the fiber orientation distribution in the fabric plane. A unique theoretical analysis of planar fluid transport using D’Arcy’s law with two-dimensional directional permeability in nonwoven structures is presented which takes into account the fiber orientation distribution of the structure.

Tunable drug-loading capability of chitosan hydrogels with varied network architectures

Advanced bioactive systems with defined macroscopic properties and spatio-temporal sequestration of extracellular biomacromolecules are highly desirable for next generation therapeutics. Here, chitosan (CT) hydrogels were prepared with neutral or negatively charged cross-linkers in order to promote selective electrostatic complexation with charged drugs. CT was functionalized with varied dicarboxylic acids, such as tartaric acid, poly(ethylene glycol) bis(carboxymethyl) ether, 1,4-phenylenediacetic acid and 5-sulfoisophthalic acid monosodium salt (PhS), whereby PhS was hypothesized to act as a simple mimetic of heparin. Attenuated total reflectance Fourier transform infrared spectroscopy showed the presence of CO amide I, N-H amide II and CO ester bands, providing evidence of covalent network formation. The cross-linker content was reversely quantified by proton nuclear magnetic resonance on partially degraded network oligomers, so that 18 mol.% PhS was exemplarily determined. Swellability (SR: 299 ± 65-1054 ± 121 wt.%), compressibility (E: 2.1 ± 0.9-9.2 ± 2.3 kPa), material morphology and drug-loading capability were successfully adjusted based on the selected network architecture. Here, hydrogel incubation with model drugs of varied electrostatic charge, i.e. allura red (AR, doubly negatively charged), methyl orange (MO, negatively charged) or methylene blue (MB, positively charged), resulted in direct hydrogel-dye electrostatic complexation. Importantly, the cationic compound, MB, showed different incorporation behaviours, depending on the electrostatic character of the selected cross-linker. In light of this tunable drug-loading capability, these CT hydrogels would be highly attractive as drug reservoirs towards e.g. the fabrication of tissue models in vitro.

Triple-helical collagen hydrogels via covalent aromatic functionalisation with 1,3-phenylenediacetic acid

Chemical crosslinking of collagen is a general strategy to reproduce macroscale tissue properties in physiological environment. However, simultaneous control of protein conformation, material properties and biofunctionality is highly challenging with current synthetic strategies. Consequently, the potentially-diverse clinical applications of collagen-based biomaterials cannot be fully realised. In order to establish defined biomacromolecular systems for mineralised tissue applications, type I collagen was functionalised with 1,3-Phenylenediacetic acid (Ph) and investigated at the molecular, macroscopic and functional levels. Preserved triple helix conformation was observed in obtained covalent networks via ATR-FTIR (AIII/A1450 ~ 1) and WAXS, while network crosslinking degree (C: 87-99 mol.-%) could be adjusted based on specific reaction conditions. Decreased swelling ratio (SR: 823-1285 wt.-%) and increased thermo-mechanical (Td : 80-88 °C; E: 28-35 kPa; σmax : 6-8 kPa; εb : 53-58 %) properties were observed compared to state-of-the-art carbodiimide (EDC)-crosslinked collagen controls, likely related to the intermolecular covalent incorporation of the aromatic segment. Ph-crosslinked hydrogels displayed nearly intact material integrity and only a slight mass decrease (MR : 5-11 wt. %) following 1-week incubation in either PBS or simulated body fluid (SBF), in contrast to EDC-crosslinked collagen (MR : 33-58 wt. %). Furthermore, FTIR, SEM and EDS revealed deposition of a calcium-phosphate phase on SBF-retrieved samples, whereby an increased calcium phosphate ratio (Ca/P: 0.84-1.41) was observed in hydrogels with higher Ph content. 72-hour material extracts were well tolerated by L929 mouse fibroblasts, whereby cell confluence and metabolic activity (MTS assay) were comparable to those of cells cultured in cell culture medium (positive control). In light of their controlled structure-function properties, these biocompatible collagen hydrogels represent attractive material systems for potential mineralised tissue applications.

Controlled Morphology and Mechanical Characterisation of Electrospun Cellulose Acetate Fibre Webs

The purpose was to interpret the varying morphology of electrospun cellulose acetate (CA) fibres produced from single and binary solvent systems based on solubility parameters to identify processing conditions for the production of defect-free CA fibrous webs by electrospinning. The Hildebrand solubility parameter () and the radius of the sphere in the Hansen space () of acetone, acetic acid, water, N,N-dimethylacetamide (DMAc), methanol, and chloroform were examined and discussed for the electrospinning of CA. The Hildebrand solubility parameter () of acetone and DMAc were found to be within an appropriate range for the dissolution of CA. The suitability of the binary solvent system of acetone: DMAc (2 : 1) for the continuous electrospinning of defect-free CA fibres was confirmed. Electrospun webs exhibited improved tensile strength and modulus after heat and alkali treatment (deacetylation) of the as-spun material, and no major fibre morphological degradation occurred during the deacetylation process.

Multi-scale mechanical characterization of highly swollen photo-activated collagen hydrogels

Biological hydrogels have been increasingly sought after as wound dressings or scaffolds for regenerative medicine, owing to their inherent biofunctionality in biological environments. Especially in moist wound healing, the ideal material should absorb large amounts of wound exudate while remaining mechanically competent in situ. Despite their large hydration, however, current biological hydrogels still leave much to be desired in terms of mechanical properties in physiological conditions. To address this challenge, a multi-scale approach is presented for the synthetic design of cyto-compatible collagen hydrogels with tunable mechanical properties (from the nano- up to the macro-scale), uniquely high swelling ratios and retained (more than 70%) triple helical features. Type I collagen was covalently functionalized with three different monomers, i.e. 4-vinylbenzyl chloride, glycidyl methacrylate and methacrylic anhydride, respectively. Backbone rigidity, hydrogen-bonding capability and degree of functionalization (F: 16 ± 12–91 ± 7 mol%) of introduced moieties governed the structure–property relationships in resulting collagen networks, so that the swelling ratio (SR: 707 ± 51–1996 ± 182 wt%), bulk compressive modulus (Ec: 30 ± 7–168 ± 40 kPa) and atomic force microscopy elastic modulus (EAFM: 16 ± 2–387 ± 66 kPa) were readily adjusted. Because of their remarkably high swelling and mechanical properties, these tunable collagen hydrogels may be further exploited for the design of advanced dressings for chronic wound care.

Photo-active collagen systems with controlled triple helix architecture

The design of photo-active collagen systems is presented as a basis for establishing biomimetic materials with varied network architecture and programmable macroscopic properties. Following in-house isolation of type I collagen, reaction with vinyl-bearing compounds of varied backbone rigidity, i.e. 4-vinylbenzyl chloride (4VBC) and glycidyl methacrylate (GMA), was carried out. TNBS colorimetric assay, 1H-NMR and ATR-FTIR confirmed covalent and tunable functionalization of collagen lysines. Depending on the type and extent of functionalization, controlled stability and thermal denaturation of triple helices were observed via circular dichroism (CD), whereby the hydrogen-bonding capability of introduced moieties was shown to play a major role. Full gel formation was observed following photo-activation of functionalized collagen solutions. The presence of a covalent network only slightly affected collagen triple helix conformation (as observed by WAXS and ATR-FTIR), confirming the structural organization of functionalized collagen precursors. Photo-activated hydrogels demonstrated an increased denaturation temperature (DSC) with respect to native collagen, suggesting that the formation of the covalent network successfully stabilized collagen triple helices. Moreover, biocompatibility and mechanical competence of obtained hydrogels were successfully demonstrated under physiologically-relevant conditions. These results demonstrate that this novel synthetic approach enabled the formation of biocompatible collagen systems with defined network architecture and programmable macroscopic properties, which can only partially be obtained with current synthetic methods.

Biomimetic wet-stable fibres via wet spinning and diacid-based crosslinking of collagen triple helices

One of the limitations of electrospun collagen as bone-like fibrous structure is the potential collagen triple helix denaturation in the fibre state and the corresponding inadequate wet stability even after crosslinking. Here, we have demonstrated the feasibility of accomplishing wet-stable fibres by wet spinning and diacid-based crosslinking of collagen triple helices, whereby fibre ability to act as bonemimicking mineralisation system has also been explored. Circular dichroism (CD) demonstrated nearly complete triple helix retention in resulting wet-spun fibres, and the corresponding chemically crosslinked fibres successfully preserved their fibrous morphology following 1-week incubation in phosphate buffer solution (PBS). The presented novel diacid-based crosslinking route imparted superior tensile modulus and strength to the resulting fibres indicating that covalent functionalization of distant collagen molecules is unlikely to be accomplished by current state-of-the-art carbodiimide-based crosslinking. To mimic the constituents of natural bone extra cellular matrix (ECM), the crosslinked fibres were coated with carbonated hydroxyapatite (CHA) through biomimetic precipitation, resulting in an attractive biomaterial for guided bone regeneration (GBR), e.g. in bony defects of the maxillofacial region.

Directional Permeability in Homogeneous Nonwoven Structures Part II: Permeability in Idealised Structures

The application of a newly derived model of directional permeability, presented in Part I of this series of papers, is described for a range of specific fibre-orientation profiles. A consideration of the permeability results predicted by both the new model and existing theoretical and empirical models indicates that it is of practical use and is generally applicable to fibrous media of either low or high porosity.