Journal club report - Device for testing tennis shoe-court interactions

One of the challenges faced by tennis players is the interaction of their shoes with the court.  Tennis is played on a range of surfaces with different properties. For example, it it much easier to slide across a clay court than a hard court due to the lower friction with the less rough surface.  Researchers at the University of Sheffield investigated tennis shoe-court interactions and subsequently developed a portable mechanical device that can assess the quality of a tennis court which will be utilised by the International Tennis federation.

The rubber used to make tennis shoes is viscoelastic which means if a force is briefly applied to the material it is deformed and then returns to its original shape but when a force is applied for longer the material is deformed permanently.  The rubber soles are compressed against the surface of a tennis court and the rubber molds its shape to the roughness of the surface which is the cause of the friction. Therefore, when designing the test it was important to understand how load, surface roughness, shoe orientation, temperature and contact area contribute to the frictional force and a lab-based test rig was built for initial tests.  The researchers concluded that friction is mainly affected by sole surface temperature and surface roughness, and also the presence of a tread in the shoe sole.

In the design of the portable test device, the main features considered were reliability, size and weight for transportation and the ability to represent match play conditions well. Also, the test shoe needed to be interchangeable to allow testing of different sole materials and sizes. The final design consists of a test shoe slider attached to a sled with weights mounted on it, a pneumatic ram which provides a horizontal force to drive the sled. The horizontal force is applied until the test shoe slider is initiated and friction is measured.

The researchers hope that with further development the device could become part of a standard test to assess the quality of courts for elite tennis competitions and eventually be used to improve the quality of tennis courts for all players.

Original Paper:

D. Ura, M Carré, Development of a Novel Portable Test Device to Measure the Tribological Behaviour of Shoe Interactions with Tennis Courts, Procedia Engineering, 147, 550-555, 2016.

Anti-microbial Graft Co-polymer Gels

Antimicrobial-gels-384x384.png

In 2008, researchers at the University of Sheffield published work regarding the synthesis of a triblock copolymer which formed a biocompatible gel suitable for use in wound dressings.1Unexpectedly, these gels also exhibited antimicrobial activity towards a range of microorganisms.2 This work has recently been further studied in a collaboration between the Department of Chemistry, the School of Clinical Dentistry and the Department of Engineering Materials at the University of Sheffield. This study aimed to understand more about the relationship between the polymer structure or architecture and its antimicrobial activity. This exciting research has potential be used in the future to provide alternatives to standard antibiotic treatments in a society where they have become much overused and antibacterial resistance is a growing problem.3

The original material synthesised had an ABA structure comprising of blocks of 2-hydroxypropyl methacrylate (HPMA) and 2-(methacryloyloxy) ethyl phosphorylcholine (MPC).An analogous graft-copolymer has been synthesised where PHPMA chains have been attached onto a PMPC chain via RAFT polymerisation, a form of controlled free-radical polymerisation used frequently by the research group of Professor Steve Armes in the Department of Chemistry. As well as this, AB diblock copolymers of the same monomers have been synthesised and shown to form self-assembled ‘worm-gels’. This novel material is biocompatible and has also been used in this study to determine if it is due to the gel property of polymers with this composition which leads to their antimicrobial properties.

Biological assays with staphylococcus aureus have been carried out on all of these PHPMA-PMPC materials to determine more about the mechanism of antimicrobial activity. Results have shown that the ‘worm-gels’, although biocompatible gels, were not antimicrobial. However, the graft copolymer gels have been shown inhibit bacterial growth and further studies by transmission electron microscopy show that this is likely due to the hydrophobic PHPMA chains in the gel penetrating and therefore causing damage to the bacterial membrane. Another interesting property of this graft copolymer gel is its thermoreversible gelation behaviour, making it applicable in the formation of antibacterial smart wound dressings.

Original article: Antimicrobial Graft Copolymer Gels. A. C. Harvey, J. Madsen, C. W. I. Douglas, S. MacNeil, S. P. Armes, Biomacromolecules, 2016, 2710-2718

Article by Rheanna Perry; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

(1)        Madsen, J.; Armes, S. P.; Bertal, K.; Lomas, H.; MacNeil, S.; Lewis, A. L. Biomacromolecules 2008, 9 (8), 2265–2275.

(2)         Bertal, K.; Shepherd, J.; Douglas, C. W. I.; Madsen, J.; Morse, A.; Edmondson, S.; Armes, S. P.; Lewis, A.; MacNeil, S. J. Mater. Sci. 2009, 44 (23), 6233–6246.

(3)           Harvey, A. C.; Madsen, J.; Douglas, C. W. I.; MacNeil, S.; Armes, S. P. Biomacromolecules 2016, 17 (8), 2710–2718.

Polymer Centre Highlight – Self-Assembled Graphene Oxide–Gelatin Nanocomposite Hydrogels

Hydrogels consist of a network of hydrophilic polymer chains held together by cross-links. These cross-links prevent the individual motion of polymer chains leading to the formation of a ‘gel’. Hydrogels closely resemble biological tissues due to their high water content and environmental sensitivities. This makes them attractive for biomedical applications, including tissue engineering and drug delivery.

The cross-links between polymer chains can be either physical or chemical. Physically cross-linked gels tend to be weak and the physical cross-links between the polymer chains are reversible. The weak mechanical properties limit the use of physically cross-linked gels in wider applications.

Chemically cross-linked gels are formed following a chemical reaction, where a chemical cross-linking agent is added to a solution of polymer chains.  By varying the quantity of cross-linking agent, the degree of cross-linking can be altered to optimise the gel properties for different applications. Although chemically cross-linked gels have attractive properties, the toxicity of cross-linking agents limits their use in biomedical applications.

Polymer centre academic Dr. Biqiong Chen has attempted to overcome these boundaries by synthesising novel, physically cross-linked, graphene oxide (GO)-gelatin nanocomposite hydrogels by self-assembly.

Both gelatin hydrogels and graphene-based materials have previously been studied for applications in drug delivery and tissue engineering. Gelatin is a denatured biopolymer, derived from collagen (found in skin and muscle). It has advantageous properties such as being biocompatible, biodegradable and low cost. GO, the oxidised form of graphene, has low toxicity. Gelatin-functionalised GO nanosheets are non-toxic and can be removed from the body by metabolism. Although these gels are physically cross-linked, multiple hydrogen-bonding and electrostatic interactions between the gelatin chains and GO sheets increase the strength of the 3D network significantly. These gels also exhibit self-healing properties.

The GO-gelatin hydrogels are also pH responsive and can be used for pH-sensitive drug release. At acidic pH (pH 1.7) the GO sheets form tightly packed aggregates preventing drug release. At neutral pH (pH 7.4) the pore size increases, promoting the diffusion of an encapsulated drug from the hydrogel to its surrounding liquid environment. This pH-sensitive behaviour would allow selective drug release into the intestine (pH 6.6-7.5) with minimal release in the stomach (pH 1.0-2.5). Conventional drug delivery methods can carry drugs to a specific location but are unable to protect the drug against the acidic and enzymatic environment of the stomach which can lead to the drug being released early or being altered. In contrast, these GO-gelatin hydrogels can protect the drug from enzymatic attack in the stomach. This allows the drug to be maintained and then released in a more controlled manner.

This research was published by Dr Biqiong Chen’s research group in the Journal of Polymer Science Part B and was supported by the University of Sheffield. Original Article: Y. Piao, B. Chen,  J. Polym. Sci. Part B: Polym. Phys, 2014, 53, 356 -367.

Article by Amy Cockram; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

An application of the Shear-Induced Polarized Light Imaging (SIPLI) Technique

Rheology is the study of flow and/or deformation of materials under applied forces. Using a plate-plate geometry, where the top and bottom features are flat plates, the measurement of rheological properties such as shear viscosity, normal stress, dynamic modulus and phase angle are enabled. Although this rotational rheology technique is well established, there is continuously growing interest in the combination of rotational rheology with other characterisation techniques to allow simultaneous measurements of viscoelasticity and additional physical properties.

Oleksandr Mykhaylyk et al. recently reported a new technique, shear-induced polarised light imaging (SIPLI), which combines rotational rheology with a reflection polariscope in order to study the behaviour of macromolecules and nanoparticles through changes in the birefringence properties of a material. Birefringence occurs when there is shear-induced orientation of particles which creates optical anisotropy. Birefringent materials change the orientation of the polarised light used to illuminate the sample, allowing it to pass through an analyser which is orthogonal to the axis of polarised light. The result of shear-induced orientation is a characteristic Maltese cross pattern. Conversely, non-birefringent samples do not change the orientation of plane-polarised light and thus the resulting polarised light image appears dark.

One application for SPILI presented by Mykhaylyk et al., is for the study of thermo-responsive block copolymer micelles. A number of particle morphologies can be produced from the self-assembly of amphiphilic block copolymers in a solvent selective for one of the blocks. These include spheres and vesicles, which form free-flowing liquids, and worms, which form free-standing physical hydrogels. Unlike the non-birefringent nature of the sphere and vesicle morphologies, the uniaxial anisotropy of the worm morphology results in birefringence. Consequently, the thermo-reversible worm-to-sphere transition, and associated reversible de-gelation, of a poly(glycerol monomethacrylate)-block-poly(2-hydroxypropyl methacrylate) hydrogel is an ideal system for SIPLI characterisation.

The rheology data presented in the study confirms the expected reduced viscosity on cooling to 5 °C associated with degelation during the worm-to-sphere morphology transition. Simultaneously, a loss of the Maltese cross pattern is observed due the formation of isotropic spherical particles. On heating, the opposite sphere-to-worm transition occurs resulting in the reappearance of the Maltese cross pattern and an increase in viscosity due to regelation. Here, SIPLI has allowed the study of structural-rheological property relationships of thermo-responsive block copolymer micelles. This demonstrates just one application of SIPLI, with many other advantages and applications of SIPLI discussed in the original article.

Original Article: Applications of Shear-Induced Polarized Light Imaging (SIPLI) Technique for Mechano-Optical Rheology of Polymers and Soft Matter Materials O. O. Mykhaylyk, N. J. Warren, A. J. Parnell, G. Pfeifer and J. Laeuger, J. Polym. Sci. Part B Polym. Phys., 2016, 1–20.

Article by Sarah Byard; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Repeatable pre-cracking preparation for fracture testing of polymeric materials

A team of researchers from the University of Sheffield’s Department of Mechanical Engineering, including Polymer Centre academic Dr Rachel Tomlinson, have recently developed a method for creating repeatable pre-cracked polymer specimens for fracture testing.

Currently, specimens are prepared manually using a razor blade to create a small pre-crack from a notched sample. These samples are then tested to destruction to obtain fracture properties of the material. The procedure is heavily dependent on the operator and produces pre-cracks of varying length, angle and crack-front shape; all of these factors affect the repeatability and reliability of the subsequent test, as well as posing several safety concerns.

Using a technique first proposed by Tamura et al.[1], Dr Tomlinson’s team developed a methodology to produce consistent pre-cracks that meet the requirements of the various test standards for measuring fracture toughness. The technique involves applying loading in 2 directions; a tensile load to propagate a crack from the notch manufactured during specimen creation, and a compressive load applied across the width of the specimen at the desired distance from the notch to act as a barrier to further crack growth (Figure 1). A variety of compressive loads were investigated to find a suitable load for the test material which prevented crack propagation past the desired length, without causing the crack to then travel parallel to the direction of the compressive load. Once this was established, 40 test samples were prepared using this method and the pre-crack length and shape recorded. The results showed that the developed method was very repeatable, with 35 of the specimens accurate to within 10 microns of the desired crack length. A comparison of the crack fronts produced using the current and proposed methods showed that the developed method also produces a significantly straighter crack front, minimising the need to average the crack length across the width of the specimen and saving additional time.

Original Publication: Nithiananthan Kuppusamy, Rachel A. Tomlinson, Repeatable pre-cracking preparation for fracture testing of polymeric materials, Engineering Fracture Mechanics, Volume 152, February 2016, Pages 81-87, ISSN 0013-7944

Article by Ryan Brown; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

[1] Tamura K, Hasjimoto S. A precrack introducing method in CT-specimens for measuring KIC values of brittle materials. In: 15th International conference of experimental mechanics. ICEM15. Portugal; 2011. p. 7

Influence of Surface Wettability on Microbubble Formation

Over the last 20 years bubbles and their properties have been of high interest in the field of engineering and have found their way into a variety of applications such as biofuel production, medical imaging, and drug delivery. Lately, there has been a surge of interest in the study of microbubbles which are found in the size range of 1-999μm. The appeal of microbubbles comes from the fact that they possess a high surface area for a given gas volume and therefore enable high levels of mass transfer which is linked with the interfacial area between two phases.

Investigation into microbubble formation is essential in order to fully utilise their properties in applications. Many features of bubble formation have previously been explored and have been found to have a large impact, these include pore size and orientation, and flow rate.  The university and Polymer centre academic Dr Jonathan Howse et al. present an investigation into how the wettability of the diffuser surface impacts upon the dynamics of bubble formation process. Effects were studied at the orifice as well as upon the resultant bubble cloud produced by the wetting variations. The study comprises a thorough investigation into bubble formation by examining bubbles formed from a single pore, multiple well-defined pores, and sintered diffusers with a random network of pores.

Various surfaces were investigated during the course of this study which displayed contact angles from 107.9° to 13.1°. It was found that when a surface displays a contact angle lower than 90° the bubbles that are emitted are significantly smaller than those emitted from a surface with a contact angle above 90°. This data indicates that a contact angle of 90° is a switching point where the bubble size vastly changes.

Original article: Influence of Surface Wettability on Microbubble Formation, D. J. Wesley, R. M. Smith, W. B. Zimmerman, J. R. Howse, Langmuir, 2016, 32, 1269-1278.

Article by Thomas Neal; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Photoluminescent and superparamagnetic quantum dots

Quantum dots are semiconducting nanostructures which emit radiation over a narrow spectrum. This is caused by the restricted movement of excitons, formed when electrons are excited from the valence band into the conduction band leaving holes. When excitons relax light is emitted over a narrow spectrum due to the small crystal size, typically 2-50 nm, with the wavelength of light emitted dependent on the size.

Recent work by Biqiong Chen and Shelia MacNeil at the Polymer Centre, in collaboration with various researchers, have harnessed the narrow spectrum emission of quantum dots for the potential of medical imaging alongside drug delivery and photothermal therapy.

The GO quantum dots, composed of reduced graphene oxide-iron oxide, combined the superparamagnetic properties of iron oxide with the drug loading capabilities of graphene oxide. The quantum dots were found to be inherently photoluminescent and superparamagnetic alongside being biocompatible making the materials ideal for use as medical devices.

Photoluminescence is useful in providing detection of the materials by fluorescent imaging. Superparamagnetism allows the magnetic moment of the quantum dots to be changed on a nanoscale. Hence, drug release from quantum dots can be stimulated by applying an external magnetic field for targeted drug delivery. Superparamagnetism also allows imaging by magnetic resonance imaging (MRI) to aid the visualisation of tumours and image guided surgeries.

The GO quantum dots were loaded with the model drug lidocaine hydrochloride. The loading ratio of drug to quantum dots was 0.31:1 which was fully released in phosphate buffered saline solution over 8 hours indicating efficient drug elution.

Testing of the GO quantum dots over a range of concentrations with varying laser powers showed a maximum temperature increase of 50 °C. This large temperature increase would be useful for treating tumours by photothermal therapy, where heat is applied directly to tumours to damage the cancerous cells.

Original article: Photoluminescent and superparamagnetic reduced graphene oxide-iron oxide quantum dots for dual-modality imaging, drug delivery and photothermal therapy, R. Justin, K. Tao, S. Román, D. Chen, Y. Xu, X. Geng, I. M. Ross, R. T. Grant, A. Pearson, G. Zhou, S. MacNeil, K. Sun and B. Chen, Carbon N. Y., 2016, 97, 54–70.

Article by Jasmine Lord; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Structural Colour in the Natural World

Polymer centre academics, including Dr. Andrew Parnell, Prof. Patrick Fairclough and Dr. Oleksandr Mykhaylyk, have recently been involved in a collaboration that has helped shed more light on the origin of structural colour in the natural world. Structural colour refers to cases where colour arises from the scattering of light by nanoscale structural elements, as opposed to absorption of light by pigment molecules, and it is common to many species of animal. Achieving colour in this way has a number of benefits, for example it avoids the need for constant replenishing of degraded pigment.
It has been known for a long time that birds make use of structural colour in their feathers, although the mechanism for this has also long been disputed. More recently, it was suggested that the colour arises from a number of ‘spongy’, relatively disordered networks of beta-keratin and air. The difference in refractive index between the two phases leads to coherent scattering of light, with the exact wavelengths scattered depending on the dimensions of the feather’s nanostructure. The team at the University of Sheffield were specifically interested in the origin of the non-iridescent blues and whites found on the feathers of the Eurasian Jay, as well as several other birds with similar colouration from a wide geographical range.

In the study, the feathers were imaged using both optical and electron microscopy, although the most significant results were obtained by scanning with Small Angle X-ray Scattering (SAXS). This technique allows non-destructive probing of the nanoscopic length scales relevant to the optically-active structures. The colours of the feathers could be correlated to the size of the beta-keratin domains in the scattering structures, and it was shown that the variation in colour seen along individual barbs on the feather was due to continuous variation of the dimensions of the keratin ‘sponge’. However, the exact biological mechanism which allows this degree of control during the feather’s growth is still unknown.

Their results also help explain why non-iridescent green colours in the natural world are rarely generated by structural elements alone, despite being obviously useful for camouflage. The keratin structures were found to scatter relatively broad portions of the spectrum: ‘structural blue’ can be obtained by scattering the blue and the non-visible, ultraviolet portions of the spectrum, and ‘structural white’ can obtained by scattering light across the whole of the visible spectrum, but green light occupies too narrow a set of wavelengths to be scattered selectively. However, one solution to this is to use a combination of ‘structural blue’ and yellow pigment, with the green tree frog being a good example.

Original publication: Parnell, A. J., Washington, A. L., Mykhaylyk, O. O., Hill, C. J., Bianco, A., Burg, S. L., Parker, A. R. (2015). Spatially modulated structural colour in bird feathers. Scientific Reports5, 18317. http://doi.org/10.1038/srep18317

Article by Rhys Williams; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Encapsulation and Thermally Triggered Release from Polymer Vesicles

University of Sheffield and Polymer Center academic Professor Steven Armes et al. have demonstrated the encapsulation of silica nanoparticles and bovine serum albumin within synthetic polymeric vesicles, and the consequent triggered release of the payload.
The encapsulation and subsequent release of active ingredients on the micro or nanoscale has drawn considerable academic interest over the recent years. Such processes find application in a wide range of industrial formulations, from medicine to laundry science and agrochemicals.

Similarly, the ability of certain polymers to self-assemble in solution to form hollow spheres, known as vesicles or polymersomes, provides us with a convenient vessel within which to encapsulate desired active ingredients. These vesicles contain an inner lumen encased by a spherical polymeric bilayer, somewhat similar to the way eukaryotic cells are encased by a lipid bilayer.

In the recent work reported by Professor Armes, the particular block copolymer used to produce these vesicles was poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate), which abbreviated to PGMA-PHPMA. The use of a technique known as polymerisation-induced self-assembly (PISA) enabled the convenient synthesis of these PGMA-PHPMA vesicles at relatively high concentrations and short reaction times.
Poly(2-hydroxypropyl methacrylate) has an unusual property: in water it becomes more soluble at lower temperatures. Furthermore, by cooling a solution of PGMA-PHPMA vesicles down to around 0 °C, the PHPMA block becomes increasingly solvated which causes the vesicles to break apart. The ability to disintegrate the encapsulating vesicles by cooling provides a convenient thermal trigger to release any payload that may be loaded within them. By cooling the silica loaded vesicles in ice for 30 minutes, it was demonstrated that the vesicles disintegrated and the silica nanoparticles were released back into solution.
The encapsulation and release process was characterised by a number of analytical techniques, including transmission electron microscopy (TEM), disc-centrifuge photosedimentometry (DCP) and small-angle x-ray scattering (SAXS). In addition, this encapsulation process was not limited to inorganic silica nanoparticles; biological material could also be encapsulated. Bovine serum albumin (BSA), a model globular protein, was successfully encapsulated in the PGMA-PHPMA vesicles and then released upon cooling.

For the original publication please see C. J Mable, S. P. Armes et al. J. Am. Chem. Soc. 2015, 137, 16098 or visit http://pubs.acs.org/doi/abs/10.1021/jacs.5b10415

Article by Matt Rymaruk; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.

Polymer Centre Highlight: A Greener Route to Green Energy

Polymer centre academics; Dr Alan Dunbar, Dr Ahmed Iraqi, Dr Alastair Buckley and Prof David Lidzey have reported the manufacture of organic photovoltaic devices from non-halogenated binary solvent blends. This work was carried out in collaboration with Dr Andrew Pearson from the Optoelectronics Group at Cambridge University.

Organic photovoltaic devices (OPVs) have seen recent improvements in the power conversion, now being able to achieve 9 % efficiency. The performance achievable is approaching the benchmark which would allow OPVs to become commercially viable. OPVs use a conjugated polymer and fullerene derivative to act as a semi-conducting layer. However, currently OPVs require the use of halogenated solvents to dissolve and then deposit this essential semi-conducting layer. In order to produce environmentally acceptable OPVs, a suitable non-halogenated solvent must be found for the semi-conducting layer though the majority do not readily dissolve in such solvents.

Hansen solubility parameters were used to predict solvent systems that would dissolve the organic semi-conductor. This was achieved by using a system with similar solubilising properties to that of the halogenated solvent. The solvent system used was carbon disulphide (CS2) and acetone. Both are used commercially and have a lower toxicity than the halogenated solvents previously used. The solvent blend had a solubility limit of 20 mg ml-1 compared to 10 mg ml-1 of the halogenated solvent. This increase is attributed to blending allowing for a closer match to the solubility parameters of the organic semi-conductor.

OPVs produced using the solvent blend achieved power conversion values higher than those obtained from using a halogenated solvent, for both conjugated polymers used.

Original publication: Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends, Griffin, J.; Pearson, A. J.; Scarratt, N. W.; Wang, T.; Dunbar, A. D. F.; Yi, H.; Iraqi, A.; Buckley, A. R.; Lidzey, D. G., Org. Electron. 2015, 21, 216-222.

Article by Luke Fox; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.