In 1999 I wrote a review article on auxetic materials for C&I magazine (1999, 10, 384) in which I introduced the novel elastic property of a negative Poisson’s ratio that characterises an auxetic material. Put simply, such a material becomes thicker widthwise when stretched lengthwise, and thinner when compressed. This is counter to the response of many common materials, which become thinner when stretched.
So what progress has been made since then, particularly towards commercial applications for these fascinating materials? The CHISMACOMB and REACTICS consortia are examples of industry-led projects in which the emphasis is on developing commercial auxetic material-based products and processes. Other efforts to move from the academic laboratory to market include the formation of university spin-out companies – including Auxetix from Ken Evans’ group at the University of Exeter and Auxetic Technologies from our own group at the University of Bolton, UK.
The portfolio of patents, whilst relatively modest, has grown significantly over recent years. Figure 1 shows the number of patents by year of priority filing, for example, identified using the search terms ‘auxetic’ and/or ‘negative + Poisson + ratio’ in the title or abstracts of patents held by the European Patent Offi ce. Multiple applications for the same invention have been excluded.
The distribution starts in 1986 with Rod Lakes’ invention of the process to make the first auxetic foam material, closely followed by Evans’ patents in 1989 relating to auxetic honeycombs as core materials in sandwich panel components (with Brian Caddock) and auxetic microporous polymers (with Kim Alderson). It is notable that the concerted attempts by sector network AuxetNet and others (see Box) around 2003 to engage industry appear to coincide with a growth in patent application filings: 71% of the patent portfolio in Figure 1 were filed in the most recent five years for which public domain data exist (2004-2008) and this trend continues. The highest number of fi lings per year occurred in 2007 and 2008, with more than twice as many filed in 2008 compared with 2007.
A third of all patent applications in Figure 1 were filed initially in the UK; matched by a similar proportion filed in the US; 20% from other European countries; and 12.5% from China, Japan and the Republic of Korea. A significant number of the applications relate to developments in materials and processes, but applications are also specific to the biomedical, aerospace, automotive, military, leisure, chemical engineering, construction, apparel and energy sectors.
As an example of developments in the biomedical sector, my earlier C&I feature noted a patent application – US 5108413, filed in 1990 – relating to the use of auxetic PTFE as the expansion member in a dilator device for coronary angioplasty and related procedures. Auxetic expansion members have several advantages over conventional balloon catheters and avoid the need to inflate a balloon. Advanced Cardiovascular Systems filed a patent in 2005 for a dilator Counter-intuitive: auxetic materials become thicker width-wise when stretched employing expanded ultra-high molecular-weight polyethylene (UHMWPE), including auxetic variant, as a liner component for an intraluminal catheter suitable for angioplasty and bio stent deployment, for example. Because UHMWPE is stronger than PTFE, a thinner lining is possible – so maximising the diameter of the inner lumen.
Based on similar principles, biomedical devices startup Auxetica filed patents in 2002 on the design of auxetic liner structures for medical stents. Other auxetic stents are based on origami principles – origami stent grafts – and use the small volume of a folded crease structure to deliver the device to the correct location. The creases disappear when the stent is deployed. These have a number of advantages over stents comprising strut structure and cover components and are easier to manufacture.
Other biomedical patent filings relating to auxetic materials include ophthalmic devices, annuloplasty prosthesis and compression bandages. Lakes – now at the University of Wisconsin – has also designed and performed laboratory testing of an auxetic artifical intervertebral disc intended to relieve pain through the prevention of spinal disc bulge impingement on surrounding nerves.
Particularly in the biomedical arena, auxetic materials are already starting to make an impact. But what about the developments in auxetic materials, properties and processes over the past 10 years? Progress in manufacturing auxetic foams has included adapting Lakes’ batch process to produce highly anisotropic foams – with mechanical and structural properties dependent on direction – which leads to the prospect of a semi-continuous conversion process.
In work at Bolton, originally funded by Sara Lee Branded Apparel (SLBA), we have also recently extended this process to produce both flat and curved thin – typically 1-5mm thick – foam displaying auxetic characteristics, for use as inserts in intimate apparel garments for example. Large area auxetic thin sheet materials can be produced, but making large thick samples suitable for mattresses and wrestling mats is still a challenge. Joseph Grima at the University of Malta has recently developed an alternative process in which the role of temperature in Lakes’ process is replaced by a solvent stage.
Other advances likely to lead to more foam applications include the development of multifunctional auxetic foams. By soaking auxetic foams in electrorheological and magnetorheological fluids, Fabrizio Scarpa of the University of Bristol, UK, has developed auxetic foams that change stiffness when electric or magnetic fields are applied. Significant magnetostrictive effects – changes in size/shape in response to a magnetic field – and increases in viscoelastic damping and refractive index have been observed. These foams have potential as active acoustic absorbers in ‘smart’ sound or vibration control applications.
The theme of multi-functionality was explored further in the CHISMACOMB project, which has led to a range of advanced honeycomb structures, including the first honeycomb with embedded sensors and actuators. This multifunctional honeycomb platform may find use in structural health monitoring of sandwich panel components in aerospace and marine applications; and in active microwave screens for selective absorption of electromagnetic waves. Fabrication routes for commercial production have been developed, including snap-fit, resin transfer moulding and composite winding methods.
Over in the textiles sector, materials have been developed for high performance and intelligent textiles applications. At Bolton, Kim Alderson has developed a continuous melt extrusion process for producing auxetic polymer monofilaments. We have used these fibres to demonstrate beneficial anchoring properties, with potential as biomedical sutures and in advanced fibre-reinforced composites. A common failure mechanism in composites is debonding of the fibre-matrix interface, leading to the tendency of the fibre reinforcement to ‘pull out’ of the matrix with an accompanying loss of load-bearing capability. The anchoring properties of auxetic fibres would tend to lock the fibres into the matrix when debonding has occurred, leading to enhanced fibre-pullout resistance.
Evans and Patrick Hook at Exeter University, UK, have also produced an auxetic double helix yarn (DHY). These and the Bolton group monofilaments have been produced on commercial-scale production facilities at Auxetix and Shakespeare Monofilament, respectively, as part of the REACTICS project. Chris Smith of Exeter University led the development of a flexible auxetic composite containing a DHY fabric, and the Bolton group has worked with Auxetic Technologies to develop a concept for using auxetic fibres to control thermal expansion in carbon-epoxy composite laminates. In this concept, the Poisson’s expansion of the auxetic counteracts stresses in carbon-reinforced expoxy composites, which can induce failure, microcracking and distortion. It therefore has potential for improving the performance and reducing the weight, material requirements and cost of components in the aerospace, automotive and manufacturing sectors.
In another study initially funded by SLBA, we have developed an auxetic solid warp knit fabric from conventional commercially available conventional fibres on commercial knitting machinery. Alternative auxetic knit fabrics have been produced by Samuel Ugbolue at the University of Massachusetts, US, and by Hu Hong of the Hong Kong Polytechnic University, China. Evans and Hook have made auxetic fabrics from their DHY fibres. Auxetic fabrics will find use in apparel applications, blast protection curtains and in ‘smart’ bandages for controlled release of active pharmaceutical ingredients from fibre micropores which open in response to wound swelling, for example (C&I, 2010, 13, 10).
Second-generation auxetic fibres, with improved mechanical properties and/or reduced fibre diameter/microstructure, are under development and will require molecularly engineered auxetic polymers. The liquid crystalline polymer approach developed by Anselm Griffin of Georgia Institute of Technology, US, arguably remains the most likely route. In an alternative approach, at Bolton we have worked with the UK’s Northwest Composites Centre to identify mechanisms for auxetic behaviour in nanostructures in naturally-occurring auxetic polymers, such as crystalline cellulose, reported in a study by Marko Peura and colleagues at the University of Helsinki, Finland, and are now planning to make synthetic equivalents with chemists in the Knowledge Centre for Materials Chemistry in Liverpool, UK.
Auxetic nanostructures remain a key focus of activity. In 2000, in a Malta-Exeter-Bolton collaboration, researchers reported detailed computer simulations predicting auxetic behaviour in a number of zeolites. This was subsequently confirmed in experimental work led by Jay Bass at the University of Illinois, US. Zeolites are used in industrial molecular sieving and catalysis applications and at Bolton we have used computer simulations to confirm that the auxetic effect leads to control of guest molecules within the zeolitic cage structures.
Ray Baughman at the University of Texas at Dallas, US, together with researchers in Brazil, have also reported the development of ‘auxetic buckypaper’, made by drying a slurry of carbon nanofibres comprising single-walled as well as multiwalled nanotubes (SWNTs and MWNTs, respectively). The resulting buckypaper was likened to a compressible ‘wine-rack’ structure, which has either a positive or negative Poisson’s ratio depending on the proportion of MWNTs in the paper (Science, 2008, 320, 504). The group foresees applications in composites, artificial muscles and mechanical and chemical sensors.
Now is an exciting time for the field of auxetic materials. Auxetics are increasingly recognised as integral components of the smart and advanced materials technologies required for future economic growth. Judging by the patent portfolio analysis and industrial research activity, we are in the period of take-off for auxetic materials-based technologies and applications, helped by both the growth in the numbers of materials and by the development of viable manufacturing routes.
My 1999 review article concluded by stating future development would require multidisciplinary research into multi-functional auxetics, with especially exciting areas being biomedical and nanotechnology applications. This has not only been proven to be true, but is as valid in 2011 as it was in 1999 – only the range of potential applications has expanded.
Andy Alderson is professor of materials physics in the Institute for Materials Research and Innovation, University of Bolton, UK.
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A buoyant international community
A well-connected international community now exists in the field of auxetics. In the UK, Kim Alderson of the University of Bolton led the EPSRC-funded Auxetic Materials Network of academics and industrialists (AuxetNet). During 2003-2006, AuxetNet sought to promote research into auxetics, focus UK activities and align these within the international arena. Activities undertaken by AuxetNet included awareness-raising events targeted at industry, and proposal generation for industry-led consortia. Two major outcomes from this latter activity were the 10-partner EU 6th Framework Programme CHISMACOMB project, led by Fabrizio Scarpa of the University of Bristol, and the nine-partner UK Technology Strategy Board REACTICS project, led by the author.
Last year saw the seventh event in the annual series of international conferences and workshops dedicated to auxetics and related systems, spearheaded by Krzysztof Wojciechowski of the Institute of Molecular Physics, Polish Academy of Science. The next event in this series will take place on 6-9 September 2011 in Szczecin, Poland. In the UK, the Materials Knowledge Transfer Network will be hosting a one-day workshop on 16 March 2011 for industry on the use of auxetic materials in biomedical applications (https://ktn.innovateuk.org/web/materialsktn).