Smart Solutions from Auxetic Materials
The array of potential applications for these smart materials is outlined by Professor Andy Alderson of the University of Bolton.
What could we use a material for, which in hollow cylinder form, increases wall thickness or porosity in response to an increase in internal pressure? What about a material or structure that can be deployed from a small volume to large volume on application of a simple stretch in one direction? Or how could we employ a material that behaves as the perfect anchor by locking into a surrounding matrix when pulled? These are some of the features of auxetic materials. These characteristics mean that auxetics have a role to play as we develop improved products and processes. Auxetic materials offer the potential to develop medical devices with enhanced functionality or optimised capabilities.
Most common materials become thinner when stretched. An auxetic material does the opposite: it becomes thicker widthwise when stretched lengthwise (Figure 1), and thinner when compressed. Auxetic polymers, composites, metals and ceramics exist.1
The auxetic effect is an intrinsic property of materials arising from features of the nano, meso- or micro-structures of the materials. The effect can be designed into macroscale structures up to and including civil engineering structures. Naturally occurring auxetic materials are known and include certain forms of skin and bone.
Figure 1: Undeformed ( left) and axially stretched (right) foams showing transverse
contraction and expansion charachteristics of conventional (top) and
auxetic (bottom) materials.
Technically, the auxetic effect corresponds to a negative value of the Poisson’s ratio (v) of the material or structure. Other material properties can be enhanced as a consequence of a negative v,for example, indentation resistance, fracture toughness and shear rigidity. Consequently, potential applications will exploit the auxetic property itself or other properties that are enhanced by the auxetic effect, or both. In this article, I shall review some of the medical device applications being considered for auxetics.
Expanded polytetrafluoroethylene (ex-PTFE) displays the auxetic effect when pre-conditioned by the application of through-thickness compression.2
In addition to possibly being a better match to the Poisson’s ratio of natural artery material, an auxetic ex-PTFE arterial prosthesis will perform in a different manner to one formed from a conventional material such as a fibrillar polyurethane material.
A blood vessel made of non-auxetic material will undergo a thinning of the material in response to a pulse of blood flowing through it (Figure 2a). Eventual rupture of the vessel with potentially catastrophic results may occur due to material thinning in extreme cases. An auxetic arterial material will become thicker in response to a pulse of blood flowing through it (Figure 2b), thereby mitigating against rupture as a result of wall thinning.
Schematics of medical devices
the auxetic effect: (a) pulse of blood flowing through
non-auxetic arterial prosthesis; (b) an
equivalent auxetic aterial
(c) dilator device with auxetic
expansion member: (d)
demonstrating anchoring potential
Dilators and stents
The use of auxetic PTFE as the expansion member in a dilator device for coronary angioplasty and related procedures has been proposed.3
Lateral expansion of the PTFE sheath to open the lumen of an artery occurs in response to axial expansion applied by movement of a central guide wire using a simple finger–thumb mechanism (Figure 2c), similar in principle to operation of a hypodermic syringe. The advantages of using auxetic expansion members over conventional balloon catheters include
- voiding the need to inflate a balloon (removing the need to control inflation pressure and the potential for leak of the inflation medium)
- easy manual control by the physician of radial expansion
- the ability to produce dilators of various diameters for patient-specific and/or controlled distention operation.
A dilator has been reported,4
which employs expanded ultra-high molecular-weight polyethylene (UHMWPE), including auxetic variant,5
as a liner component. UHMWPE is stronger than PTFE and allows a thinner lining to maximise the diameter of the inner lumen with respect to the outer profile of the catheter. UHMWPE also provides high lubricity, flexibility, abrasion resistance and lower processing temperature than PTFE. It also allows catheter sterilisation by gamma rays or electron beam, thus reducing manufacturing time compared with the PTFE-based device, which requires gas sterilisation.
The design of auxetic truss-like liner structures for stents has been reported.6
Other deployable auxetic stents include origami stent grafts,7
which utilise the small volume of a folded crease structure to deliver the device to the correct location. The creases disappear when the stent is deployed. Advantages over deployable stents comprising strut structure and cover components are that
- issues of geometrical incompatibility between cover and stent are avoided
- manufacturing costs are reduced (no cover-to-stent attachment)
- cover-sliding with respect to struts is eliminated
- uneven distribution of stresses, entanglement or rupture during expansion are mitigated.
Copper foams have been used to demonstrate that auxetic materials have potential in anchoring and fixation devices.8
Insertion through an axial compression applied to an auxetic anchor device is facilitated by lateral contraction due to the auxetic effect. Similarly, extraction of the device due to applied axial tension is resisted through lateral expansion, which tends to lock the device into the surrounding material. More recently, at Bolton, we have produced auxetic polypropylene (PP) monofilaments and used these in single fibre pull-out tests to demonstrate beneficial anchoring properties.9
The auxetic fibre specimens withstood more than twice the maximum load and required up to three times more energy to extract the fibre than the equivalent positive Poisson’s ratio fibre specimens (Figure 2d). Auxetic fibres have potential as biomedical sutures and ligament/muscle anchors.
Prosthetic limb sockets are susceptible to variations in stump volume, which leads to prosthetic limb loosening and deleterious consequences in terms of wearer compliance, skin irritation, tissue breakdown and discomfort. In this respect auxetic materials
- Undergo higher volume change under mechanical loading than conventional materials
- undergo synclastic (dome shape) curvature when subject to out-of-plane bending (Figure 3a), rather than the anticlastic (saddle-shape) curvature adopted by normal materials (Figure 3b)
- densify under the location of an indenter to offer increased resistance to indentation
- provide enhanced energy absorption capabilities.
Auxetic materials, therefore, offer potential as a lining material for prosthetic limb sockets. They can provide
- fixation and adjustable volume control in response to variations in stump volume over a period of time
- the ability to conform optimally to the contours of the dome-shaped stump and prosthetic limb socket
- improved support where cushioning is required
- reduced transmission of vibrations and loads.
Figure 3: Pure bending of (a) auxetic material (synclastic curvature) and (b) conventional material (anticlastic curvature)
The ability to design auxetic behaviour into the structure of a fabric opens up potential for use in smart compression bandages. Here, swelling of the limb will cause circumferential extension of the bandage and, therefore, a thickening of the auxetic bandage structure (analogous to Figure 2b), leading to an increase in pressure applied to the limb.
Similarly, swelling of an infected wound would cause an auxetic bandage to stretch, leading to increased porosity and breathability to expedite the wound-healing process.
Another smart bandage application relates to the controlled release of guest active pharmaceutical ingredients (APIs) from within the microstructure of host auxetic fibres forming the bandage itself.10
Opening of the auxetic fibre micropores, in response to fibre stretching due to wound swelling, enables release of the API onto the wound (Figure 4). Subsequent reduction in wound swelling relaxes the fibres and switches off release of the API.
Figure 4: Schematic of smart bandage concept comprising auxetic fibres containing active pharmaceutical ingredients in the fibre micropores
The above smart bandage applications have moved a step closer to reality with recent developments in auxetic fabrics. At Bolton we have developed a solid warp knit fabric using conventional commercially available fibres on commercial knitting machinery (Figure 5a).11
Alternative auxetic knit fabrics have been reported elsewhere,12,13
and auxetic fabrics have been made from carefully arranged double helix yarns, which are themselves auxetic.14
We have recently incorporated our auxetic monofilaments into conventional knit and woven fabric structures (Figures 5b and 5c, respectively), and the release of guest material from an auxetic host has been demonstrated in honeycomb and foam materials,15,16
as well as in computer simulations of auxetic molecular sieves.17
Figure 5: Auxetic textiles: (a) auxetic knitted fabric (conventional) yarns);(b) conventional 1x1 rib knitted fabric (auxetic monofilaments); (c) conventional 2 ends x 2 picks woven fabric (auxetic monofilaments)
Other auxetic applications
Other suggested uses for auxetics include
- total hip arthroplasty where enhanced indentation resistance and fracture toughness are expected to lead to auxetic linings having improved wear resistance)
- ultrasonic sensors and imagers18 for enhanced electromechanical coupling in piezoceramic composites
- ophthalmic devices (double curvature)19
- artificial intervertebral discs (prevention of disc bulge)20
- annuloplasty prostheses.21
These exciting new materials have clear potential in medical device applications. The challenge now is to develop commercially viable auxetic materials in appropriate form. Given the many benefits of using auxetic materials, this is a challenge that is being addressed and will surely be overcome in a number of applications in the future.
The author gratefully acknowledges funding for his current activities into biomedical device applications of auxetics from the Marriott Trust of the Rotary Club, Bolton Le Moors.
1. A. Alderson, "Triumph of Lateral Thought,” Chem. Ind., 384–391 (1999).
2. B.D. Caddock, K E. Evans, "Microporous Materials with Negative Poisson’s Ratios, I. Microstructure and Mechanical Properties,” J. Phys. D: Appl. Phys., 22, 1877 (1989).
3. R.E. Moyers, US Patent 5108413, 1992.
4. M. Chludzinski, E. Hammill, US Patent 6837890, 2005.
5. K.L. Alderson, K.E. Evans, "The Fabrication of Microporous Polyethylene having a Negative Poisson's Ratio,” Polymer, 33, 4435 (1992).
6. R. Hengelmolen, GB Patent 0217973, 2002.
7. Z. You, K. Kuribayashi, in "Origami,” Ed. R. J. Lang, A. K. Peters Ltd, Natick, MA, USA, 117–126 (2009).
8. J.B. Choi, R.S. Lakes, "Design of a Fastener Based on Negative Poisson’s Ratio Foam,” Cellular Polymers, 10, 205–212 (1991).
9. V.R. Simkins et al., "Single Fibre Pullout Tests on Auxetic Polymeric Fibres,” J. Mat. Sci., 40, 4355–4364 (2005).
10. A. Alderson, K.L. Alderson, "Expanding Materials and Applications: Exploiting Auxetic Textiles,” Technical Textiles International, 14, 6, 29–34 (2005).
11. M. Starbuck, S.C. Anand, N. Ravirala, K.L. Alderson, A. Alderson, International Patent Publication WO 016690, 2008.
12. S. Ugbolue et al., US Patent 0936857, 2007.
13. Y. Liu, H. Hu, J.K.C. Lam and S. Liu, "Negative Poisson’s Ratio Weft-Knitted Fabrics,” Textile Research Journal, 80, 9, 856–863 (2010).
14. W. Miller, et al., "The Manufacture and Characterisation of a Novel, Low Modulus, Negative Poisson’s Ratio Composite,” Composites Science and Technology, 69, 651–655 (2009).
15. A.Alderson et al., "An Auxetic Filter: A Tuneable Filter Displaying Enhanced Size Selectivity or De-Fouling Properties,” Ind. Eng. Chem. Res., 39, 654665 (2000).
16. A. Alderson et al., "Mass Transport Properties of Auxetic (Negative Poisson’s Ratio) Foams,” Phys. Stat. Sol. B 244, 3, 817 (2007).
17. A. Alderson et al., "Modelling of the Mechanical and Mass Transport Properties of Auxetic Molecular Sieves: An Idealised Inorganic (Zeolitic) Host-Guest System,”Molecular Simulation, 31,13, 889 (2005).
18. W.A. Smith, US Patent 5334903, 1994.
19. J.H. Shadduck, US Patent 021139, 2005.
20. E.O. Martz et al.,"Design of an Artificial Intervertebral Disc Exhibiting a Negative Poisson's Ratio,” Cellular Polymers, 24(3), 127–138, (2005).
21. G. Burriesci and G. Bergamasco, US Patent 162112, 2007.
Professor Andy Alderson is Professor of Materials Physics at the
Institute for Materials Research & Innovation, University of Bolton, Deane Road, Bolton BL3 5AB, UK,
+44 (0)1204 903 513,
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