Auxetic materials have a negative Poisson's ratio, meaning they become fatter rather than thinner when stretched. This allows auxetic materials to have applications as smart filters, dressings, and foams. Auxetic foams are typically created from conventional foams by applying heat or solvents during compression to induce a re-entrant structure. Once made, auxetic foams can be reconverted to conventional foams when exposed to solvents again, and this conversion/reconversion process can be repeated multiple times.

2. Introduction

3. Auxetics Conventional vs. Auxetic + ve Poisson’s ratio - ve Poisson’s ratio Poisson’s ratio: P ULL P ULL P ULL P ULL x y

4. Properties and applications Auxetic nails Gets shorter & thinner whilst ‘going in’ Gets longer & fatter whilst ‘going out’

5. Properties and applications Auxetic materials are harder to indent … ... In auxetics, the material tends to go towards the point of impact to become denser.

6. Properties and applications A saddle shape is formed when bending a conventional foam.

7. Properties and applications A dome shape is formed when bending an auxetic foam.

8. Properties and applications Smart filters Smart filters are made of auxetic structures. The stress applied determines the pore size and thus which particles are filtered. PULL PULL

9. Properties and applications Smart dressings are a smart way to administer drugs to freshly made wounds. When wounded a smart dressing is applied, as the wound swells it pulls the bandage, opening the microstructure and thus releasing the medicine found inside it. As the wound cures the swelling decreases releasing less medicine. Smart dressings a bandage made from an auxetic microstructure impregnated with a healing drug.

10. Auxetic foams

11. Process First manufactured by Rod Lakes, University of Wisconsin, Madison, (R. Lakes, Science, 235 (1987) p. 1038-1040.) Produced from commercially available conventional foams through a process involving: Volumetric compression of ~30% in volume Heating to the polymer’s softening temperature Cooling whilst remaining under compression

12. Typical Procedure Starting from: Reticulated 30 ppi polyester polyurethane Cut conventional foam in the shape of a cuboid of size 35 mm x 35 mm x 105 mm long; Press sample into a mould of dimensions 25 mm x 25 mm x 75 mm (28.6 % strain along each axis); Heat at 200 °C for 10 minutes, Remove from mould Stretch Replace in the mould. Cool to room temperature Heat for 1 hour at 100 °C x 2 Taken from: Smith, Grima, Evans, Acta Mater . 48 (2000) p.4349-4356. Technique adapted from: Chan and Evans, J. Mater. Sci. , 32 (1997) p. 5945-5953 .

13. New approach Uses solvent instead of heat Process involves Wetting foam with appropriate solvent Compressing the foam volumetrically by 30% Allowing the foam to dry well

14. Typical Process Starting from: Reticulated 30 ppi polyurethane foam (Dongguan Dihui Foam Sponge, China) Cut conventional foam in the shape of a cylinder of diameter 40mm and length 84mm Wet the foam with acetone Remove excess solvent Press sample into a mould of diameter 26 mm and length 55mm (~35 % strain along each axis); Allow the sample to dry completely before removing from mould

15. Result JN Grima, D Attard, R Gatt and RN Cassar, Adv . Eng. Mater. , 21 (2009)

16. Models

17. Re-entrant structures Uniaxial loading Uniaxial loading Compression/ heating process conventional auxetic LJ Gibson and MF Ashby, Cellular Solids , Cambridge Uni. Press, 1997. IG Masters and KE Evans, Composite Struct, 35 (1996) 403. KE Evans, A Alderson and FR Christian, J. Chem. Soc. Faraday Trans., 91 (1995) 2671.

18. 3D Re-entrant structures dodecahedron foam models tetrakaidecahedron foam models conventional Re-entrant (KE Evans, MA Nkansah and IJ Hutchinson, Acta Metall. Mater ., 2 (1994) 1289) (JB Choi, RS Lakes, J Compos. Mater ., 29 (1995) 113.)

19. Missing rib model CW Smith, JN Grima and KE Evans, Acta Mater. , 48 (2000) 4349. Uniaxial loading Compression/ heating process Uniaxial loading conventional auxetic

20. Rotating rigid units (a) (idealised form … rotating triangles model ) Uniaxial loading Uniaxial loading Compression/ heating process conventional auxetic JN Grima, A Alderson and KE Evans, J. Phys. Soc . Jpn , 74 (2005) 1341.

21. Reconversions

22. Process Expose auxetic foam to solvent Resulting foam is conventional with comparable dimensions to the original foam Auxetic foams made by the thermal method also lose their auxeticity when in contact with a solvent

23. Result and Implications The conversion / re-conversion process can be repeated for a number of times Auxetic foams should not be used in applications where they come into contact with solvents Conversion to auxetic Re -Conversion to conventional

24. Acknowledgments … The financial support of the Malta Council for Science and Technology and of the Malta Government Scholarship Scheme (Grant Number ME 367/07/17) is gratefully acknowledged. We also thank the ICMAT 2009 Organising committee for their financial assistance

25. Thank You !