25msp05 Spotlight 2 Proof
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View 25msp05 Spotlight 2 Proof as PDF for free.
More details
- Words: 1,991
- Pages: 3
[in the SPOTLIGHT] Fabrizio Scarpa
Auxetic Materials for Bioprostheses
I
Recently, the term auxetic has also been used to indicate classes of materials exhibiting negative thermal expansion and negative stiffness. Although these properties can be present at the same time with NPR in a solid, their description is beyond the scope of this article. AUXETIC BEHAVIOR The auxetic behavior is provided by the particular arrangement of the microstructure geometry and by the specific mechanical deformation occurring in the elements composing the microstructure of the material. The simplest microstructure geometry in auxetic materials is the inverted hexagon cell shown in Figure 1, also referred to as butterfly or bow-tie. In a normal
hexagonal cell, lateral contraction occurs when the cell is pulled along perpendicular direction. In an inverted hexagon cell, lateral expansion occurs when the cell is pulled along perpendicular direction. Other microstructure geometries in auxetic materials are those made of rotating nodes and ligaments (exhibiting what is referred to as chiral symmetry), rotating rectangles and triangles, or interconnected units with kinematic transmission provided by shear forces, like the hexachiral honeycomb of Figure 1. For hexachiral structures, the rotation of the nodes leads to the bending of the connected struts, providing the NPR behavior.
IE E Pr E oo f
n a recent article, BBC News Online reported on the creation of a smart curtain that can provide a new type of protection from explosions. The curtain is built using materials that are referred to as auxetic materials. These have interesting properties that are already known in the biomedical field and have been used in the construction of prostheses. In this article, we describe the properties of auxetic materials, their biomedical applications, and the challenges faced by signal processing in these applications. WHAT ARE AUXETIC MATERIALS? In elasticity theory, Poisson’s ratio (PR) quantifies—to some extent—the behavior of a material. An isotropic, three-dimensional (3-D), and homogeneous material can exhibit a Poisson’s ratio between −1 and +0.5. The upper (positive) bound corresponds to rubber-like materials. The lower (negative) bound corresponds to materials with infinite shear stiffness. Materials with negative values of the Poisson’s ratio (NPR) are referred to as auxetic. Their name, which comes from Greek and means “that can expand,” indicates that these materials can expand in all directions when stretched along one direction. In general, NPR materials exhibit unusually large volumetric transformation, conserving the shape of their microstructures. Clearly, this behavior is different from that of “conventional” materials (see Figure 1). Although NPR behavior has been recorded for naturally-occurring materials, most of the solids and structures exhibiting auxetic behavior have been produced artificially. Open cell foam was the first type of solid produced featuring a NPR value equal to −0.7 [1].
Digital Object Identifier 10.1109/MSP.2008.926663
(continued on page 125)
(a)
(b)
(c)
(d)
[FIG1] Conventional and auxetic structures. (a) Conventional honeycomb; (b) auxetic bow-tie honeycomb; (c) auxetic foam; (d) auxetic hexachiral honeycomb.
IEEE SIGNAL PROCESSING MAGAZINE [128] SEPTEMBER 2008
©2008 BRITISH CROWN COPYRIGHT
[in the SPOTLIGHT]
continued from page 128
Under mechanical deformation, all auxetic materials have a common deformation characteristic to an ellipsoidal shape, referred to as the synclastic curvature. Consequently, a plate made of auxetic material will take up a dome-shaped configuration when bent, with no crimps induced by in-plane buckling. By contrast, a nonauxetic plate (i.e., with a positive Poisson’s ratio) will assume a saddle shape when bent. This characteristic of auxetic materials can be used to design and build domes of other structures with complex curvatures and shapes.
AUXETIC APPLICATIONS A considerable amount of work is currently focusing on auxetic materials and their applications. These efforts, which are mainly driven by army, maritime, and aerospace commercial sectors and materials companies, include large scale production of auxetic honeycomb structures, characterization of chiral honeycomb structures [2], and development of
chiral honeycomb structures for sandwich applications. A sandwich structure is formed by a honeycomb or cellular core placed between two face plates. Further applications involve microwave absorbers, and structural health monitoring [2]; development of complete auxetic sandwich structures for multifunctional applications and evaluation in crash conditions, vibration damping, and thermal management; feasibility evaluation of auxetic configurations for testing structural integrity, and for morphing applications (e.g., for wing and other aerospace test cases) by the U.S. Army [3]. The deformation characteristics of auxetic materials have made them good candidates in biomedical applications. Auxetic textiles made by yarns arranged have been recently patented for smart bandages [4]. According to levels of humidity associated to the swollen state of the wound, the bandage could make use of its deformation mechanism to release drug suspensions to accelerate the ailment of the injury. However, prosthetics have been by far the most important class of
IE E Pr E oo f
AUXETIC PROPERTIES There are several properties of auxetic materials that have been documented using experimentation. We provide a high-level summary of the most important properties: Compressive strength and shear stiffness: Increased in auxetic honeycombs (“bow-tie”) compared to conventional counterparts. Indentation resistance: Enhanced indentation resistance in auxetic materials, which is important for damage toler-
ance. The auxetic material tends to wrap around the indenter, inducing an increased hardness of the structure. Resilience: Increased over conventional foams as shown by crashworthiness tests. Remarkable results with thermoplastic open cell foams, which belong to one of the most tested auxetic materials classes, i.e., the auxetic polymers. Energy dissipation levels: Up to 15 times higher than in conventional foams and five to six times higher than in conventional foams with high density. Acoustic absorption at low frequencies (between 400 and 800 Hz): Higher in auxetic materials than in conventional foams by an average factor of two.
[conference SPOTLIGHT]
continued from page 120
papers were then assigned to lecture sessions and poster sessions depending on the topic and suitability for oral presentation versus poster presentation, and with the goal of assembling sessions with welldefined themes. In addition to the regular sessions, there are six special sessions, covering timely topics such as 3-D graphics and compression, connected filters, super resolution, landmark shape sequence analysis, privacy protection, and image aesthetics. Papers submitted to the special sessions went through the same diligent review process as the papers submitted to the general conference. The number of student paper submissions has grown to about 770 papers contributing to stiff competition for the selection of best student papers. It also shows the attractive-
ness and vibrancy of the areas supported by ICIP. PLENARY TALKS Three prestigious plenary speakers will offer in-depth views on some of the most fascinating topics. ■ Prof. Marc Levoy of Stanford University will discuss the emerging area, “New Techniques in Computational Photography.” ■ Prof. V.S. Ramachandran, University of California, San Diego, will present a stimulating discourse, “From Molecules to Metaphor.” ■ Prof. Alan C Bovik of the University of Texas, will present his views on image quality in the talk titled, “Quality Is in the Eye of the Beholder.”
SOCIAL EVENTS In addition to the usual coffee and tea breaks, ICIP 2008 will host a students’ reception on Monday, 13 October and a reception with sumptuous Mexican fare on Tuesday, 14 October for all of the conference attendees. SEE YOU IN SAN DIEGO We are looking forward to welcoming you to sunny San Diego and offering you a memorable scientific, cultural, and culinary experience. As has been done in the past, a one-price passport registration system will permit the attendees to enjoy all of the technical sessions, breaks, and reception. Please take a moment to register today on http://www. icip2008.org/.
IEEE SIGNAL PROCESSING MAGAZINE [125] SEPTEMBER 2008
[in the SPOTLIGHT]
continued from page 125
biomedical applications to date that make use of auxetic materials.
THE AUXETIC BEHAVIOR IS PROVIDED BY THE PARTICULAR ARRANGEMENT OF THE MICROSTRUCTURE GEOMETRY AND BY THE SPECIFIC MECHANICAL DEFORMATION OCCURRING IN THE ELEMENTS COMPOSING THE MICROSTRUCTURE OF THE MATERIAL.
pose challenges in terms of SP, particularly at the level of signal interpretation and the signal control units. For instance, although the overall mechanical response would be similar for auxetic and conventional prosthesis, the control signals would be quite different because of their different behaviors. In addition, the mass properties of auxetic foams and solids are quite peculiar, with increased absorption of water and other liquids. This suggests that auxetic fabrics could be used as covers for electrocardiogram (ECG) electrodes to reduce the loss of resistivity of the sensors by sweating from the human skin, yet requires incorporation of these properties in system modelling and SP.
IE E Pr E oo f
AUXETIC BIOPROSTHESES Three major auxetic bioprostheses are the following: a) Artificial intervertebral disks: Sandwich elliptical plates with an auxetic core have been recently proposed as artificial intervertebral disks. The core, made of reentrant prismatic honeycomb cells, allows an auxetic effect through the thickness of the plates, providing the lateral compression of the artificial disk under compressive loading. b) Annuloplasty prostheses: Auxetic effects can be also achieved in tubular truss-like structures having a cellular topology similar the ones of honeycombs and chiral assemblies. Recently, European patents [5] have proposed auxetic solutions for annuloplasty prostheses, i.e., prostheses that provide plastic repair of a cardiac valve. The rational behind these designs is the lack or crimping in an auxetic tube, leading to the development of high-flexibility stems for cardiac applications. c) Cushion pads and knee prosthetics: Open-cell polyurethane foams are commonly used to manufacture cushion pads for seats. However, a positive Poisson’s ratio open cell foam features uneven distribution of normal stresses when indented, with a localization of the peak stresses around the contact area. By contrast, an auxetic (NPR) foam wraps around the indenter, providing a more uniform stress distribution with peak stresses lowered by an average factor of three. These characteristics have made NPR foams a good choice as cushion pads for people with disabilities or medical conditions. In addition, the same equipartition of mechanical stresses and mechanical resilience have recommended them for use in knee prosthetics.
SP CHALLENGES There are several challenges related to auxetic materials and their biomedical applications. Some of the most important ones are related to material characterization and to directional conductivity in applications that can use auxetic conductive fibers and tendons.
Although applications often assume a linear behavior of the material under test, the response of the auxetic poroelastic foam pads features nonlinear characteristics based on the input force and the frequency bandwidth of the excitation. This behavior not only poses challenges in terms of material characterization, but also suggests that refined SP methods need to be incorporated in applications that handle auxetic porous materials. In principle, there is no impediment on using conductive fibers, piezoresistive, or optical wires in auxetic materials. For instance, textile auxetic fabrics made of yarns of conductive fibers would show piezoresistive properties. By extending the fabrics, the fiber cross section would increase. This anisotropy of the auxetic textile could be beneficial as compared to isotropic fabric configurations. Similarly, auxetic fibers and tendons could exhibit enhanced piezoresistivity and sensing capabilities in theory, which have yet to be confirmed experimentally. However, the same properties of auxetic materials that make them interesting for such applications
CONCLUSIONS The smart sensors and prosthetic applications of auxetic materials are still in the beginning stages. Initial evidence and theoretical considerations lead to believe that auxetic materials have a remarkable potential for these applications and the biomedical field.
AUTHOR Fabrizio Scarpa ([email protected]) is reader of smart structures with the Department of Aerospace Engineering, University of Bristol, United Kingdom.
REFERENCES
[1] R. Lakes, “Foam structures with a negative Poisson’s ratio,” Science, vol. 235, pp. 1038–1040, 1987. [2] European CHISMACOMB project [Online]. Available: www.chismacomb.net
[3] A. Spadoni, M. Ruzzene, and F. Scarpa, “Dynamic response of chiral truss-core assemblies,” J. Intell. Mater. Syst. Struct., vol. 17, no.11, pp. 941–952. 2006.
[4] K.L. Alderson and V.R. Simkins, “Auxetic materials,” U.S. Patent 6878320, Dec. 21, 2004.
[5] G. Burriesci and G. Bergamasco, “Annuloplasty prosthesis with an auxetic structure,” European Patent EP 05425926.2. Dec. 28, 2005. [SP]
IEEE SIGNAL PROCESSING MAGAZINE [126] SEPTEMBER 2008
Auxetic Materials for Bioprostheses
I
Recently, the term auxetic has also been used to indicate classes of materials exhibiting negative thermal expansion and negative stiffness. Although these properties can be present at the same time with NPR in a solid, their description is beyond the scope of this article. AUXETIC BEHAVIOR The auxetic behavior is provided by the particular arrangement of the microstructure geometry and by the specific mechanical deformation occurring in the elements composing the microstructure of the material. The simplest microstructure geometry in auxetic materials is the inverted hexagon cell shown in Figure 1, also referred to as butterfly or bow-tie. In a normal
hexagonal cell, lateral contraction occurs when the cell is pulled along perpendicular direction. In an inverted hexagon cell, lateral expansion occurs when the cell is pulled along perpendicular direction. Other microstructure geometries in auxetic materials are those made of rotating nodes and ligaments (exhibiting what is referred to as chiral symmetry), rotating rectangles and triangles, or interconnected units with kinematic transmission provided by shear forces, like the hexachiral honeycomb of Figure 1. For hexachiral structures, the rotation of the nodes leads to the bending of the connected struts, providing the NPR behavior.
IE E Pr E oo f
n a recent article, BBC News Online reported on the creation of a smart curtain that can provide a new type of protection from explosions. The curtain is built using materials that are referred to as auxetic materials. These have interesting properties that are already known in the biomedical field and have been used in the construction of prostheses. In this article, we describe the properties of auxetic materials, their biomedical applications, and the challenges faced by signal processing in these applications. WHAT ARE AUXETIC MATERIALS? In elasticity theory, Poisson’s ratio (PR) quantifies—to some extent—the behavior of a material. An isotropic, three-dimensional (3-D), and homogeneous material can exhibit a Poisson’s ratio between −1 and +0.5. The upper (positive) bound corresponds to rubber-like materials. The lower (negative) bound corresponds to materials with infinite shear stiffness. Materials with negative values of the Poisson’s ratio (NPR) are referred to as auxetic. Their name, which comes from Greek and means “that can expand,” indicates that these materials can expand in all directions when stretched along one direction. In general, NPR materials exhibit unusually large volumetric transformation, conserving the shape of their microstructures. Clearly, this behavior is different from that of “conventional” materials (see Figure 1). Although NPR behavior has been recorded for naturally-occurring materials, most of the solids and structures exhibiting auxetic behavior have been produced artificially. Open cell foam was the first type of solid produced featuring a NPR value equal to −0.7 [1].
Digital Object Identifier 10.1109/MSP.2008.926663
(continued on page 125)
(a)
(b)
(c)
(d)
[FIG1] Conventional and auxetic structures. (a) Conventional honeycomb; (b) auxetic bow-tie honeycomb; (c) auxetic foam; (d) auxetic hexachiral honeycomb.
IEEE SIGNAL PROCESSING MAGAZINE [128] SEPTEMBER 2008
©2008 BRITISH CROWN COPYRIGHT
[in the SPOTLIGHT]
continued from page 128
Under mechanical deformation, all auxetic materials have a common deformation characteristic to an ellipsoidal shape, referred to as the synclastic curvature. Consequently, a plate made of auxetic material will take up a dome-shaped configuration when bent, with no crimps induced by in-plane buckling. By contrast, a nonauxetic plate (i.e., with a positive Poisson’s ratio) will assume a saddle shape when bent. This characteristic of auxetic materials can be used to design and build domes of other structures with complex curvatures and shapes.
AUXETIC APPLICATIONS A considerable amount of work is currently focusing on auxetic materials and their applications. These efforts, which are mainly driven by army, maritime, and aerospace commercial sectors and materials companies, include large scale production of auxetic honeycomb structures, characterization of chiral honeycomb structures [2], and development of
chiral honeycomb structures for sandwich applications. A sandwich structure is formed by a honeycomb or cellular core placed between two face plates. Further applications involve microwave absorbers, and structural health monitoring [2]; development of complete auxetic sandwich structures for multifunctional applications and evaluation in crash conditions, vibration damping, and thermal management; feasibility evaluation of auxetic configurations for testing structural integrity, and for morphing applications (e.g., for wing and other aerospace test cases) by the U.S. Army [3]. The deformation characteristics of auxetic materials have made them good candidates in biomedical applications. Auxetic textiles made by yarns arranged have been recently patented for smart bandages [4]. According to levels of humidity associated to the swollen state of the wound, the bandage could make use of its deformation mechanism to release drug suspensions to accelerate the ailment of the injury. However, prosthetics have been by far the most important class of
IE E Pr E oo f
AUXETIC PROPERTIES There are several properties of auxetic materials that have been documented using experimentation. We provide a high-level summary of the most important properties: Compressive strength and shear stiffness: Increased in auxetic honeycombs (“bow-tie”) compared to conventional counterparts. Indentation resistance: Enhanced indentation resistance in auxetic materials, which is important for damage toler-
ance. The auxetic material tends to wrap around the indenter, inducing an increased hardness of the structure. Resilience: Increased over conventional foams as shown by crashworthiness tests. Remarkable results with thermoplastic open cell foams, which belong to one of the most tested auxetic materials classes, i.e., the auxetic polymers. Energy dissipation levels: Up to 15 times higher than in conventional foams and five to six times higher than in conventional foams with high density. Acoustic absorption at low frequencies (between 400 and 800 Hz): Higher in auxetic materials than in conventional foams by an average factor of two.
[conference SPOTLIGHT]
continued from page 120
papers were then assigned to lecture sessions and poster sessions depending on the topic and suitability for oral presentation versus poster presentation, and with the goal of assembling sessions with welldefined themes. In addition to the regular sessions, there are six special sessions, covering timely topics such as 3-D graphics and compression, connected filters, super resolution, landmark shape sequence analysis, privacy protection, and image aesthetics. Papers submitted to the special sessions went through the same diligent review process as the papers submitted to the general conference. The number of student paper submissions has grown to about 770 papers contributing to stiff competition for the selection of best student papers. It also shows the attractive-
ness and vibrancy of the areas supported by ICIP. PLENARY TALKS Three prestigious plenary speakers will offer in-depth views on some of the most fascinating topics. ■ Prof. Marc Levoy of Stanford University will discuss the emerging area, “New Techniques in Computational Photography.” ■ Prof. V.S. Ramachandran, University of California, San Diego, will present a stimulating discourse, “From Molecules to Metaphor.” ■ Prof. Alan C Bovik of the University of Texas, will present his views on image quality in the talk titled, “Quality Is in the Eye of the Beholder.”
SOCIAL EVENTS In addition to the usual coffee and tea breaks, ICIP 2008 will host a students’ reception on Monday, 13 October and a reception with sumptuous Mexican fare on Tuesday, 14 October for all of the conference attendees. SEE YOU IN SAN DIEGO We are looking forward to welcoming you to sunny San Diego and offering you a memorable scientific, cultural, and culinary experience. As has been done in the past, a one-price passport registration system will permit the attendees to enjoy all of the technical sessions, breaks, and reception. Please take a moment to register today on http://www. icip2008.org/.
IEEE SIGNAL PROCESSING MAGAZINE [125] SEPTEMBER 2008
[in the SPOTLIGHT]
continued from page 125
biomedical applications to date that make use of auxetic materials.
THE AUXETIC BEHAVIOR IS PROVIDED BY THE PARTICULAR ARRANGEMENT OF THE MICROSTRUCTURE GEOMETRY AND BY THE SPECIFIC MECHANICAL DEFORMATION OCCURRING IN THE ELEMENTS COMPOSING THE MICROSTRUCTURE OF THE MATERIAL.
pose challenges in terms of SP, particularly at the level of signal interpretation and the signal control units. For instance, although the overall mechanical response would be similar for auxetic and conventional prosthesis, the control signals would be quite different because of their different behaviors. In addition, the mass properties of auxetic foams and solids are quite peculiar, with increased absorption of water and other liquids. This suggests that auxetic fabrics could be used as covers for electrocardiogram (ECG) electrodes to reduce the loss of resistivity of the sensors by sweating from the human skin, yet requires incorporation of these properties in system modelling and SP.
IE E Pr E oo f
AUXETIC BIOPROSTHESES Three major auxetic bioprostheses are the following: a) Artificial intervertebral disks: Sandwich elliptical plates with an auxetic core have been recently proposed as artificial intervertebral disks. The core, made of reentrant prismatic honeycomb cells, allows an auxetic effect through the thickness of the plates, providing the lateral compression of the artificial disk under compressive loading. b) Annuloplasty prostheses: Auxetic effects can be also achieved in tubular truss-like structures having a cellular topology similar the ones of honeycombs and chiral assemblies. Recently, European patents [5] have proposed auxetic solutions for annuloplasty prostheses, i.e., prostheses that provide plastic repair of a cardiac valve. The rational behind these designs is the lack or crimping in an auxetic tube, leading to the development of high-flexibility stems for cardiac applications. c) Cushion pads and knee prosthetics: Open-cell polyurethane foams are commonly used to manufacture cushion pads for seats. However, a positive Poisson’s ratio open cell foam features uneven distribution of normal stresses when indented, with a localization of the peak stresses around the contact area. By contrast, an auxetic (NPR) foam wraps around the indenter, providing a more uniform stress distribution with peak stresses lowered by an average factor of three. These characteristics have made NPR foams a good choice as cushion pads for people with disabilities or medical conditions. In addition, the same equipartition of mechanical stresses and mechanical resilience have recommended them for use in knee prosthetics.
SP CHALLENGES There are several challenges related to auxetic materials and their biomedical applications. Some of the most important ones are related to material characterization and to directional conductivity in applications that can use auxetic conductive fibers and tendons.
Although applications often assume a linear behavior of the material under test, the response of the auxetic poroelastic foam pads features nonlinear characteristics based on the input force and the frequency bandwidth of the excitation. This behavior not only poses challenges in terms of material characterization, but also suggests that refined SP methods need to be incorporated in applications that handle auxetic porous materials. In principle, there is no impediment on using conductive fibers, piezoresistive, or optical wires in auxetic materials. For instance, textile auxetic fabrics made of yarns of conductive fibers would show piezoresistive properties. By extending the fabrics, the fiber cross section would increase. This anisotropy of the auxetic textile could be beneficial as compared to isotropic fabric configurations. Similarly, auxetic fibers and tendons could exhibit enhanced piezoresistivity and sensing capabilities in theory, which have yet to be confirmed experimentally. However, the same properties of auxetic materials that make them interesting for such applications
CONCLUSIONS The smart sensors and prosthetic applications of auxetic materials are still in the beginning stages. Initial evidence and theoretical considerations lead to believe that auxetic materials have a remarkable potential for these applications and the biomedical field.
AUTHOR Fabrizio Scarpa ([email protected]) is reader of smart structures with the Department of Aerospace Engineering, University of Bristol, United Kingdom.
REFERENCES
[1] R. Lakes, “Foam structures with a negative Poisson’s ratio,” Science, vol. 235, pp. 1038–1040, 1987. [2] European CHISMACOMB project [Online]. Available: www.chismacomb.net
[3] A. Spadoni, M. Ruzzene, and F. Scarpa, “Dynamic response of chiral truss-core assemblies,” J. Intell. Mater. Syst. Struct., vol. 17, no.11, pp. 941–952. 2006.
[4] K.L. Alderson and V.R. Simkins, “Auxetic materials,” U.S. Patent 6878320, Dec. 21, 2004.
[5] G. Burriesci and G. Bergamasco, “Annuloplasty prosthesis with an auxetic structure,” European Patent EP 05425926.2. Dec. 28, 2005. [SP]
IEEE SIGNAL PROCESSING MAGAZINE [126] SEPTEMBER 2008
