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re, favourable for both hard- and soft-tissue applications as well as the design

ous composite materials for tissue engineering and functional coatings for metallic implants are furtherdevelopments anticipated to be introduced in next generation orthopaedic medicine.

2011 Elsevier Ltd. All rights reserved.

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Corresponding author.

Composites Science and Technology 71 (2011) 17911803

Contents lists available at SciVerse ScienceDirect

Composites Science and Technology

journal homepage: www.elsevier .com/ locate/compsci techE-mail address: M.Scholz@bristol.ac.uk (M.-S. Scholz).2.4. Dental applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17953. Soft-tissue applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17964. Tissue engineering applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17965. Special prosthetics for application in professional sports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17976. Commercial prosthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17997. Biomimetic sensors, actuators and artificial muscles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1800

7.1. Biomedical applications of ionic polymer-metal composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18007.2. Challenges faced with ionic polymer-metal composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1800

8. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18009. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1801

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1801References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1801A. PolymersB. InterfaceB. Mechanical propertiesOrthopaedics

Contents

1. Introduction . . . . . . . . . . . . . . . . .2. Hard-tissue applications . . . . . . .

2.1. Bone fracture repair . . . . .2.2. Total knee replacement. . .2.3. Total hip replacement . . . .0266-3538/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.compscitech.2011.08.017. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1792

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1795. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1795Keywords:A. Polymer-matrix composites (PMCs)

of prostheses. In particular, the development of specically designed carbon bre sports prostheses nowallows lower-limb amputees to actively participate in competitive sports. Sensory feedback systems, por-patibility. They are, therefoeceived in revised form 18 August 2011ccepted 19 August 2011vailable online 30 August 2011

polymer composite materials has, in recent years, led to technological advances across a wide range ofapplications in modern orthopaedic medicine and prosthetic devices. Composites typically possess asuperior strength to weight characteristic compared to monolithic materials and offer excellent biocom-i c l e i n f o

history:d 21 April 2011

a b s t r a c t

The use of bre reinforced composite materials for biomedical purposes is reviewed. The development of.A. Muahi, M.F. Pernice, S.I. Rae, J.A. Trevarthen, S.C. White, P.M. Weaver, I.P. Bonddvanced Composites Centre for Innovation and Science (ACCIS), University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, UK. Scholz , J.P. Blancheld, L.D. Bloom, B.H. Coburn, M. Elkington, J.D. Fuller, M.E. Gilbert,he use of composite materials in modern orthopaedic medicinend prosthetic devices: A reviewReviewll rights reserved.

1792 M.-S. Scholz et al. / Composites Science and Technology 71 (2011) 179118031. Introduction

Advances in the development of composite materials have, inrecent years, enabled major improvements in the design of modernorthopaedics and prosthetic devices. Composites are engineeredmaterials made from two or more constituents, each offering dif-ferent physical properties, which can be combined synergistically.Properties and architectures of biological materials can thus be re-ected more accurately by means of tailoring [1,2]. Fibre reinforcedpolymer composites are, at present, the most widely used multi-phase materials in orthopaedics. In addition, most of todaysupper- and lower-limb prostheses are now made from compositeswith underlying polymer matrix. These types of materials arefavourable due to their exceptional strength to weight characteris-tics [3] as well as their superior biocompatibility [4,5].

The simultaneous exhibition of relatively low elastic modulusand high strength is of particular importance in the case of directskeletal attachment (osseointegration) of articial limbs [3,5].The comfort and ease of control of an articial limb critically de-pend on the quality of the interface between stump and prosthesis[3,6]. For example, a mismatch in stiffness between implant andhost tissue can cause severe stress shielding [5]. In order to mini-mise such effects, the development and application of biocompat-ible materials is essential.

The biocompatibility of a material indicates its ability to per-form in conjunction with a living system [5]. Specically, one dis-tinguishes between the chemical, biological and physicalsuitability of a material (surface compatibility) and its compatibil-ity in terms of mechanical properties (structural compatibility)such as stiffness, strength and optimal load transmission, at theimplant/tissue interface [5,7]. Optimal material performance isachieved by maximising both surface and structural compatibilitywhilst maintaining the materials ability to withstand the some-times harsh in vivo environment; levels of pH can range from 19 [5]. Nevertheless, it is not solely biocompatibility that inuenceswhether implants or osseointegrated prostheses are accepted bythe human body but many other aspects, including surgical tech-nique and patients health, must also be considered [5].

In nature, mechanical function and structural support is pro-vided by the musculoskeletal system, consisting of hard- andsoft-tissues. The most frequently implanted tissue after blood isbone [8] with approximately 500,000 operations a year in the USalone [9]. A good understanding of bone in terms of its structureand physical behaviour is therefore needed in order to successfullydesign various types of implants. Bone is a natural, highly hierar-chical composite material made up of collagen brils withhydroxyapatite (HAP) nanocrystals interspersed along the collagenbres. Despite the high elastic modulus of HAP (100 GPa) [10],the combined elastic modulus of cortical bone falls between 10and 20 GPa [5,11,12], a relatively low elastic modulus comparedto that of metals, conventionally used for bone xation; titaniumand stainless steel having elastic moduli of around 118 GPa [13]and 206 GPa [14], respectively. The stiffness, strength and fracturetoughness of metals, technical ceramics, composites and bre rein-forced plastics in relation with bone are illustrated schematicallyin Fig. 1ac. It is clear that appropriately tailored polymer matrixcomposites can match the properties of bone and so have signi-cant potential as a replacement material. Of course, other aspectsincluding biocompatibility, practicality and costs are also impor-tant drivers. Finally, it is noted that bone remodelling takes placeaccording to the principles set out in Wolffs Law: Bone is depos-ited and reinforced at areas of greatest stress [15].

The environmental impact on prosthetics due to everyday activ-

ities such as walking, running, stretching, jumping and climbingmust be considered in order to ensure practicability. Stresses expe-rienced by bones in the human body are of the order of 4 MPa,while tendons and ligaments may be subjected to stresses as highas 80 MPa [5]. Moreover, loads vary and are applied repeatedly;stress cycles of nger or hip joint motion are estimated to be ofthe order of 1 106 cycles a year [5]. Consequently, prostheticsthat are to be in direct skeletal contact require a low elastic mod-ulus to be structurally compatible, but a high level of strength toensure practicability and durability. Additionally, surface compat-ibility must also be achieved. For instance, designing a prostheticdevice from purely polymeric materials may seem appropriate(due to their low elastic modulus), however, their low strength im-pairs their usability [5]. Equally, the performance of metallic arti-cial limbs in terms of surface compatibility often proves to beunsatisfactory [5].

Fibre reinforced polymer composites are able to accomplishboth low elastic modulus as well as high strength, in an efcientmanner (Fig. 1). Furthermore, corrosion and fatigue resistancecharacteristics are greatly improved due to the application of com-posite materials [16]. The properties of these materials can beadapted in a number of ways. For instance, changing the arrange-ment of reinforcing bres or varying the volume fraction canchange the material characteristics signicantly [5,17]. Thus, brereinforced polymer composites in particular have the potential tobe highly biocompatible, whilst maintaining suitable mechanicalproperties as it is possible to produce a wide range of materialarchitectures. Moreover, making implants for bone replacementor xation from bre reinforced polymers permits the creation ofuseful X-ray, magnetic resonance imaging (MRI) and computedtomography (CT) images, commonly used for diagnostic interpre-tation [7].

Conceptually, one distinguishes between three classes of bio-materials: biologically inert materials (rst generation), bioactiveand biodegradable materials (second generation), and materialsaiming to achieve specic cellular responses on a molecular level(third generation) [18,19]. First generation materials are, at pres-ent, still being widely used, specically in hard-tissue applications,and their research and development continues. Second and thirdgeneration materials are not intended to simply replace whatevolved from previous generations but meant to create categori-cally novel, ameliorated methods of treatment. Composites arethought to be particularly benecial in the development of com-plex, third generation biomaterials forming biodegradable scaf-folds and hierarchically organised structures [12,19,20].

With the incorporation of more expensive materials in a designand the increasing complexity of manufacture, costs clearly consti-tute a limiting factor to the commercial availability of state of theart technology. Moreover, it should be noted that prior to a productreaching successful commercialisation numerous trials are re-quired due to a necessarily slow acceptance of new devices partic-ularly within surgical practices [5,21,22]. The consideration of lesseffective but nancially more feasible solutions using traditionalengineering materials must, therefore, not be neglected as compos-ites are often only available at relatively high economic cost.

Within this review, the uses of bre reinforced polymer com-posite materials are discussed separately for hard- and soft-tissueas well as tissue engineering applications. With particular empha-sis on elite sports, the recent impact of composites on the design ofupper- and lower-limb prostheses is reviewed. The use of compos-ites within the eld of biomimetic sensors and actuators is brieyexamined together with their potential applications. Future objec-tives for next generation orthopaedic medicine are briey outlinedprior to an analysis of the key innovations responsible for the re-

cent technological advances in modern orthopaedics.

e anM.-S. Scholz et al. / Composites Scienc2. Hard-tissue applications

Hard-tissue applications of bre reinforced composite materialsinclude: skull reconstruction, bone fracture repair, total knee, an-kle, hip and other joint replacements, as well as dental applica-tions. An overview of some exemplar applications is given below.

Fig. 1. Comparison of (a) stiffness, (b) strength and (c) fracture toughness for metals, tech[2331]. CF: carbon bre, GF: glass bre, PA12: polyamide12, PC: polycarbonate, PE: polypoly(l-lactic acid), PP: polypropylene, PSU: polysulfone, PTFE: polytetrauoroethylene, Pd Technology 71 (2011) 17911803 17932.1. Bone fracture repair

There are two distinct types of bone fracture repair, namelyexternal and internal xation.

External xation keeps the bone fragments aligned by means ofcasts, splints, braces or similar xation systems. Traditional casting

nical ceramics, composites and bre reinforced plastics with respect to those of boneethylene, PEEK: poly ether ether ketone, PLGA: poly(l-lactic-co-glycolic acid), PLLA:UR: polyurethane.

ced

1794 M.-S. Scholz et al. / Composites Science anmaterials are generally made from plaster of calcium sulphate with

Fig. 3. Bone fragment xation via plates and screws. Reproduwoven cotton fabrics or fabrics of glass and polyester bres provid-ing the necessary reinforcement [5]. To prevent scorching or weak-ening of the patients skin, breathable casts have been developed[32]. Other external xation systems (Fig. 2) have only recentlygained popularity as lightweight carbon bre designs have becomemore readily available. In the particular case of carbon bre ortho-ses a 10% reduction in patient heart rate and oxygen consumptionwas reported, following weight savings of around 29% compared tostainless steel equivalents [33]. Furthermore, due to the low den-sity of carbon bre reinforced plastic, an improvement in agility,gait and walking speed can be noticed [34]. While previous xation

Fig. 2. External bone xation systems. Adapted from DePuy [35] with courtesy ofDePuy Orthopaedics, Inc.systems, made from stainless steel or aluminium caused artefacts

from DePuy [35] with courtesy of DePuy Orthopaedics, Inc.

d Technology 71 (2011) 17911803in radiographs, non-metallic designs permit the more effectiveuse of medical imaging [5], a technique often used to monitorthe healing process. The major shortcoming of all external systemscurrently lies within their inability to seamlessly adjust the stiff-ness analogous to human muscle.

Internal xation entails the use of implants such as plates,screws, pins and wires holding the bone fragments in place; for ri-gid xation, plates and screws are more commonly used (Fig. 3). Avariety of polymer composites including both thermoset and ther-moplastic materials are available for these applications. Polymercomposites are divided into two classes: avital/avital and vital/avi-tal. Avital/avital composites comprise non-living matrix and rein-forcement phases only, while vital/avital composites are madefrom both living as well as non-living materials. In the group ofavital/avital composites one distinguishes further between non-resorbable, partially-resorbable and fully-resorbable compositions[5,36]. Resorbable bone plates have become of particular interestbecause they permit the gradual increase of stress on the bone asit progresses through healing; thus stress shielding can be reducedand osteopenia may be eradicated. Fully-resorbable composite im-plants provide a major advantage over conventional metallic platesas they do not need to be removed during a second surgical proce-dure. A number of resorbable polymers including poly(l-lactic acid)(PLLA), poly(glycolic acid) (PGA) or their copolymers and poly(l-lactic-co-glycolic acid) (PLGA) have readily been approved for hu-man clinical uses [37]. However, it generally proves difcult toachieve non-toxic degradation at an acceptable rate and yet retaingood mechanical properties. To enhance mechanical performance,resorbable polymers have recently been bre-reinforced (makingthem partially-resorbable) and nite element analysis has been ap-plied to assist in optimising volume fractions [36].

In the manufacture of early, non-resorbable composite boneplates the material combinations of carbon bre/epoxy (CF/epoxy)and glass bre/epoxy (GF/epoxy) were trialled. Due to concerns

analyse the performance of novel, carbon bre/polyamide12 (CF/PA12) and carbon bre/polyamide 12/hydroxyapatite (CF/PA12/HAP) composite hip stems [47,5658]. Bone density distributionshave been obtained and are presented visually, at a proximal sec-tion, in three cases (Fig. 5): for an intact femur, a femur with CF/PA12 stem, and a femur with Ti6-Al4V stem. Compared withtheir metallic counterparts, the new materials have been predictedto result in more adaptive bone remodelling producing around 2040% greater bone density in the proximal femur and 1020% lessbone density in the distal end [57]. Stress shielding effects andfracture risk are thus thought to be reduced.

In addition to design and mechanical performance, xationtechniques play an important role in total hip replacements. Thedevelopment of composite materials for use in hip joint xationcurrently concentrates mostly on the approaches of cementingand bone ingrowth [5255].

2.4. Dental applications

In dental applications, including orthodontic archwires, brack-ets and dental posts, conventional restorative materials such asamalgam, gold, alumina, zirconia and many others have essentiallybeen fully replaced by composite resins. Modern composite resins

e an High strength, elastic modulus, fracture toughness, and fatigueresistance to sustain mechanical reliability whilst resistingdeformation under loading; loads in the body range from 3 kNduring normal walking to 8 kN when jogging or stumbling.

High corrosion resistance for bioinertness and biocompatibilityin vivo.

High hardness and good surface nish supporting wear resis-tance in the long-term together with low friction.

Good wetting at the bearing surface/synovial uid interface forlubrication in the body.

2.3. Total hip replacement

Total hip replacements are the most common joint replace-ments with more than 150,000 operations a year in the US alone[5]. At present, metal hip implants are still predominantly usedas clinical success rates of 93% at 10 years and 85% at 15 yearspost-surgery are high [47]. Nevertheless, problems of prosthesisloosening and induced non-physiological stresses in the bone, di-rectly affecting bone remodelling, have been reported repeatedly,following the implantation of articial hip joints. Modern prosthe-raised over the toxicity of monomers in partially cured epoxy resin,however, researchers have largely turned their attention to ther-moplastic composites and carbon bre/poly ether ether ketone(CF/PEEK) designs in particular [5,38]. CF/PEEK is considered tobe biologically inert yet able to maintain high mechanical strengthand good fatigue resistance at plate thicknesses comparable tothose of conventional metallic types approximately 3.8 mm[39]. At present, inadequate manufacturing methods producingpoor quality CF/PEEK bone plates pose the major challenge [39].A rst step to overcome these issues has been taken by Fujiharaet al. [39,40] in fabricating knitted and braided CF/PEEK compres-sion plates. Aspects such as axial tension following pre-loading ofthe healing callus as well as local stresses at the screw holes mustalso be considered. The mechanical design of multilayer knittedfabric-reinforced laminates on the basis of progressive failure anal-ysis has been discussed by Huang et al. [41].

Finally, intramedullary nails may be used for stabilising longbone fractures such as those of the femoral neck or intertrochan-teric bone [5]. Carbon bre reinforced liquid crystalline polymer(LCP/CF) and GF/PEEK material combinations have been investi-gated for this less common application [5,42,43].

2.2. Total knee replacement

The application of composites in total knee replacements hasproven to be extremely difcult. Attempts to improve wear resis-tance of ultra high molecular weight polyethylene (UHMWPE) bymeans of reinforcing carbon bres failed due to poor bonding be-tween the two constituents [5]. Similarly, other efforts such asreinforcing UHMWPE with UHMWPE bres did not lead to in-creases in wear resistance; however, improved stiffness, strengthand creep resistance were achieved [5]. In a recent study Utzschne-ider et al. [44] tested the wear resistance of crosslinked polyethyl-ene in a number of different knee joints and reported statisticallysignicant lower wear rates compared to UHMWPE. In contrast,Fisher et al. [45] have suggested alterations in knee joint designsto achieve reduced wear.

A number of desirable characteristics for materials employed asthe articulating surfaces in any type of total joint replacement havebeen summarised by Rahaman et al. [46] and are laid out below:

M.-S. Scholz et al. / Composites Sciencses (Fig. 4) made from polymer composites have the potential tomeet spatially varying mechanical requirements such as strengthand stiffness. Consequently, regions of high stress concentrationcan be minimised and stress can be transferred more effectivelyat the bone/prosthesis interface [5].

However, while a more compliant hip implant reduces stressshielding it must maintain a minimum level of stiffness in ordernot to cause residual pain due to low-amplitude oscillatory micro-motion [49]. Micromotion describes the deformation and relativemovement between prosthesis and bone under loading. Further,investigations on the response of laminated composite prosthesesthat were, in shape, based on the standard metal alloy hip design,suggest that these may not be structurally adequate [50]. Finiteelement analysis is, therefore, often employed to test and optimisearticial hip joints. Following multiscale, structural analysis, a setof guidelines for designing future composite hip implants has beenproposed by Srinivasan et al. [51].

Computational methods have also been applied to simulate and

Fig. 4. Reduced stiffness, composite femoral stem. Adapted from Zimmer [48] withpermission of Zimmer, Inc.d Technology 71 (2011) 17911803 1795have a refractive index matching that of enamel and can thus notonly restore the function but also the appearance of biological tis-sue [59]. In addition, composite post-insertion proves less time

) an

e anconsuming, making surgical procedures less traumatic for the pa-tient [60]. The main shortcoming of these materials is their life-time; issues such as polymerisation shrinkage, shrinkage-stressdevelopment, thermal expansion mismatch, wear resistance or

Fig. 5. Cross-sectional view of the bone density distribution in units of g/cm3 for (aAdapted from Bougherara et al. [57] with permission of John Wiley and Sons.

1796 M.-S. Scholz et al. / Composites Scienctoxicity are yet to be resolved, leaving large scope for advancement[59,61]. Furthermore, typical orthodontic prosthetic compositeshave poor thermal conductivity leading to altered perception oftaste [62,63]. A viable solution to this problem has been proposedby Messersmith et al. [62] who incorporated ceramic microwires ofaluminium oxide into poly(methyl methacrylate) (PMMA) in orderto attain a higher diffusivity.

3. Soft-tissue applications

The application of bre reinforced composites to soft-tissue has,over the past two decades, proven to be difcult. Many kinds ofsoft-tissue, including skin, nerves, tendons, ligaments and vasculargrafts, do not obey Hookes law when subjected to a physiologicalload; instead, their stressstrain behaviour is represented by anon-linear, convex, j-shaped curve [5].

Composite materials to replace soft-tissue are generallyfounded on naturally occurring polymer-based systems such aspolysaccharide proteins [64]. Moreover, the intrinsic properties(molecular weight, charge, etc.) of the constituent parts areincreasingly important when developing these materials [64]. Inthe case of vascular grafts, for example, the optimisation of poros-ity plays an important role. Excessive porosity may lead to bloodleakage, but at the same time a certain threshold is required in or-der to encourage tissue growth and acceptance by the host tissue[5]. The suitability of different polymer composites for soft-tissueapplications has been reviewed in recent articles by Silva et al.[64] and Boccaccini et al. [65].

The structure of natural extracellular matrix (ECM) can be mim-icked by manufacturing bres that are within a similar size range.Cells seeded onto brous scaffolds tend to adhere to nanobreswhose culture modulates cell morphology and cytoskeletalorganisation [66,67]. Despite numerous manufacturing techniques,including self-assembly, phase separation, melt-blown and tem-plate synthesis, electrospinning has emerged as the leading tech-nique for nanobre scaffold fabrication as it is cost effective and

intact femur, (b) a femur with CF/PA12 stem, and (c) a femur with titanium stem.

d Technology 71 (2011) 17911803provides a user friendly approach, allowing versatility [66,68]. Itfurther permits scaffold renement by aligning bres to createstructural anisotropy with the forming network. Nevertheless,problems relating to inadequate cell inltration are often encoun-tered with this technique, clearly limiting their in vivo application.It has recently been suggested that enhanced cell inltration maybe achieved through the selective removal of sacricial bres inelectrospun, bre-aligned composite scaffolds [69].

4. Tissue engineering applications

Fibrous, scaffold-like structures are of particular interest notonly in the eld of soft-tissue engineering but moreover for thespecic purposes of advanced bone tissue engineering. Tradition-ally, defective bones are managed by employing well establishedtreatment methods such as autografting, allografting, the applica-tion of vascularised grafts, and bone marrow replacement. Presentapproaches are increasingly focused on the application of biocom-patible, osteoinductive, osteoconductive and mechanically com-patible scaffolding constructs, potentially able to integrate withnative tissue whilst stimulating contiguous bone formation[70,71]. Scaffolding material may further act to carry implantedbone cells and other agents [71]. Some scaffolds capable of drugdelivery, and able to locally release both growth factors and antibi-otics, promise to enhance bone ingrowth in conjunction withwound healing [7276].

Modern bone tissue engineering provides a number of propi-tious advantages [70]:

The amount of donor tissue required over any given periodof time would be reduced, as skeletal cells may be engi-neered in vitro.

e anM.-S. Scholz et al. / Composites Scienc The integration of biomaterials, whose mechanical proper-ties closely match those of the defective bone, may in thefuture see a strong decrease not only in the rate of implantfailure but also with respect to secondary surgery.

Affections could be relieved and diseased tissue potentiallycured if early treatment with mesenchymal stem cells waspursued. Consequently, the need for life-long treatment forthe patient would be reduced.

To optimise the integration process of newly formed bone intothe surrounding tissue, scaffolds synthesised for osteogenesis areexpected to closely match bone morphology, structure and func-tion [77]. A series of three-dimensional composite scaffolds origi-nating from PGA and b-tricalcium phosphate (b-TCP) compoundshas recently emerged, following a study by Cao and Kuboyama[77]. Their properties were found to mimic the natural bone com-ponents whilst permitting a close t of samples to bone defects.Further, advances in the bone formation rate have been observed.Fig. 6 compares, over a 90 day period, the extent of bioresorptionand bone remodelling for commercial HAP, PGA/b-TCP (1:1), andPGA/b-TCP (1:3). It was concluded that PGA/b-TCP scaffolds in aweight ratio of 1:3 prove most effective, clearly demonstratingtheir ability for mineralisation, osteogenesis and biodegradationat a rate comparable to that of tissue regeneration.

Further composite materials based on poly(e-caprolactone)(PCL) and esters of hyaluronic acid have been derived by Guarinoet al. [78]. Their work primarily focused on the fabrication of repro-ducible scaffolding networks that are tailorable with respect toboth physical and chemical properties. Scaffolds are moreoveranticipated to maintain structural integrity under load-bearingconditions and for a predictable period of time [73,78]. Impor-tantly, it was noted however, that a compromise must be reached

Fig. 6. Tartrate-resistant acid phosphatase haematoxylin counterstaining micrographs o90 days post-surgery, respectively. Images are taken at a magnication of 200, insetsnucleus; NB: new bone; M: material; black scale bar: 200 lm; red scale bar: 50 lm. Reproof the references to colour in this gure legend, the reader is referred to the web versiod Technology 71 (2011) 17911803 1797between the material chemistry and the structural design of poly-meric composite scaffolds, if any dynamic changes associated withthe progressive evolution of the hydrolytic degradation mecha-nism are to be predicted.

Besides, a number of nanometric bioactive glasses have recentlyattracted signicant attention. Specically, osteochondral, carti-lage and dentin regeneration but also bone tissue engineering arereported to be major contenders for the application of novel com-posite materials, that bring together biodegradable polymers withnanosize bioactive glass particles or bres [65]. Complementarydesign features on the nanoscale are particularly thought to createextra exibility in adapting both elastic modulus and strength. Anexemplar graphic of typical submicron bioactive glass bres is pro-vided in Fig. 7. A scanning electron micrograph, characteristic ofthe morphology associated with poly(hydroxybutyrate-2-co-2-hydroxyvalerate)/biomimetically synthesised nano-sized bioactiveglass (PHBV/BMBG) porous composites immersed in simulatedbody-uid (SBF) is shown in Fig. 8.

While mechanical strength constitutes the primary design dri-ver in the case of bone tissue engineering, scaffold structures serv-ing to support nerve regeneration are additionally required toenable the direction of the axonal growth cone to the distal stumpvia biological signalling [64,81,82]. Accordingly, design complexityis promoted further and the need for multifunctional materialsolutions readily becomes apparent.

5. Special prosthetics for application in professional sports

The amputees ability to participate in competitive sport wasrevolutionized by the introduction of carbon bre reinforced poly-mer (CFRP) composites into the structural design of articial limbs

f (a) commercial HAP, (b) PGA/b-TCP (1:1), and (c) PGA/b-TCP (1:3) at 0, 14, 30, andare enlarged with a zoom factor of 600. Red colour: osteoclast; blue colour: cellduced from Cao and Kuboyama [77] with permission of Elsevier. (For interpretationn of this article.)

e an1798 M.-S. Scholz et al. / Composites Scienc[83]. CFRP is an extremely lightweight material and making use ofits great exibility yet high strength, it is possible to embed an en-ergy return system within lower-limb prostheses [8486]. Theneed for such a system was recognised following advances in gaitanalysis and biomechanics, and was rst introduced in the Seattle

Fig. 8. Scanning electron micrographs of PHBV/BMBG composite scaffolds immersed inwall prior to immersion, (c) scaffold at 8 h immersion, (d) porous network at 24 h imm

Fig. 7. Scanning electron microscopy images of (ac) electrospun submicron bioactive get al. [79] with permission of Springer.d Technology 71 (2011) 17911803foot in 1981 [87]. Shifting body weight onto the CFRP structure in-duces compressive loads and thus energy is stored. Lifting the bodyweight off results in decompression, allowing the material to re-turn to its original shape; consequently, energy is returned. Asthe dynamic response of the amputee/prosthetic system depends

SBF. (a) Porous structure before immersion, (b) locally enlarged morphology of poreersion. Adapted from Zheng et al. [80] with permission of Trans Tech Publications.

lass 70S30C bres at different magnication, (d) a single bre. Reproduced from Lu

e anM.-S. Scholz et al. / Composites Sciencon patient height, weight and activity level, it is important to takethe systems natural frequency into consideration when designingany energy store and return (ESAR) foot. In a simple model of thebehaviour, one could assume ideal conditions i.e. if there is no en-ergy loss within the structure, the CFRP foot may be treated like aperfect spring, and Hookes law can be applied. Clearly, more real-istic scenarios have to account for friction as well as other forms ofenergy loss including heat and noise. Brggemann et al. [88] mea-sured the dynamic hysteresis for the case of ssurs Cheetah foot(Fig. 9a). They calculated the energy return to be approximately95%. Previous models such as the Silent Ankle Cushion Heel(SACH), Seattle and Flex Foot have been reported to attain energyefciencies of only 31%, 52% and 84% respectively [89]. Of course,these values depend directly on the accuracy of the model andare likely to be an overestimate due to idealisations.

The rst below-knee prosthesis made purely from CFRP theFlex Foot (Fig. 10) was developed by Philips in 1985 [85,90]. Its

Fig. 9. Examples of different sprint foot designs: (a) Cheetah (ssur), (b) Flex-Run (ss

Fig. 10. Schematic representation of the Flex Foot design, as derived by Phillips[90].appearance in professional sport followed shortly after at theParalympic Games in 1988 [86]. Since then a diverse range of mod-els has been developed to better meet the various needs of ampu-tees. To further customise these designs, adjustments may be madeby differing laminate lay up, bre orientation and/or laminatethickness.

A selection of different sprint foot designs is exemplied inFig. 9. Modern sprint prostheses no longer incorporate a heel aselite runners have been observed to only run on their toes [91] and generally comprise an articulated, long keel design [92]. Workby Nolan [86] suggests that the design of CFRP sprint prosthesescan be optimised using a mathematical expression for the sprint

ur), (c) Flex-Sprint (ssur), (d) C-Sprint (Otto Bock), (e) Sprinter (Otto Bock) [86].d Technology 71 (2011) 17911803 1799speed in terms of foot shape and stiffness.Despite the numerous, specialised designs todays sports pros-

theses are almost all made from CFRP, in particular those used inrunning and jumping events [86]. Recent devices enable athletesto complete the 100 m in just under 11 s; approximately one sec-ond off the mens Olympic record [3,86].

In 2008, when Oscar Pistorius, a bilateral lower limb amputee,requested to compete in both the Olympic and Paralympic Games,the International Association of Athletics Federation initially ruledthat modern prostheses provide an unfair advantage [85], suggest-ing that current designs of composite prostheses may even be ableto outperform the human limb. This decision has since been over-turned as research has in fact shown that, despite great advances inthe design of lower limb prosthetics, biomechanical asymmetriesstill lead to inferior performance of lower-limb prostheses in bothwalking and running [85,9395]. Nevertheless, it clearly demon-strates how bre composite materials have greatly inuenced thedevelopment of modern articial limbs.

While composites are predominantly found in lower limb pros-thetics their application is by no means limited to this market. Togive an example, Riel et al. [96] developed an arm prosthetic kit forracingcyclists, partlymade frombreglassandnylonwhichwassuc-cessfully used by a Canadian athlete at the 2008 Paralympic Games.

6. Commercial prosthetics

As a disproportionate number of lower-leg amputees live indeveloping countries, where the traditional causes of amputation

candidate for applications in this eld as a direct consequence oftheir combined sensing and actuating characteristics. Furthermore,IPMCs full the need for a lightweight material, able to undergolarge bending deformations. Because IPMCs do not rely on the pro-vision of power from external sources [111], enhanced practicabil-ity may also be achieved.

Other applications have been explored by Fang et al. [113] whodeveloped an IPMC actuator for active catheter systems as well asNguyen et al. [114] in the fabrication of a ap valve IPMC micro-pump. Li et al. [104] very recently investigated the radius controlof biomedical active stents using helical IPMC actuators.

7.2. Challenges faced with ionic polymer-metal composites

Despite the many advantages of IPMCs, a number of concernsare yet to be addressed ahead of clinical approval and broad com-mercial application. One of the major problems relates to the lim-itation of accuracy and positioning bandwidth at high operatingfrequencies due to the exhibition of relaxation behaviour and non-linearities in IPMCs [103]. Furthermore, IPMCs are sensitive tochanges in stress/strain as well as temperature, pressure, andhumidity [99,101,111]. Subsequent instabilities in terms of ther-mal, mechanical and chemical properties may also be observeddepending on the application. Typical Naon-based IPMC actuatorsoften come at a high price, thus recent developments have led toalternative novel actuators based on sulfonated poly(ether etherketone) (SPEEK) and poly(vinylidene uoride) (PVDF) [100].

8. Future directions

Fig. 11. GF/HDPE prosthetics as developed by Dubois [97]. Reproduced from Dubois[97] with permission of Dubois.

e anBiomedical applications of IPMCs include articial ventricular,sphincter and ocular muscles, articial smooth muscle actuators,correction of refractive index in the human eye, peristaltic pumps,incontinence assist devices, and surgical tools [104,108].

Shahinpoor [109] has developed an implantable, electricallycontrolled, heart compression device that, based on the sensingcapabilities of IPMCs, allows for the continual monitoring of ap-plied ventricular stroke volume and/or pressure. The apparatus islightweight and external to the heart so as to avoid thrombogene-sis and other complications. Through the ability of IPMCs to act asboth a sensor and an actuator, actuation and control of the deviceare readily enabled. A simpler IPMC heart assist device, in the formof a compression band, has also been proposed [108,110].

The potential benets of utilising IPMCs for the development ofmodern sensory feedback systems, specically in hand prostheses,have been investigated by Biddis and Chau [111]. Direct sensoryfeedback is essential in equipping future upper limb prostheticswith attributes such as reex capabilities, adaptive grasp, preven-tion of slip and tactile exploration [111,112]. Integrating these fea-While the eld of biomimetic actuators and polymer-based sen-sors is immature, electro-active polymers are becoming progres-sively more popular as components of prostheses as well asimplant feedback systems and biochips for personalised medicine[99]. Among the vast variety of electrically conducting polymericmaterials, ionic polymer-metal composites (IPMCs) have recentlyattracted particular attention owing to their combined sensingand actuating ability as well as an array of prominent propertiesincluding softness, exibility, lightweight, excellent biocompatibil-ity, large bending deformation, low power consumption and highfrequency operation [100104]. Furthermore, IPMCs have beenshown to have a high level of performance under wet conditions[101,104]. For details regarding the chemical structure, manufac-turing techniques, computer modelling and industrial applicationsof IPMCs the reader is referred to a series of review articles byShahinpoor and Kim [105108]. Some exemplar biomedical appli-cations are discussed below, alongside a number of challengeswhich IPMCs are facing in their development.

7.1. Biomedical applications of ionic polymer-metal compositesare often added to by war and in particular land mines, custom t,highly expensive CF/epoxy prosthetics are not always feasible[3,97]. Less expensive polypropylene plastic and glass bre/high-density polyethylene (GF/HDPE) types (Fig. 11) have thus beenderived at the expense of comfort, mechanical performance andmanufacturing reliability [3,97]. Furthermore, knitted fabrics madefrom Kevlar bres suspended in an ultraviolet curable resin haveevolved in order to ease the conformation of direct sockets to thepatients stump [98]. Nevertheless, advances being made in thedevelopment of costly prosthetics specically designed for eliteand highly specialised applications are widely accepted to contrib-ute to both the recovery of previously inconceivable levels of sport-ing and recreational function as well as the quality of commerciallyavailable prostheses in general [86,89].

In a push to rapidly produce highly tailored designs, South et al.[84] recently investigated selective laser sintering (SLS) tech-niques. One should note that despite the radically different charac-teristics of Rilsan D80 in comparison with CFRP, prosthetic feetwith similar mechanical properties could be achieved.

7. Biomimetic sensors, actuators and articial muscles

1800 M.-S. Scholz et al. / Composites Scienctures into modern prostheses is likely to provide the amputee withmore control and thus the ability to respond to the external envi-ronment in a more natural way. Clearly, IPMCs appear a promisingd Technology 71 (2011) 17911803Extensive research is currently focused on the development ofvarious porous composite materials [12,20,54,78,115] that are

neering [73]. In some cases these may be established via theintroduction of bre reinforcement; however, depending on the

e anporosity of the material, bre reinforcement does not always im-prove compressive strength [116]. Instead, techniques such as par-ticle reinforcement [54] and the inclusion of nano bres [117] orcarbon nanotubes [118,119] within the scaffold structure havebeen derived. At the same time, however, the degree of bioactivityis directly dependent on volume fraction, size, shape, and arrange-ment of inclusions, with greater volume fraction and higher surfacearea to volume ratio resulting in enhanced bioactivity [73]; in anumber of cases the incorporation of brous reinforcements maythus remain favourable over that of particles [73]. The successfulemployment of glass bre reinforced, porous implants in rabbitshas already been reported by Tuusa et al. [120].

Another area of ongoing research is concentrating on the devel-opment of functional coatings for deposition on metallic implants,noting that metallic implants are still widely employed due to theirexcellent mechanical properties and low cost. A number of com-posite coatings including mixtures of alumina and zirconia [121]and nanocomposite lms made from zirconium nitride and silver[122] or diamond-like TiC/a-C [123] have recently been tested.These types of coatings are anticipated to improve the wear andcorrosion resistance of metallic implants; wear and corrosion areconsidered to be the main causes of degradation in hip and kneeimplants [121].

Novel advances in composite usage have also been realisedwithin orthotics. Following a recent study by Stodolak et al.[124], composite membrane implants based on sodium alginate -bres may lead to enhancements in glaucoma surgery. Glaucomadescribes a form of eye disease caused by an increase in intraocularpressure, and should medicine therapy fail in controlling the eyepressure, surgery is required [124]. Under in vitro conditions,Stodolak et al. found their newly developed membrane implantsto have relatively high strength and elasticity, however, their suit-ability for glaucoma treatment is yet to be conrmed throughin vivo testing.

Finally, a combination of nanoengineering through to innova-tive manufacturing and material processing techniques may, inthe future, allow inherent material weaknesses to be overcomesuch that tailored material property sets (e.g. stiffness, strengthand toughness) may be targeted at an effective cost.

9. Conclusions

Composites are now widely applied across all areas of modernorthopaedic medicine. Specically, key technological advanceshave been seen in dentistry and the design of lower-limb sportsprostheses. The main advantages in using bre-reinforced compos-ites for orthopaedic purposes are associated with their exceptionalspecic strength characteristics and biocompatibility. Followingthe wide range of applications, there is signicant scope for contin-uing innovation and improvement of present techniques. The ratethought likely to prove extremely benecial, particularly in tissueengineering applications. Their sponge-like structure allows forhuman mesenchymal stem cells to grow into the scaffolding[12,54] and as a result, implant loosening and stress shieldingcan be reduced signicantly [12]. In addition, cellular structuresimply material and weight savings, both undoubtedly desirableattributes.

While some porous materials exhibit excellent properties interms of their biocompatibility they often do not meet the mechan-ical requirements that make them suitable for bone tissue engi-

M.-S. Scholz et al. / Composites Sciencof advancement will, therefore, mainly depend on nancial con-straints and the concurrency of multidisciplinary efforts. Currently,intense areas of research are underway in the development of bothporous materials and coatings that are better suited to biomedicalapplications, as well as advanced manufacturing and material pro-cessing techniques. Future generations of prosthetic devices areaiming to provide enhanced control of upper-limb prosthesesthrough the inclusion of sensory feedback systems, offering ampu-tees the prospect of a more natural response.

Acknowledgments

The authors gratefully acknowledge the support of the EPSRCunder its ACCIS Doctoral Training Centre grant, EP/G036772/1.

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M.-S. Scholz et al. / Composites Science and Technology 71 (2011) 17911803 1803

The use of composite materials in modern orthopaedic medicine and prosthetic devices: A review1 Introduction2 Hard-tissue applications2.1 Bone fracture repair2.2 Total knee replacement2.3 Total hip replacement2.4 Dental applications

3 Soft-tissue applications4 Tissue engineering applications5 Special prosthetics for application in professional sports6 Commercial prosthetics7 Biomimetic sensors, actuators and artificial muscles7.1 Biomedical applications of ionic polymer-metal composites7.2 Challenges faced with ionic polymer-metal composites

8 Future directions9 ConclusionsAcknowledgmentsReferences