Design and fabrication of CoCrMo alloy based novel structures for load bearingimplants using laser engineered net shaping

Félix A. España, Vamsi Krishna Balla, Susmita Bose, Amit Bandyopadhyay ⁎

W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, USA

a b s t r a c ta r t i c l e i n f o

Article history:

Received 14 November 2008Received in revised form 10 July 2009Accepted 13 August 2009Available online 26 August 2009

Keywords:

Rapid manufacturingLaser engineered net shaping (LENS™)Porous metalsMetal implantsImplant design

Designing load bearing implants with the desired mechanical and biological performance and to fabricate netshape, functional implants with complex anatomical shapes is still a challenge. In addition, patient specificload bearing implants with the possibilities of guided tissue regeneration are gaining significant interest inorthopedics. Novel design approaches and fabrication technologies that can achieve balanced mechanicaland functional performance in mono-block implants are necessary to accomplish these objectives. In thisarticle we give an overview of our novel design concepts for load bearing metal implants and demonstratethe manufacturing of unitized implant structures with and/or without porosity using laser engineered netshaping (LENS™) — a solid freeform fabrication technique. We have fabricated porous metal implants withdesigned porosities up to 70 vol.% in various biomedical metals/alloys, such as Ti, Ti6Al4V, NiTi and CoCrMo,and tailored their effective modulus to suit the modulus of human cortical bone, thus eliminating stress-shielding. Unitized structures with functionally graded CoCrMo alloy coating on porous Ti6Al4V alloy havebeen fabricated using LENS™ to minimize wear induced osteolysis. Finally, this technology can also be usedto fabricate porous, net shape implants with functional gradation in structure and/or composition to mimicnatural bone. Since the LENS™ fabrication does not change the chemistry of the biocompatible alloys theinherent in vitro and in vivo biocompatibility will remain the same and therefore, we have not provided anybiocompatibility results in this article. This article provide an insight into the important aspects of LENS™fabrication and properties of CoCrMo alloy structures, which can potentially eliminate long standingchallenges in load bearing implants such as total hip prosthesis to increase their in vivo life time.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Metallic biomaterials are extremely successful in restoring lostfunctions of human bone under high loads. However, metals arebioinert and have a considerably higher stiffness than natural bonewhich significantly reduces the implant's in vivo lifetime. For example,total hip replacement (THR) surgeries are being performed more onyounger patients below the age of sixty, which expose the implant togreater mechanical stress over a longer period of time due to theiractive lifestyle. Short life of current THR implants, between 7 and12 years, is generally due to the aseptic loosening of the implant,which occurs due to (i) mismatch of the Young's modulus betweenbone (10–30 GPa) and metallic implant materials (110 GPa for Tiand 248 GPa for CoCrMo alloy) leading to stress-shielding, (ii) poorinterfacial bond between the host tissue and the implant due tobioinert surface, (iii) wear induced osteolysis and aseptic loosening inmetal-on-polymer implants, and (iv) absence of high recoverablestrain (~ 2%) as well as hysteresis similar to natural bone. For these

reasons there is a considerable demand for improved THR's, andsimilar load bearing implants, which can last longer in vivo.

In order to increase the in vivo lifetime of metal implants, one can(i) decrease its effective modulus to match that of bone, and (ii)increase the interfacial bond between living cells and implantmaterials via compositional or structural modification. Use of porousmetals in place of fully dense material can effectively reduce themodulus mismatch [1–3]. Also, the interconnected porosity increasesthe bone-implant interfacial bond by bone ingrowth through thepores. Several processing routes have been used to fabricate surfacetreated or fully porous metals for biomedical applications. Inconventionally sintered metals [4–7] porosity characteristics such aspore size, shape, volume fraction, and distribution are difficult tocontrol, which have major influence on mechanical and biologicalproperties. Other fabrication techniques that use foaming agents ormolten metal suffer from typical limitations such as contamination,impurity phases and limited part geometries. Overall, the parts fabri-cated using above processes usually suffer from loss of physical pro-perties due to stress concentrations at the porous interface,microstructure changes, and surface contamination [4,5,7]. Anotherserious concern limiting the life of THR is relatively high wear rate ofultra-high-molecular-weight polyethylene (UHMWPE) liner leading

Materials Science and Engineering C 30 (2010) 50–57

⁎ Corresponding author. Fax: +1 509 335 4662.E-mail address: amitband@wsu.edu (A. Bandyopadhyay).

0928-4931/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.msec.2009.08.006

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to osteolysis and aseptic loosening [8]. In the past, ceramic coatingshave been proposed, as being wear-resistant and a metal ion releasebarrier for THR. These coatings were obtained using different vapordeposition techniques, such as physical vapor deposition, ionimplantation, sputtering and the coatings consisted of diamond-likecarbon [9,10] or nitrides [11,12]. These coatings have found littleapplication in the field of THR due to their inherent brittlenessand catastrophic fracture possibilities. Therefore, there is a growinginterest in wear-resistant metal/alloy coatings for THR because oftheir excellent toughness coupled with high wear resistance. It isclear from the foregoing discussion that improved fixation andincreased longevity are still important performance criteria in thedevelopment of orthopedic prostheses, which mandates the devel-opment of innovative designs and use of advanced manufacturingtechniques.

2. Novel macro/micro structural design for load bearing implants

The need for adequate mechanical and functional propertiescoupled with manufacturing flexibility for a wide range of metallicimplant materials demands the use of novel designs. Innovativedesigns such as functionally graded acetabular shells with openporosity on one side to improve cell–material interactions and a hardcoating on the other side (in contact with femoral head) to increasethe wear resistance, can significantly improve the implant's in vivo

life by completely eliminating the need for acetabular liners. Byeliminating the liner, the diameter of femoral head can be increased,which in turn provides greater maneuverability and stability for theimplant assembly. A schematic of such gradient structure is shownin Fig. 1A. Similarly, graded structures with gradual increase in theconcentration of bioactive calcium phosphate based ceramic frommetallic implant's core to the surface can significantly increaseosseoconductivity, while simultaneously reducing the interfacialproblems such delamination during fabrication or in service. While awear-resistant alloy coating on metal substrates seems plausible,there is only onemetallic alloy combination, i.e. CoCrMo and Ti6Al4V,suitable for surgical implant, which shows metallurgical incompat-ibility [13]. Although functionally graded coatings (FGCs) can over-comemetallurgical incompatibility, it is difficult, if not impossible, tofabricate net shape implants/structures with spatial gradation incomposition and structure with conventional processing routes. Theuniform structural/compositional change across the interface inFGCs provides unique functionality and performance for biomedicalapplications [14,15]. For example, implants with gradients inporosity and pore sizes, as shown in Fig. 1B, that can allow on oneside of the implant high vascularization and direct osteogenesis,while promoting osteochondral ossification on the other, areappealing in terms of reproducing multiple tissues and tissueinterfaces on the same implant. Porosity is not always beneficial forTHR, as it can significantly decrease thewear resistance of biomedicalmetals/alloys. While fully dense CoCrMo alloy based implants canprovide the best wear resistance, its high stiffness (248 GPa) canpose problems due to stress-shielding. Therefore, the only way toreduce the stiffness of CoCrMo alloy without sacrificing its wearresistance is to fabricate an implant with fully dense outer surfaceand isolated/interconnected porosity inside. Finally, if one implantwith various sections having different internal/external macrostruc-tures, such as fully dense, fully porous, and porous core with solidwalls, can be designed and fabricated, then the implant can possessite specific functions at different location on the same implant. Anexample of this functionally designed hip stem is shown schemat-ically in Fig. 1C.

The above designs enable implants with designed macro- andmicroporosity to achieve desired mechanical and functional perfor-mance. Although such innovative implant designs can potentiallytriple the in vivo life of load bearing implants, fabricating them is a

real hurdle. Functional implants with these complex designs can befabricated using layered manufacturing processes, generally knownas solid freeform fabrication (SFF) and one such process is laserengineered net shaping (LENS™). This process uses a focused laserbeam as a heat source to melt metallic powder and create a solid,three-dimensional object. A schematic representation of the LENS™process is shown in Fig. 2. Initially, a three-dimensional model of acomponent to be built is generated using CAD, subsequently acomputer program slices the model into a number of horizontalcross-sections or layers. These cross-sections are sequentiallycreated on a substrate producing a three-dimensional object.More detailed description of the process is provided elsewhere[2]. Being a CAD and layer based manufacturing process LENS™gives significant advantage over conventional manufacturingmethods in terms of tailoring microstructure, shape, size andinternal architectures particularly of porous structures in oneoperation by controlling different process parameters. Multiplepowder feeding and closed loop melt pool control systems facilitatethe fabrication of components with functional gradient in compo-sition and/or porosity across the section. Since the fabrication iscarried out in a protective atmosphere with oxygen content lessthan 10 ppm, LENS™ processed materials retain the purity of feed-stock powder, which is extremely important during processing ofmaterials for biomedical applications. LENS™ process involves highsolidification cooling rates (103 to 105K/s) leading to severalmicrostructural benefits [16]. Many metallic, intermetallic, ceramic,and composite powders have been successfully processed usingLENS™ [17–20].

3. Implant manufacturing

3.1. Net shape porous implants

Fig. 3 shows the two types of porosity, namely inter-particleporosity and tool path based porosity that can be introduced in theparts using LENS™. The extent of powder melting in each track/scandecides the achievable porosity in the final part depending on laserenergy input, which can be controlled by changing LENS™ processparameters such as laser power (P), scan spacing (h), powder feedrate, laser scan speed (v) and Z-increment or layer thickness (t). Sincethe parts aremade via layer wise deposition, and each layer consists ofa number of consecutive overlapping tracks/scans, the final density ofa LENS™ processed part can be considered as an average of thedensity of each track/scan. The total energy input per volume of eachtrack/scan (E, J/mm3) as a function of processing parameters can beevaluated from [21]:

E =P

v⋅h⋅t: ð1Þ

At appropriate LENS™ process parameters one can achieveoptimal lowest energy input ensuring lower working temperaturesand small amount of liquid phase around the powder particles dueto partial melting of the powder. These surface melted powders jointogether leaving some inter-particle porosity. Fig. 3 shows theformation of porosity in each track. Particle bonding in this case isa direct result of localized melting and resolidification as againstsolid state sintering in powder metallurgical route. Therefore, theinherent brittleness associated with solid state sintered metalpowders is completely eliminated, potentially enhancing implant'sfatigue life. Structures with different porosity parameters andinternal architecture with designed gradient across the part canalso be fabricated by optimizing the distance between twosuccessive metal roads (laser scans) and the thickness of eachmetal layer. Moreover, by changing the deposition angles of laser

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scans for each layer, the tool path based pores can be oriented layerby layer leading to a three-dimensional interconnected porosity.Fig. 4 shows typical porous titanium samples and actual hip stemsproduced via LENS™. These samples show the net shape fabricationcapability of LENS™.

3.2. Functionally graded/unitized structures

Independently controllable multiple powder feeders in LENS™enable variation of composition and porosity simultaneously in oneoperation to manufacture novel implant structures. Functionally

Fig. 1. Schematic showing novel implant designs.

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graded structures with hard and wear-resistant CoCrMo alloy coatingon porous Ti6Al4V alloy with metallurgically sound interface havebeen produced using LENS™ [22]. In vitro biocompatibility studyshowed that these coatings were non-toxic and biocompatible [22].Composition gradation in the transition region was achieved bygradually increasing the feed rate for CoCrMo alloy and accordinglydecreasing the feed rate of Ti6Al4V alloy powder over 5–7 layers ofdeposition. Elimination of intermetallic compounds in these coatings,due to rapid cooling rates, is beneficial in terms of better wearresistance and biocompatibility [23]. Gradient coatings with 86%CoCrMo in the top surface showed ~184% increase in surface hardness.Moreover, the porosity on the Ti6Al4V alloy side, which will be incontact with the bone, can improve bone tissue ingrowth [3,5] andhard coating on the other side increases the wear resistance of thestructure in contact with CoCrMo femoral heads.

LENS™ also has the capability to fabricate two separate partssimultaneously in one operation, to make unitized structures, whichcan potentially eliminate (i) interfacial problems associated withsharp interface between conventionally assembled parts, and (ii)precision machining and assembling of the components fabricatedseparately by conventional routes. For example consider a typical dogbone implant, as shown in Fig. 5A, which consists of two parts namelyporous sleeve, to enhance osseointegration, and solid core to supportthe mechanical loads. Such implants conventionally manufacturedand assembled usually suffer from low fatigue strength due to sharpinterface between the porous sleeve and the fully dense core. UsingLENS™ we have manufactured these two parts simultaneously in onestep making the implant a unitized structure, shown in Fig. 5B,without sharp interface, which can potentially increase the in vivo life

time of load bearing metal implants. From these works it is apparentthat LENS™ can be used to manufacture a variety of load bearingimplants with tailored microstructures and compositions whilemaintaining the size and the shape for specific applications orpatients.

4. Case study: CoCrMo alloy structures

CoCrMo alloy powder, Stellite® 21, (Stellite Coatings, Goshen, IN,USA)with particle size between 45 and 150 μmwas used in this study.The nominal chemical composition (wt.%) of CoCrMo alloy powder,conforming to ASTM F75, was 27 Cr, 5.5 Mo, 2.75 Ni, 15 Fe, 0.25 C, 1Mn, 1.5 Si and balance Co. The substrates used were rolledcommercially pure Ti plates of 3 mm thickness (President TitaniumCo., MA, USA). LENS™-750 (Optomec Inc. Albuquerque, NM, USA)with 500 W Nd-YAG laser system was used to fabricate porousCoCrMo structures. Two laser power levels of 200 and 250 W wereused to partially melt the alloy powder during the deposition processto create porous structures. In this work, scan speeds of 15 and20 mm/s and powder feed rates of 40, 50, and 60 g/min were used tostudy their influence on the porosity. As stated earlier, porositysignificantly reduces the wear resistance of CoCrMo alloy basedimplants, which are used for self-bearing applications due to itsoutstanding wear resistance. However, one concern about CoCrMoalloys is its high stiffness. Therefore, methodologies to reduce CoCrMoalloy stiffness while maintaining its wear resistance are extremelyimportant. The present CoCrMo alloy samples contain fully denseouter surface to retain its inherent wear resistance and a porous coreinside the structure, which reduces the effective stiffness of the

Fig. 2. Schematic depiction of LENS™ process.

Fig. 3. Porosity formation during LENS™ processing.

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structure to match that of human cortical bone. Dense outer surfacewas fabricated by decreasing the scan speed, by 5 mm/s, duringdeposition of outer surface/contour. Fig. 6 shows the cross-sectionalimage of such CoCrMo alloy structures.

Cylindrical samples with 7 mm diameter were fabricated for com-pression testing and microstructural evaluation. Bulk density, whichincludes both open and closed pores, of the samples was determinedby measuring physical dimensions and mass of the samples. Apparentdensity, which includes closed pores, was also measured. The fractionof open and closed pores in the samples was calculated from the bulkand apparent densities. Microstructural studies were performed usinglight microscope and field emission scanning electron microscope(FESEM, FEI – Quanta 200F). Polished specimens were electrolyticallyetched with 5% HCl aqueous solution at 3 V for about 10 s. Constituentphases in the laser processed samples were identified using a SiemensD 500 Kristalloflex diffractometer with Cu Kα radiation (1.54056 Å)at 20 kV between the 2θ range of 20 and 80°, and compared withthose of as-received powder. Three samples corresponding to eachdensity were compression tested using servo hydraulic axial/torsionmaterials test system machine with 250 kN capacity at a strain rateof 10−3s−1. Polytetrafluoroethylene (PTFE) was used as a lubricantbetween the sample and compression tools to reduce friction. Young'smodulus and compression strength was determined from the stress-strain plots derived from load-displacement data recorded duringcompression testing. Vicker's microhardness measurements (Leco, M-400G3 model) were performed on the laser processed CoCrMo alloysamples using 200 g load for 15 s. An average value of 10 measure-ments on each sample was reported.

Samples for in vitro biocompatibility test were sterilized byautoclaving at 121 °C for 20 min. In this study the cells used werean immortalized, cloned osteoblastic precursor cell line 1 (OPC1),

whichwas derived from human fetal bone tissue [24]. OPC1 cells wereseeded onto the samples placed in 12-well plates. Initial cell densitywas 5.0×104 cells per well. A 1 ml aliquot of McCoy's 5A medium(enriched with 5% fetal bovine serum and 5% bovine calf serum, andsupplemented with 4 μg per ml of fungizone) was added to each well.Cultures were maintained at 37 °C under an atmosphere of 5% CO2.

Fig. 4. LENS™ processed porous titanium structures. (A) Samples with total porosity>50 vol.% are fabricated using tool path based porosity. (B) Net shape, functional hipstems with designed porosity fabricated using LENS™.

Fig. 5. Typical dog bone implant. (A) CAD model. (B) LENS™ processed porous sleeve(left) and solid core (middle), which require further finishing and assembly. Unitizedstructure (right) with porous sleeve and fully dense core can be fabricated in one stepusing LENS™.

Fig. 6. Typical LENS™ processed CoCrMo alloy sample with fully dense outer wall andporous matrix inside. The relative bulk density of this structure was 82%.

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Medium was changed every 2–3 days for the duration of theexperiment. The MTT (Sigma, St. Louis, MO) solution of 5 mg ml−1

was prepared by dissolving MTT in PBS, and filter sterilized. The MTTwas diluted (50 μl into 450 μl) in serum-free, phenol red-freeDulbeco's minimum essential medium. Then 500 μl of diluted MTTsolution was added to each sample in 12-well plates. After 2 h ofincubation, 500 μl of solubilization solution made up of 10% Tri-ton X-100, 0.1 N HCl and isopropanol was added to dissolve the

formazan crystals. Then 100 μl of the resulting supernatant wastransferred into a 96-well plate, and read by a plate reader at 570 nm.

4.1. Porosity characteristics

Relative density of laser processed porous CoCrMo alloy samplesvaried from 81% to 90% depending on laser processing parameters.As-processed samples showed rough surface which can enhance thelong-term stability of the implants by providing anchorage forbiological fixation and by enabling stresses to be transferred from theimplant to the bone. However, for wear resistance applications finalfinishing is required. Experimental data related to the influence oflaser parameters on the porosity of CoCrMo alloy samples, as shownin Table 1, indicate that total porosity of CoCrMo alloy samples can betailored by changing the LENS™ processing parameters.

The porosity decreased with a decrease in the scan speed, powderfeed rate and by increasing the laser power. The working tempera-tures decreases with decreasing laser energy input (by decreasinglaser power and/or increasing the scan speed or feed rate) leading to apartial melting of powders during LENS™ deposition. These partially

Table 1

Influence of laser parameters on the porosity of CoCrMo alloy samples.

LENS™parameter

Scanspeed,mm/s

Powderfeed rate,g/min

Laserpower,W

Bulkdensity,%

Comments

Power 200 15 60 ⁎⁎⁎ 82.5 Porosity decreases withincreasing power250 87.5

Scanspeed

15 ⁎⁎⁎ 50 250 88.4 Porosity increases withincreasing scan speed20 87.0

Powderfeedrate

40 20 ⁎⁎⁎ 250 90.4 Porosity increases withincreasing powder feedrate

60 88.2

*** No results.

Fig. 7. (A) X-ray diffraction pattern of laser processed structures and as-received CoCrMo alloy powder. (B) Microstructure of as-received CoCrMo alloy powder. (C) Microstructureof laser processed CoCrMo alloy.

55F.A. España et al. / Materials Science and Engineering C 30 (2010) 50–57

melted powders join together in the presence of the liquid metal atthe particle–particle interfaces, leaving some inter-particle porosity.On the other hand, increasing the energy input, by increasing the laserpower, increases the working temperatures and consequently meltsthe powder completely leading to a dense deposit/layer. In addition,at high working temperatures the flow of liquid metal to fill anyresidual inter-particle pores becomes easier and promotes higherdensification than at low working temperatures. Increasing the pow-der feed rate increases the volume of powder in the laser–materialsinteraction zone, i.e., thicker tracks, leading to a decrease in the laserenergy input. The low energy absorbed by the powder particlesresults in partial melting of the powder, consequently high porosity inthe samples. At constant powder feed rate and scan speed, decreasingthe laser power from 250 W to 200 W, resulted in high porosity in thesamples. In the same manner, porosity increased as the scan speedwas increased and other parameters being held constant. This isattributed to the fact that as the scan speed is increased, the inter-action time between the powder and laser is reduced. Therefore, theinstantaneous laser energy absorbed by the powder also decreases

leading to relatively less amount of liquid phase around the metalpowders. As a result, the particle rearrangement, which is consideredresponsible for high sintered density in liquid phase sintering, will beless in this case resulting in high porosity in the samples processedat high scanning speeds. Finally, in general, high porosity sampleswere achieved as the powder feed rate was increased while keepingthe laser power and scan speed constant. As the powder feed rateis increased there is more volume of powder in the laser–materialinteraction zone. This will decrease the laser energy density of thepowder leading to only partially melted material and then conse-quently to high porosity samples. Similar observations were made inlaser processed Ti samples [2].

Vicker's microhardness measurements of LENS™ processed po-rous CoCrMo alloy samples fabricated under various processingconditions indicated an average hardness of 361±11 Hv. X-ray dif-fraction results, as shown in Fig. 7A, of laser processed CoCrMo alloysamples show that all major peaks correspond to as-received powder.However, presence of (102) and (101) in laser processed in place of(200) and (220) of as-received powder indicate the presence of some

Fig. 8. Influence of porosity on the Young's modulus of laser processed CoCrMo alloy samples.

Fig. 9. Compressive strength of laser processed CoCrMo alloy samples as a function of sample porosity.

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texture i.e., solidification in preferred direction. Preferential grainorientation in LENS™ processed materials is common [25] due to therapid heat loss through the substrate, as observed in the present work.Moreover, the microstructural study, as shown in Fig. 7B and C,showed thicker carbide networks in laser processed sample com-pared to that observed in as-received powder. The additional peak,Cr7C3 (511), in laser processed sample could also be due to thickercarbide network present in this sample. The variations in carbidenetwork are presumably due to variations in cooling rates during laserprocessing.

4.2. Mechanical and biological properties

Mechanical properties of laser processed porous CoCrMo alloystructures are shown in Figs. 8 and 9. The data indicates that theYoung's modulus of the laser processed porous samples can be variedbetween 33 and 43 GPa by changing the LENS™ process parameters.This is a huge improvement when compared to the wrought CoCrMoalloy with a modulus of 248 GPa. Themodulus of porous CoCrMo alloyis close to that of natural bone which has a range of 3 to 20 GPa. Theelastic modulus varied depending on porosity of the structures andmodulus decreased with the increasing pore fraction. This trend isintuitive as the samples become less stiff and more easily deformed atlower load due to the decrease of the effective area. Also, the LENS™process provides additional flexibility for designers to tailor themodulus of these porous implants without changing their bulk den-sity or total pore volume by tailoring the pore shape by changingdistance between two successive laser scans in each layer duringfabrication [2]. The average compressive strength of LENS™ processedporous CoCrMo alloy structures was between 948 to 1943 MPa.

In vitro biocompatibility of laser processed CoCrMo alloy wasevaluated using OPC1 cells for 14 days of culture and compared to thatof commercially pure (CP) Ti control sample to ensure that the sam-ples are non-toxic. MTT assay of laser processed CoCrMo alloy sampleshowed an optical density of 0.065±0.01 and on CP Ti control samplea density of 0.13±0.015 was observed. Higher optical density repre-sents higher cell density on the sample surface. All laser processedCoCrMo alloy samples showed no evidence of cell death. The highconcentration of cell density on Ti sample is due to its more biocom-patible surface, as the cell material interactions depend on materialchemistry, surface morphology, surface energy and wettability [26].These in vitro biocompatibility results confirm that CoCrMo alloysamples are non-toxic and retain their biocompatibility even afterlaser processing. Current results also indicate that LENS™ processingcan utilized to fabricate net shape implants with designed porosities,which can be extended to other metallic biomaterials.

5. Conclusions

Application of LENS™ to fabricate novel porous and unitizedstructures with functional gradation in composition and/or porosity

can potentially eliminate the long standing issues such as stress-shielding, poor interfacial bond between the host tissue and theimplant, and wear induced bone loss, in load bearing implants toincrease their in vivo life time. Porosities, pore characteristics andmechanical properties of laser processed structures can be tailored tosuite various biomedical applications by changing LENS™ processparameters. Moreover, LENS™ processing retains inherent biocom-patibility of feed stock materials. Under present experimentalconditions, the average total porosity of CoCrMo alloy samples withsolid wall and porous core can be varied in the range of 10% to 18%with a modulus between 33 to 43 GPa.

Acknowledgements

Authors would like to acknowledge the financial support from theOffice of Naval Research (grant no. N00014-1-05-0583) and theNational Science Foundation (grant no. CMMI 0728348). Authorswould also like to acknowledge W. M. Keck Foundation's financialsupport to establish the Biomedical Materials Research Lab at WSU.

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