scaffold development using selective laser sintering of polyetheretherketone–hydroxyapatite...

Biomaterials 24 (2003) 3115–3123

Scaffold development using selective laser sinteringof polyetheretherketone–hydroxyapatite biocomposite blends

K.H. Tana, C.K. Chuaa,*,1, K.F. Leonga, C.M. Cheaha, P. Cheangb,M.S. Abu Bakarb, S.W. Chac

aSchool of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, SingaporebSchool of Materials Engineering, Nanyang Technological University, Singapore 639798, SingaporecNational Institute of Education, Nanyang Technological University, Singapore 639798, Singapore

Received 27 November 2002; accepted 20 February 2003

Abstract

In tissue engineering (TE), temporary three-dimensional scaffolds are essential to guide cell proliferation and to maintain native

phenotypes in regenerating biologic tissues or organs. To create the scaffolds, rapid prototyping (RP) techniques are emerging as

fabrication techniques of choice as they are capable of overcoming many of the limitations encountered with conventional manual-

based fabrication processes. In this research, RP fabrication of solvent free porous polymeric and composite scaffolds was

investigated. Biomaterials such as polyetheretherketone (PEEK) and hydroxyapatite (HA) were experimentally processed on a

commercial selective laser sintering (SLS) RP system. The SLS technique is highly advantageous as it provides good user control

over the microstructures of created scaffolds by adjusting the SLS process parameters. Different weight percentage (wt%)

compositions of physically mixed PEEK/HA powder blends were sintered to assess their suitability for SLS processing.

Microstructural assessments of the scaffolds were conducted using electron microscopy. The results ascertained the potential of SLS-

fabricated TE scaffolds.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Tissue engineering; Scaffolds; Polyetheretherketone; Hydroxyapatite; Selective laser sintering

1. Introduction

Scaffold-guided tissue engineering (TE) has beendeveloped to regenerate specific and functional humantissues or organs [1–4]. As the scaffolds form theplatform for cells to develop and to be organised intotissues and organs. TE scaffolds should facilitate thecolonisation of cells and possess properties and char-acteristics that enhance cell attachment, proliferation,migration and expression of native phenotypes. Scaffoldcharacteristics and properties such as porosity, surfacearea to volume ratio, pore size, pore interconnectivity,structural strength, shape (or overall geometry) andbiocompatibility [5–9] are often considered to be critical

factors in their design and fabrication. The limitationsand difficulties encountered with conventional fabrica-tion techniques for producing scaffolds with pores thatare appropriately and consistently sized and thoroughlyinterconnected led to the research for alternativefabrication methods that are capable of providing theuser with some control over the formation of thescaffolds’ internal microstructure.

In order to facilitate the reorganisation of cellsthrough the secretion of the cell’s native matrix,reproducible biocompatible scaffolds with unique geo-metries are needed. Many natural, e.g., collagen andchitin, and synthetic biomaterials, e.g., poly(a-hydro-xyesters) and poly(anhydrides), [10–12], have beenwidely and successfully used as scaffolding materialsbecause of their good cell-tissue biocompatibility andprocessability [13]. However despite the suitability of thescaffolding materials [14], scaffolds produced to date arefar from ‘‘ideal’’ due to the limitations encounter withthe fabrication techniques [15,16].

*Corresponding author. Tel.: +65-6790-4897; fax: +65-6795-7329.

E-mail address: [email protected] (C.K. Chua).1Present address: 50 Nanyang Avenue, School of Mechanical and

Production Engineering, Nanyang Technological University, Singa-

pore 639798, Singapore.

0142-9612/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0142-9612(03)00131-5

1.1. Applications of RP techniques for tissue engineering

Rapid prototyping (RP) is a key group of prototypingtechnologies with the ability to rapidly fabricatecomplex three-dimensional (3-D) physical structures[17]. RP rely on the use of model data created in acomputer aided design (CAD) solid modelling environ-ment to generate 3-D physical objects. RP systemsprocess CAD data by mathematically slicing thecomputer model of the final desired object into thineven layers. The RP systems then utilise the slice data toreplicate or reconstruct a physical object through layer-by-layer manufacturing whereby solid material layersare fabricated, stacked vertically and bonded to theprevious layer to give rise to the physical object.

In scaffold production, conventional fabricationtechniques such as fiber bonding, gas foaming, emulsionfreeze drying, melt moulding, membrane lamination,solvent casting, etc. [13,18–21] have been developed andused for producing TE scaffolds which have been testedand applied with varying degree of success. Despite thewide variety of techniques available, conventionallyproduced scaffolds lack consistency and reproducibilityin structural and mechanical properties that aregenerally required in TE applications. In addition, theextensive use of organic solvents with most of thesetechniques are not desirable as residual traces left behindafter processing can result in adverse toxic effects invitro and elicit inflammatory responses in vivo [13]. Dueto such limitations, the potential in using RP istremendous as most RP techniques are able to fabricateintricate 3D reproducible structures without the use oforganic solvent. RP techniques that have been exploredor being developed for scaffold fabrication can beclassified under three different categories, namely, solid

[22,23], liquid [24,25] or powder-based [26,27] techni-ques depending on the physical form of the materialstocks or building materials utilised in these processes.The research presented in this paper explored theviability of using a powder-based RP technique,Selective Laser Sintering (SLS) for fabricating polymericand composite TE scaffolds with desirable macro- andmicro-structural characteristics.

1.2. Selective laser sintering

The SLS technique employs a carbon dioxide laserbeam to sinter thin layers of powdered polymericmaterials to form solid three-dimensional objects. Theobject is built layer-by-layer from CAD data files in theindustry-standard (STL file format). During SLSfabrication, the laser beam is selectively scanned overthe powder surface following the cross-sectional profilescarried by the slice data. The interaction of the laserbeam with the powder raises the powder temperature tothe point of melting and causes the particles to be fusedtogether to form a solid mass. Subsequent layers arebuilt directly on top of previously sintered layers withnew layers of powder being deposited via a roller on topof the previously sintered layer. Fig. 1 shows the processchain of the SLS technique.

As commercially available SLS modelling materialsused by SLS are non biocompatible in nature, SLSapplication in TE scaffold production remains limited.Lee et al. [28,29] looked into the development ofbioceramic scaffolds that can aid the regeneration ofhard tissues for recovery of bone defects and injuriesvia laser sintering of hydroxyapatite (HA) powdersthat were coated with a secondary polymeric binder,poly(methylmethacrylate) (PMMA). A slurry comprising

Fig. 1. Schematic layout of the SLS process.

K.H. Tan et al. / Biomaterials 24 (2003) 3115–31233116

of predetermined mixing ratios of ceramic particles andPMMA was sprayed dried to obtain the PMMA-coatedHA powders. In addition, diluted methanol was used inthe fabrication process. Since the use of organic solventsis highly undesirable for the processing of TE scaffoldsas mentioned, the research work presented in this papercircumvents the use of solvents by utilising purebiopolymer powders (polyetheretherketone, PEEK)and physically blended mixtures of PEEK and HApowders. The feasibility of sintering such powder blendsand the influence of SLS process parameters on thesintering quality and resulting microstructure of thesintered specimens were studied.

2. Materials and methods

2.1. Polyetheretherketone (PEEK) and hydroxyapatite

(HA) powders

PEEK exists as a semi-crystalline polymer at roomtemperature and possess pairs of ether linkages in itschain backbone. Its relatively high melting point,TmE343�C, and glass transition temperature,TgE143�C, makes it a suitable and stable polymer tobe processed at high temperatures. Its mechanicalproperties remain stable at high temperature of about200�C for prolonged periods of time [30]. PEEK exhibitsexcellent chemical resistance and is almost insoluble inmost solvents. An added advantage of PEEK inbiomedical application is that being radio opaque, theimplanted scaffolds allow for further investigation bymeans of X-ray. Furthermore, it can be readily sterilisedin a steam autoclave or by radiation without encounter-ing any significant degradation in its inherent properties.

PEEK powder with a specified average particle size of25 mm marketed under the brand name PeeKTM 150XF(Victrex Plc, Lancashire, UK) was used for the purposeof this research. PeeKTM 150XF is a low viscosity gradepolymer that is used mainly for powder coating and asfillers. The cytotoxicity of PEEK has been reportedelsewhere [31,32]. HA powders used in this research aresold under the brand name CAMCERAM II HA (CamImplants B.V., Netherlands). The HA powders meet theASTM F 1185-88 requirements and have a particle sizedistribution with at least 90wt% below 60 mm, asdetermined by Coulter Counter analysis. The averagematerial density is specified as 3.05 g/cm3.

2.2. Physical blending of PEEK and HA powders

Mixtures of PEEK/HA powders were produced byphysically blending pure PEEK and HA powders indifferent weight percentages using a roller-mixer. Inproducing the blends, PEEK is the base material andHA powders were added and dispersed into the PEEK

powders to achieve powder blends having 10, 20, 30 and40wt% HA contents. As PEEK is bioinert, the additionof HA particles will increase the bioactivity of theoverall implant. The prepared powder blends wereprocessed on a commercial SLS system, Sinterstation2500 (3D Systems Inc., Valencia, CA) [33], to producetest specimens. Except for changing the process para-meters on the operating software of the SLS system, nomodifications were made on the SLS system forprocessing the new materials. Since PEEK has a muchlower melting point compared to HA, it is possible toinduce sintering of PEEK at temperatures near Tg andto bind and partly expose the HA particles within thesintered PEEK matrix.

2.3. Design and fabrication of test specimens

The test specimens fabricated on the SLS weredesigned as a circular disc with a diameter and thicknessof 12.0 and 0.5mm, respectively, for microscopicexamination and biocompatibility assessments. Thediscs were designed so as to fit snugly into the wells ofa standard 24-well plate. The CAD model of the testspecimen was generated using a standard CAD soft-ware, ProENGINEER Ver. 2000i (Parametric Technol-ogy Corp., Needham, MA) and exported in the STL fileformat for uploading into the SLS system. Since thequality and the degree of sintering of the build aredependent on the SLS process parameters, a study wasconducted to determine the optimal SLS processingparameters for PEEK. The influence of three main SLSprocess parameters on the degree of sintering achievewith PEEK were investigated, namely, laser power, partbed temperature and scan speed. In the researchconducted by Leong et al. [34] and Nelson [35] on thelaser-sintering of polymer powders, these three men-tioned parameters were ascertained to be mainlyresponsible for the amount of laser irradiation per unitarea received by the powders (i.e., energy density orAndrew’s Number [35]) during fabrication on a SLSsystem. Since the energy density affects the degree ofsintering encountered by the exposed powders, speci-mens with different porosity can be obtained by varyingSLS process parameters.

The settings for the SLS process parameters werekept at their default values except for laser powerand part bed temperature, which was set at 8W and110�C, respectively. Pure PEEK was then subjected tolaser-sintering at a different laser power and tempera-ture settings to determine the optimal settings forthe sintering of PEEK. By varying the 2 processparameters, five groups of specimens comprising ofdifferent blends of PEEK and HA were fabricated. Therange of laser power and the part bed temperaturesettings for fabricating the various specimens aretabulated in Table 1.

K.H. Tan et al. / Biomaterials 24 (2003) 3115–3123 3117

2.4. Thermal analysis of PEEK

To assist in the proper selection of part bedtemperature, the Tg and Tm values of the PEEK powderstock have to be determined. Differential scanningcalorimetry (DSC) performed on a PerkinElmer Inc.,DSC-7, thermal analyser was employed for the measure-ments. Repeated samples of PEEK powder (averageweight=5.5mg) were scanned from 50�C to 350�C at alinear ramp rate of 10�C/min, using nitrogen as a purgegas.

2.5. Microscopic examination

Particle size analysis for both the PEEK and HApowders and microstructural characterisation of theSLS-fabricated specimens were carried out using ascanning electron microscope (SEM), JEOL JSM-5600LV, to characterise the individual powders and toanalyse the surface morphology and microstructure ofthe sintered specimens. All SLS-fabricated specimenswere examined under high vacuum conditions at 12 kVand magnifications of 500 times.

3. Results and discussion

3.1. Thermal properties of PEEK

DSC results for PEEK indicated an averaged Tg andTm of 143�C and 342�C, respectively, which were veryclose to the figures provided by the technical specifica-tions for the PeeKTM 150XF material (Section 2.1).

With the Tg value determined, the part bed temperature(which is the temperature by which the powders are pre-heated to before laser sintering) was set at an initial trialsetting of 110�C to prevent unwanted coagulation andhardening of the powders.

3.2. Microscopic examination of powder stocks

Microscopic examinations were carried out on theindividual powder materials prior to blending, afterblending and on the polymer blend produced by laser-sintering. The results of the examinations are presentedin the following sections.

3.2.1. HA and PEEK powder before blending

Figs. 2a and b are micrographs taken for the as-received PEEK and HA powders, respectively, prior toblending. As observed in these figures, the PEEKpowders are irregular in shape compared to the sphericalHA powders. This distinct morphological featuresbetween the two materials make the identification ofHA particles in the sintered PEEK matrix much simpler.

Table 1

Specimens groups for pure PEEK and PEEK/HA polymer blends

Laser Power (W) Temperature (�C)

110 140

9 HAPEEK 0100

10 HAPEEK 0100

12 HAPEEK 0100 HAPEEK 0100

14 HAPEEK 0100

16 HAPEEK 0100 HAPEEK 0100,

HAPEEK 1090,

HAPEEK 2080,

HAPEEK 3070,

HAPEEK 4060

18

20 HAPEEK 0100

22

24 HAPEEK 0100

26

28 HAPEEK 0100

HAPEEK 0100—Pure PEEK

HAPEEK 1090—10wt% HA-90wt% PEEK




Fig. 2. Micrographs taken for the as-received (a) PEEK and (b) HA

powders.


3.2.2. HA/PEEK powder blends before laser sintering

Figs. 3a–d presents the micrographs taken for the fourdifferent powder blends containing 10–40wt% HA,respectively, prior to laser sintering.

In Figs. 3a–d, HA particles can be observed in all thefour different powder blends in increasing amounts withincreased ratio of HA content. It is worthwhile to notethat irregardless of the different quantities of powderused as samples for analysis in SEM, the HA particleswere observed on all samples indicating that mixtureswith good dispersion and distribution of HA wereobtained.

3.2.3. Sintering of pure PEEK

Before subjecting the powder blends to laser sintering,some preliminary sintering tests were carried out onpure PEEK powders to determine the range of suitableprocessing parameters to be used on the SLS system.Different specimens were produced at various parametersettings for part bed temperature and laser power.Instead of building a disc specimen with a thickness of0.5mm as described earlier, only one layer of material(0.lmm thick) was sintered to test the parametersettings. For all the experiments conducted, the scan

speed of the laser system was kept at the default value of5080mm/s (200 in/s). Figs. 4a–d presents the micro-graphs taken of the microstructure of pure PEEKspecimens that sintered at a part bed temperature sett-ing of 110�C and at different laser power settings of10–16W.

Attempts to sinter the PEEK powder at a part bedtemperature of 110�C and laser power settings below10W were unsuccessful as the irradiated laser energywas too low to result in the proper sintering of PEEKpowder. As observed in the micrographs in Fig. 4, theformation of ‘‘necks’’ between powder particles becomesprominent in specimens fabricated at laser powersettings of at least 12W. However, due to the relativelylow part bed temperature used, delamination wasobserved between the layers on the sintered PEEKspecimens. Thus, in order to obtain better structuralintegrity, the use of a much higher part bed temperaturesetting was decided upon.

For subsequent sintering experiments, the part bedtemperature was raised to 130�C and the same set ofvalues for the laser power settings were repeated. Theresults observed for specimens fabricated with the newset of parameters were similar to those obtained for the

Fig. 3. Micrograph of PEEK/HA powder blend before sintering for different weight composition; (a) 10wt% HA, (b) 20wt% HA, (c) 30wt% HA,

and (d) 40wt% HA.


previous specimens that were fabricated at a part bedtemperature of 110�C. The part bed temperature wassubsequently raised to 140�C, which is 3�C below themeasured Tg for PEEK. A higher range of laser powersettings from 9W to 28W was also employed. Micro-graphs taken for the different specimens produced fromthe new set of parameters are presented in Figs. 5a–f.

Although specimens can be produced at a laser powersetting of 9W using the higher part bed temperature of140�C, the specimens obtained were fragile. Observa-tions made from the micrograph taken for a samplespecimen (Fig. 5a) showed poor quality of sintering withlittle ‘‘necking’’ observed between the particles. How-ever comparing the micrographs in Figs. 4 and 5, itcould be deduced that the sintering of PEEK is moresuccessful at higher part bed temperatures for each ofthe different laser power settings tested as evident fromthe quantity and prominence of neck formation betweenparticles.

When the laser power was increased to 12W andhigher at a part bed temperature setting of l40�C, thesintering results obtained appeared more promising asobserved from Figs. 5c–f. The degree of necking wasmore evident in specimens fabricated at part bedtemperature of 140�C when compared to 110�C.However, delamination (edges opening up) was notednear the edge for specimens built using laser powersettings lower than 16W. Fig. 6 is a micrograph of apure PEEK specimen built using 15W at 140�C showing

signs of delamination. As delamination occurs whenadjacent layers of material are not properly bonded toone another, the laser power was further increasedgradually till 28W to eliminate the problem. It wasobserved that the degree of melting of PEEK was moreevident at higher laser power.

Specimens produced at laser power settings of 28Wappeared charred and may have degraded and are thusnot acceptable. As illustrated in Fig. 5e, the use of laserpower settings of 22W and above would result in theformation of microstructures with highly dense mor-phology. However, to allow the in growth of seeded cell,it is of interest to the authors to create porousspecimens. As such, the most favourable SLS processparameter settings for building PEEK scaffolds shouldbe 140�C for the part bed temperature and between16W to 21W for the laser power setting.

3.2.4. HA/PEEK composite obtained from laser sintering

Using the parameters determined previously, thindiscs of the polymer blend comprising of PEEK and HApowders were laser-sintered for biocompatibility assess-ments and to be used as a comparison with pure PEEK.All specimens were fabricated at a part bed temperatureand laser power setting of 140�C and 16W, respectively.Promising results were obtained as for the sinteringexperiments as illustrated by Figs. 7a–d, in which HAparticles are circled. As noted from these figures, the HA

Fig. 4. Micrographs of pure PEEK specimens at a part bed temperature of 110�C and laser power settings of (a) 10W, (b) 12W, (c) 14W, and

(d)16W.


particles can be seen embedded and partially exposed inthe PEEK matrix.

The successful sintering of the test specimens indicatesthe potential of producing PEEK scaffolds on the SLS.In addition, the successful incorporation of HA into thepolymer matrix will enhance the bioactivity of thespecimens. However, it is noted that with a decrease inthe percentage of PEEK in the composition of thepowder blend, the fabricated disc specimens were fragileto handle and this fragility made it not practical to use inlaser-sintering. In order to produce a structure withgood integrity, it was proposed that the composition ofHA in the mixture should be kept at 40wt% HA andhence no further increase in the composition of HA inthe PEEK/HA mixture was carried out.

Fig. 5. Micrograph of sintered pure PEEK specimens produced at a part bed temperature of 140�C and laser power settings of (a) 9W, (b) 12W,

(c) 16W, (d) 20W, (e) 24W, and (f) 28W.

Fig. 6. Micrograph of sintered pure PEEK taken at the edge showing

delamination.


4. Conclusion

The advantages of using SLS for TE scaffolds lie inthe ability to control pore structure for biogenesisthrough the control of polymer content and the abilityto construct complex three-dimensional structure for TEapplications. Due to its ability to process variousmaterials, the potential of using SLS is further enhancedas unconventional materials such as PEEK, which wasused in this project, was tested to establish the feasibilityof its usage for TE scaffolds. Three main parameters ofSLS, namely the part bed temperature, laser power andscan speed were investigated to study its effect on theintegrity of the test specimens fabricated for the purposeof biocompatibility of the materials used. With the scanspeed kept constant and varying part bed temperatureand laser power, several circular specimens measuring12mm in diameter and 0.5mm thick were fabricated.The results obtained showed that a low part bedtemperature should be complemented by a higher laserpower. The research has also shown promising resultof being able to laser sinter a high melting point polymerin a much lower temperature environment. Further-more, the ability to incorporate different amounts of a

bioactive material, hydroxyapatite, into the polymerblend reiterated its viability for use in TE scaffolds,especially bone scaffolds as apatite is one of thenaturally occurring components in human bones.

Acknowledgements

The authors would like to acknowledge the financialsupport of this project (ARC 18/97) from the Agency forScience, Technology and Research (A-STAR), Singa-pore.

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