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  • 04) 4

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    form hydroxyapatite in the bone cavity [47]. Thesemoldable calcium phosphate cements avoid the problem

    patite was the only product formed in the set CPC [4,10].Due to its excellent osteoconductivity, biocompatibility

    ARTICLE INof sintered hydroxyapatite implants that require thesurgeon to t the surgical site around the implant or tocarve the implant to the desired shape [8]. One calciumphosphate cement, designated as CPC [4], consists of a

    and bone-bonding ability, CPC has been used in anumber of orthopaedic and dental procedures [1116].Unfortunately, the relatively low strength and suscept-ibility to brittle fracture of CPC have severely limited itsuse to non-load-bearing applications [1115].Previous studies have used hydroxyapatite particles

    and other bioactive materials as llers in polymer-based*Corresponding author. Ofcial contribution of the NationalInstitute of Stan

    the United State

    E-mail addres

    0142-9612/$ - see

    doi:10.1016/j.bioPublished by Elsevier Ltd.

    Keywords: Bioactive composite; Nano-silica whisker fusion; Calcium phosphate cement; Strength; Elastic modulus; Bone repair

    1. Introduction

    Hydroxyapatite has been used for hard tissue repairbecause of its chemical and crystallographic similarity tothe carbonated apatite in human teeth and bones [13].Several calcium phosphate cements can self-harden to

    mixture of tetracalcium phosphate [TTCP:Ca4(PO4)2O]and dicalcium phosphate anhydrous (DCPA: CaHPO4).CPC sets in an aqueous environment to form a solidconsisting of nano-sized hydroxyapatite crystals with adiameter of approximately 50 nm and a length of about300 nm [4,9] X-ray diffraction showed that hydroxya-bNational Institute of Standards and Technology, Gaithersburg, MD 20899, USA

    Received 16 September 2003; accepted 2 December 2003


    Self-hardening calcium phosphate cement (CPC) sets to form hydroxyapatite with high osteoconductivity, but its brittleness and

    low strength limit its use to only non-stress bearing locations. Previous studies developed bioactive composites containing

    hydroxyapatite llers in Bis-GMA-based composites for bone repair applications, and they possessed higher strength values.

    However, these strengths were still lower than the strength of cortical bone. The aim of this study was to develop strong and

    bioactive composites by combining CPC llers with nano-silica-fused whiskers in a resin matrix, and to characterize the mechanical

    properties and cell response. Silica particles were fused to silicon carbide whiskers to roughen the whisker surfaces for enhanced

    retention in the matrix. Mass ratios of whisker:CPC of 1:2, 1:1 and 2:1 were incorporated into a Bis-GMA-based resin and hardened

    by two-part chemical curing. Composite with only CPC llers without whiskers served as a control. The specimens were tested using

    three-point exure and nano-indentation. Composites with whisker:CPC ratios of 2:1 and 1:1 had exural strengths (mean7SD;n=9) of (164714) MPa and (139722) MPa, respectively, nearly 3 times higher than (5475) MPa of the control containing onlyCPC llers (po0.05). The strength of the new whisker-CPC composites was 3 times higher than the strength achieved in previousstudies for conventional bioactive composites containing hydroxyapatite particles in Bis-GMA-based resins. The mechanical

    properties of the CPC-whisker composites nearly matched those of cortical bone and trabecular bone. Osteoblast-like cell adhesion,

    proliferation and viability were equivalent on the non-whisker control containing only CPC llers, on the whisker composite at

    whisker:CPC of 1:1, and on the tissue culture polystyrene control, suggesting that the new CPC-whisker composite was non-aNational Institute of Standards and Technology, Paffenbarger Research Center, American Dental Association Foundation, Gaithersburg,

    MD 20899-8546, USABiomaterials 25 (20

    Strong and bioactive composiwhiskers fo

    Hockin H.K. Xua,*, Dougldards and Technology; not subject of copyright in

    s. Tel.: +1-301-975-6804; fax: +1-301-963-9143.

    s: hockin.xu@nist.gov (H.H.K. Xu).

    front matter Published by Elsevier Ltd.


    containing nano-silica-fusedone repair

    . Smithb, Carl G. Simonb

    PRESSbone cements to improve the mechanical properties[1721]. Bis-GMA (bisphenol-a-glycidyl methacrylate)

  • ceramic whiskers and used as llers to reinforce Bis-GMA-based dental resin to extend their use to large

    INteriastress-bearing restorations [23,34]. The whiskers pos-sessed a high structural perfection resulting in superiorstrength values. While plain whiskers did not signicantlystrengthen the resin matrix, nano-silica fusion roughenedthe whisker surfaces and enhanced whisker retention inthe matrix, resulting in strong composites with substan-tially increased strength and toughness [23,34].In the present study, strong and bioactive composites

    were developed via the combination of pre-hardenedbioactive CPC particles and nano-silica-fused whiskersin a Bis-GMA resin. Flexural and nano-indentation testswere used to characterize the mechanical properties ofthe composites. Because cell culture toxicity assays arethe international standard for the initial screening ofmaterials for biocompatibility [35], in-vitro cell culturewas performed to evaluate the biocompatibility of thecomposites. It was hypothesized that combining bioac-tive CPC llers and whisker llers would render thecomposite signicantly stronger than the compositecontaining only CPC llers. It was further hypothesizedthat the composite containing CPC and whiskers wouldbe as non-cytotoxic as the composite containing onlyCPC llers.

    2. Methods

    2.1. Calcium phosphate cement (CPC) fillers

    The powder used to make the CPC specimensconsisted of a mixture of TTCP and DCPA with molarratio of 1:1 [4]. The TTCP powder was synthesized froma solid-state reaction between CaHPO4 and CaCO3(Baker Chemical Company, NJ), then ground andsieved to obtain TTCP particle sizes of 180 mm, witha median particle size of 17 mm. The DCPA powder wasground and sieved to obtain particle sizes of 0.43 mm,resin composites have been used as dental restorativematerials [22,23]. Previous studies have added bioactivellers into the resin matrix and imparted bioactivity toBis-GMA-based composites for bone repair applica-tions [2427]. Studies have shown that Bis-GMA-basedresins containing hydroxyapatite particles possessedexural strength values ranging from about 40 to60MPa [2830]. These exural strength values, whilebeing 46 times higher than that for pure CPC [31], werestill lower than the 100200MPa reported for corticalbone [32,33].Nano-sized silica particles were recently fused onto

    ARTICLEH.H.K. Xu et al. / Bioma4616with a median particle size of 1 mm. The TTCP andDCPA powders were mixed in a blender (DynamicsCorporation of America, New Hartford, CT) to formthe CPC powder, which was then blended manually byspatulation with distilled water to form a paste at apowder:water mass ratio of 3:1. The paste was placedinto a 3 4 25mm mold sandwiched between twoglass slides [9]. The assembly was incubated in ahumidor at 100% relative humidity at 37C for 4 h[36]. The hardened CPC was then demolded andimmersed in distilled water at 37C for 20 h. Thisresulted in hydroxyapatite being the only nal reactionproduct of CPC [10]. The set CPC was manually groundby mortar and pestle into a powder having particle sizesranging from about 2 to 15 mm with a mean ofapproximately 5 mm [31]. This powder, designated asCPC llers, was silanized by mixing it with 4% massfraction of 3-methacryloxypropyltrimethoxysilane and2% mass fraction of n-propylamine in cyclohexaneusing a rotary evaporator in a 90C water bath until dry[34,31]. The purpose of the silanization was to bond thellers to the resin matrix [23]. Previous studies showedthat silanization did not suppress the bioactivity of thehydroxyapatite llers [17].

    2.2. Nano-silica-fused whiskers

    Silicon carbide whiskers (Advanced Refractory Tech-nologies, Buffalo, NY) were used with diametersranging from 0.1 to 3 mm with a mean of approximately0.9 mm, and lengths ranging from about 2 to 100 mmwith a mean of 14 mm. The whiskers were mixed withnano-sized silica having a particle size of approximately80 nm (Aerosil OX50, Degussa, Ridgeeld, NJ) at awhisker:silica mass ratio of 5:1 in ethyl alcohol stirredwith a magnetic bar on a hot plate until dry. Thepurpose of this mixing was to disperse and separate thewhiskers to prevent entanglement. The dried mixturewas then heated in a furnace in air for 30min at 800C[23,34]. The purpose of the heating was to fuse the nano-silica particles onto the whiskers to facilitate silanizationand enhance the whisker retention in the matrix byproviding rougher whisker surfaces [23,34]. The nano-silica-fused whiskers were silanized in the same manneras for the CPC llers. The silanized nano-silica-fusedwhiskers are hereinafter referred to as whiskers.

    2.3. Specimen fabrication

    To study the effects of whisker:CPC ratio, thewhiskers were mixed with CPC llers at whisker:CPCller mass ratio of 1:2, 1:1, and 2:1. Each ller powderwas blended by spatulation with a resin monomerconsisting of mass fractions of 48.975% Bis-GMA,48.975% TEGDMA (triethylene glycol dimethacrylate),0.05% 2,6-di-tert-butyl-4-methylphenol (BHT), and 2%

    PRESSls 25 (2004) 46154626benzoyl peroxide (BPO) to form paste one, the initiatorpaste, of a two-part chemically activated composite[23,34]. The ller level, i.e. (whisker+CPC)/(whis-ker+CPC+resin), was xed as 60% mass fraction forall the composites [23]. Paste two, the accelerator paste,

  • INteriaconsisted of the same amount of powder mixed with aresin made of mass fractions of 49.5