Calcium Cl/OH-apatite, Cl/OH-apatite/Al2O3 and Ca3(PO4)2 fibre nonwovens: Potential ceramic components for osteosynthesis

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Abstract

Polycrystalline calcium phosphate ((Cl/OH)Ap = Ca5(PO4)3(OH/Cl); TCP = Ca3(PO4)2) fibres were prepared from aqueous solutions of calcium chloride and phosphoric acid using poly(ethylene oxide) (PEO) as spinning aid. Generation of nonwoven materials was accomplished via rotary jet spinning. Polycrystalline (Cl/OH)Ap fibres 10–25 μm in diameter were obtained with 37% ceramic yield by pyrolysis of the green fibres followed by sintering at 1150 °C in air. X-ray diffraction (XRD) analysis provided evidence for apatite formation starting at 650 °C while (Cl/OH)Ap ceramic fibres were obtained at 1100 °C via transformation through intermediate dicalcium dichloride hydrogen phosphate (Ca2Cl2(HPO4)) and calcium pyrophosphate (Ca2P2O7) phases. A glass-forming Al-based additive was applied to enhance the mechanical properties of the Cl/OH)Ap ceramic fibres and indeed resulted in the formation of (Cl/OH)Ap/Al2O3 fibres with improved mechanical stability. Finally, TCP, (Cl/OH)Ap and (Cl/OH)Ap/Al2O3 fibres were subjected to seeding with mesenchymal stem cells. Negligible cytotoxicity is observed.

Introduction

Calcium phosphate (CaO/P2O5/(H2O), CaP) ceramics show excellent behaviour in terms of biocompatibility, bioactivity and controllable rate of degradation/bone formation as artificial materials for tissue engineering in osteosynthesis. Their mechanical properties, nearly bone-like thermal capacity and controllable porosity complete the application profile of CaP ceramics for bone reconstruction. Calcium orthophosphate implant materials are presently available and used in tailored defect-dependent forms and material shapes, mainly in form of 3D open porous moulded bodies, particulates, cements, coatings and as injectable composites or self-setting cements.1, 2, 3 Despite intensive efforts, it has not yet been possible to develop an artificial CaP implant exhibiting a reversible flexibility while maintaining spongiosa-like structures with a trabeculae scaffold-like natural occurring bone tissue. Such a material would exhibit great potential for occupying bone defects in conjunction with bone cements of higher biodegradation rate or as reinforcing component of CMC-mouldings (CMC = ceramic matrix composites) for osteosynthesis.

One promising approach entails the generation of (Cl/OH)Ap ceramic fibres and their application as open porous, nonwoven materials with reversible deformation and flexible shaping. Such ceramic nonwoven materials would exhibit bio-compatible, bone analogous chemical composition and a trabeculae form as well as open porosity and reversible flexibility. A nonwoven ceramic fibre is preferred over a woven one since three-dimensional structures can be directly prepared in form of a nonwoven without the need for an additional processing of a single filament ceramic fibre. Up to now only few attempts have been made to develop suitable spinning dopes and fibre spinning processes leading directly to a nonwoven material composed of compact CaP ceramic fibres. Chemically, the majority of CaP bioceramics is based on β-tricalcium phosphate (β-TCP) and hydroxyl apatite ((OH)Ap); their preparation techniques are as manifold as their fields of application.4, 5, 6, 7 Fibrous or needle-like CaP solids are accessible through many general routes of synthesis, e.g., solid state synthesis of apatite with calcium metaphosphate and calcium oxide as well as calcium hydroxide or calcium carbonate as shown by Ota et al.8 Hideo and co-inventors claimed a fibre production process for CaP ceramic fibres through melt-spinning at temperatures above 1600 °C.9 However, due to melt preparation, the apatite phases lack any hydroxyl substitution in the apatite crystal structure and consequently any biological activity.10 Joo et al. developed a water based sol–gel system for green fibre fabrication via electrospinning leading to (OH)Ap nanofibre matrices consisting of hollow, compact or core–shell structured CaP/polymer or CaP fibres. The thermal conversion of these materials led to CaP ceramic fibre mats. Despite their excellent properties as an implant material, the spinning dopes are limited in terms of ageing and the process of electrospinning as such is restricted to small diameter fibres. Unfortunately, inorganic fibres with such small diameters are still medically monitored in terms of carcinogenic properties. Fuji and co-inventors described a slurry-based process to generate cotton-like materials via a cast-process.11 These most promising materials, however, lack structural homogeneity after sintering to ceramic CaP fibres. Another attempt to fabricate suitable fibrous materials for bone tissue engineering was carried out by Blenk.12 Coating of polyester fibres with a CaP-slurry led to hollow TCP fibres after pyrolysis and sintering. Despite all attempts to fabricate nonwoven CaP ceramic fibre materials an overall straightforward, cost-efficient way of fabrication such ceramic nonwoven implants is currently not at hand.

The aim of the present work was both to develop spinning dopes with defined properties in terms of rheology and stability and to reveal an expedient way for the direct fabrication of green-fibres (unburned precursor fibres) and their thermal conversion into nonwoven (Cl/OH)Ap ceramic fibres. Finally, preliminary cell toxicological experiments were to be carried out.

Section snippets

Spinning dope

The CaP precursor spinning dope was prepared by using calcium chloride dihydrate (CaCl2·2H2O) and phosphoric acid (H3PO4, 85 wt.%) as raw materials. To obtain a solution suitable for fibre spinning, poly(ethylene oxide) (PEO, Sigma Aldrich, >99%) was used as spinning aid and dissolved in a mixture of 60 vol.% double distilled H2O and 40 vol.% ethanol (EtOH, Th. Geyer, p.a.). The (Cl/OH)Ap precursor solution with a Ca:P molar ratio of 5:3 was prepared by dissolving CaCl2·2H2O (Sigma Aldrich, 99 

Crystallization

XRD analyses were performed on pulverized fibre samples, which were individually heated to a maximum temperature of 1300 °C. In the spinning dope, the CaP precursors were homogeneously distributed in a molecularly disperse spinning solution and immediately crystallized after fibre formation and during fibre drying (EDX of green-fibres: see Supporting Information, Fig. A.2). The inorganic precursors built intermediate crystalline phases at low temperatures (100 °C) and no formation of an amorphous

Discussion and conclusions

Polycrystalline CaP fibres became accessible from aqueous solutions of calcium chloride and phosphoric acid by a solution process. Beginning CaP phase formation in the fibres was found to occur at 650 °C for (Cl/OH)Ap after the recrystallization of CaCl2·2H2O and formation of intermediate, soluble CaP phases, i.e. CaCl2·Ca(H2PO4)2·2H2O, and CaClH2PO4·H2O, respectively. Non-stoichiometric single phase (Cl/OH)Ap was found to exist at T = 1150 °C with an overall ceramic yield of 36% at the point of

Acknowledgement

Excellent technical assistance of Helga Bach is gratefully acknowledged.

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