Formation of Inter-linked voids in Bioactive Glass 45S5 upon Crystallization

Satadru Kashyap*

Department of Mechanical Engineering, Tezpur University, Napaam, Sonitpur, Assam - 784 028, India.

ABSTRACT:

One of the most common therapeutic surface reactive bioresorbable implant material used nowadays is bioactive glass 45S5. However, being amorphous in nature, its usage in load bearing applications is ineffective due to its poor mechanical properties. A novel idea of toughening bioactive glass 45S5 would be to produce it as a glass ceramic with an optimum dispersion of a crystalline phase in the amorphous matrix so that crack deflection at and around crystals occur which would toughen bioactive glass ceramic by increasing the crack length. Partially crystallized bioactive glass therefore is a prime contender in the fabrication of a glass ceramic with improved fracture resistance. However, during crystallization it was observed that flowery patterns were produced inside the samples which upon analysis revealed to be inter-linked voids formed at the grain boundaries. These voids were formed as a result of shrinkage upon crystallization and may play influential part in the crack propagation behaviour in crystallized bioactive glass.

KEYWORDS: Crystallization; Bioactive glass; inter-linked voids; toughening

Copy the following to cite this article: Kashyap S. Formation of Inter-linked voids in Bioactive Glass 45S5 upon Crystallization. Mat.Sci.Res.India;10(1)

Copy the following to cite this URL: Kashyap S. Formation of Inter-linked voids in Bioactive Glass 45S5 upon Crystallization. Mat.Sci.Res.India;10(1). Available from: http://www.materialsciencejournal.org/?p=292

Introduction

Any novel material designed and fabricated to be used as a therapeutic bioimplant in order to replace damaged or diseased tissues or bones must fulfil certain criteria related to implant-tissue interaction in a living body. The necessary criteria for a successful bioimplant material must be high osteoconductivity, ability to promote regeneration of new tissue, compatibility with the host body, good biodegradability or bioresorbability, effective commercial sustainability and good mechanical properties.^1-3

Since its inception in the late 1960s, one of the most frequently used bioactive glass is Bioglass® 45S5-a surface reactive, bioresorbable, osteoconductive glass; which has been tried and tested over the years in its usage as a structural material for biomedical implants such as repair of orthopaedic cavitary (hip) defects, cochlear implants, spinal surgery, replacement of periodontal defects and reconstruction of maxillo-facial defects.⁴ In spite of fulfilling all other criteria, Bioglass® 45S5 has poor mechanical properties on account of its amorphous structure which restricts its effective usage in biomedical applications as a successful implant material.^5-8 Application of fundamental knowledge in developing assays for bioactive glass fabrication and characterization requires to be formulated in order to channel this gap towards an improved product with better mechanical properties. Resorbable implant materials endure some form of stress during manufacture or whilst in service. In the past, metallic alloys and ceramic composites have been used but they have issues such as toxicity and stress shielding (elastic modulus mismatch).⁹ A huge hindrance in expanding the application of bioactive glasses as implant materials has been their mechanical properties which are below par in terms of fracture toughness, strength, and impact resistance; thus rendering them unsuitable for life long service.¹⁰ Table 1 provides comparative mechanical properties of bioactive glass 45S5 and soda lime glass which illustrates the fragility of bioactive glass 45S5 making it inappropriate implant material in loading bearing conditions.^11-14

Various techniques have been put forward to toughen bioactive glass ceramics by introducing composites in the form of particles,^15-20 particle composites,²¹ glass ceramic composites²² and coatings.^23-25 Earlier, “Glass ceramics” were developed by special heat treatment of particular glass compositions to present a uniform fine dispersion of crystals in glassy phase matrix, which gave them a cutting edge over the conventional “glass” in terms of properties such as higher strength and impact resistance, low coefficient of thermal expansion, low dielectric loss and a range of optical properties from clear to opaque or white.^26-27

Hence, another thought of improving the stress carrying capability in bioactive glass ceramics would be phase transformation of amorphous structure into partially crystalline bioactive glass such that instead of straight crack propagation (usually observed in amorphous glass structure); the crystalline phases distributed in an amorphous matrix will actually deflect the crack path around them thereby increasing the crack path. This will lead to an increase in the fracture toughness. Chen et al. revealed the ability of mechanically competent crystalline sintered bioactive glass 45S5 powder to form biodegradable and bioactive calcium phosphate upon immersion in simulated body fluid²⁸ while Kashyap et al.,²⁹ deduced the formation of a biomineralized layer on partially crystallized bioactive glass 45S5 upon immersion in phosphate buffer solution. Evidently with the proof that crystalline bioactive glass 45S5 does not totally compromise the inherent bioactivity of the resorbable implant material, it is intended to increase the resistive capability of bioactive 45S5 glass ceramic to crack propagation by inducing crystallinity in them.

Kashyap et al.²⁹ previously studied the crystallization kinetics of bioactive glass 45S5 by heat treating them at 680°C for various time periods (0 to 60 minutes) so as to obtain a gradual transition from amorphous to a fully crystalline structure. It was observed that along with ‘devitrification’ occurring due to crystallization, certain flowery patterns were also produced inside the samples. Moreover, Kashyap³⁰ also reported an increase in the thermal shock and fracture resistance in partially crystallized bioactive glass 45S5 as compared to its amorphous counterpart. A bioactive glass sample heated to 680°C for 30 min (~ 68% crystallinity) generated only cracks in it upon quenching in water from that temperature while the amorphous sample shattered to pieces upon quenching from 680°C. SEM analysis of fracture surfaces in a partially crystallized sample revealed that although the sample fractured along the cracked planes, each cracked plane was blanketed with numerous small cleavage planes (mist). The numerous small cleavage planes seemed to change direction after every 30-40 microns. This suggested higher fracture resistance in partially crystalline sample as more energy was consumed for crack propagation when compared with an amorphous sample with an almost flat fracture surface showing no hindrance in crack propagation.³⁰

Materials And Methods

Testing of Samples

Dye penetrant test

A partially crystallized bioactive glass 45S5 sample was sectioned along the cross section and subsequently polished. A black dye was spread over the polished cross sectional surface of the sample. Following that, the sample was placed on a vacuum plate and covered by an inverted vacuum jar. A gasket sealed the gap between the lip of the jar and the vacuum plate. The whole system inside the jar was brought to a vacuum by using a mechanical pump (Fisher Technical Company, LAV-3). Vacuum was maintained for about 10 minutes following which the sample was taken out and the dye/ink on the cross sectional surface was wiped clean with a tissue paper. Macrographs and optical micrographs were taken with the same equipment as mentioned above for recording the Dye Penetrant Test (DPT) results.

Results And Discussions

Amorphous bioactive glass 45S5

A macrograph of an as received amorphous bioactive glass 45S5 disc is shown in Figure 1A which indicates a completely transparent sample. Moreover, an optical micrograph of an etched amorphous sample (Figure 1B) produces a rough surface due to etching of impurities or discontinuities on the surface. Images were obtained after the samples were treated as per the procedure mentioned by Kashyap et al.

Crystallization of bioactive glass 45S5 at 680°C

Partially crystallized sample Macrographs and optical micrographs of bioactive glass 45S5 sample crystallized at 680°C for 30 min is shown in Figure 2.²⁹ Crystalline phase growing in an amorphous matrix was observed which indicated ‘devitrification’ occurring in these samples. Moreover, as evident from the optical micrographs in Figure 2B, growth of crystals continued with increasing crystallization times until they impinged on one another. Another observation made was the emergence of ‘flowery patterns’ inside in the partially crystallized sample as is evident from the macrograph in Figure 2A. Images were obtained after the samples were treated as per the procedure mentioned by Kashyap et al.,²⁹.

Formation of ‘flowery patterns’ during crystallization

After crystallization heat treatment it was observed that apart, from changes in the optical properties due to devitrification (transparent to translucent), optical properties were also changed at random locations in the samples due to the formation of “flowery patterns” inside the glass ceramic which contributed to lower transparency in these locations by greater light scattering. Figure 3 shows an example of these flowery patterns formed in a bioactive glass 45S5 sample partially crystallized at 680°C for 30 minutes under reflected (Figure 3A) and transmitted lighting conditions (Figure 3B). 

During and after polishing it was found that the sample surface contained pit like structures which were difficult to grind/polish off and seemed to be deep as is evident from the optical micrographs in Figure 4 which were taken slightly over focused in order to highlight the pits and some bright parts on the subsurface around the pits.

Click on image to enlarge

 

Click on image to enlarge

SEM Analysis

Secondary electron micrographs of these pit like structures in unetched partially crystallized sample (Figure 5A) indicated that these might be interlinked voids formed in the sample since the morphology of these pit like structures showed elongated pits and even deeper holes present at the bottom of the pit as if it was interlinked with  another pit through this hole. This became more prominent in the SEM image of the pit like structures in an etched sample (Figure 5B) as it showed the pits forming at the edges of grains indicating that these depressions on the sample surface could be inter linked voids. Such voids may have formed due to shrinkage in the sample during crystallization. Due to bulk nature of the sample which hinders easy deformation, and nucleation/growth of a cluster of grains at isolated random locations, these voids were formed as a result of crystals forming and causing shrinkage along its peripheral areas, thus forming interlinked voids along the grain boundaries.

 

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