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Monitoring of hardening and hygroscopic induced strains in a calcium phosphate bone cement using FBG sensor
journal of the mechanical behavior of biomedical materials 60 (2016) 195–202
Available online at www.sciencedirect.com
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Research paper
Monitoring of hardening and hygroscopic induced strains in a calcium phosphate bone cement using FBG sensor A. Bimisa, D. Karalekasa,n, N. Bouropoulosb, D. Mouzakisc, S. Zaoutsosc a
Laboratory of Advanced Manufacturing Technologies and Testing, University of Piraeus, 18534 Piraeus, Greece Department of Materials Science, University of Patras, 26504 Patras, Greece c Department of Mechanical Engineering, Technological Educational Institute of Thessaly, 41110 Larissa, Greece b
art i cle i nfo
ab st rac t
Article history:
This study initially deals with the investigation of the induced strains during hardening
Received 5 October 2015
stage of a self-setting calcium phosphate bone cement using fiber-Bragg grating (FBG)
Received in revised form
optical sensors. A complementary Scanning Electron Microscopy (SEM) investigation was
23 December 2015
also conducted at different time intervals of the hardening period and its findings were
Accepted 25 December 2015
related to the FBG recordings. From the obtained results, it is demonstrated that the FBG
Available online 6 January 2016
response is affected by the microstructural changes taking place when the bone cement is
Keywords:
immersed into the hardening liquid media. Subsequently, the FBG sensor was used to
Calcium phosphate cement
monitor the absorption process and hygroscopic response of the hardened and dried
Fiber Bragg grating sensor
biocement when exposed to a liquid/humid environment. From the FBG-based calculated
Hygroscopic strains
hygric strains as a function of moisture concentration, the coefficient of moisture
Coefficient of moisture expansion
expansion (CME) of the examined bone cement was obtained, exhibiting two distinct linear regions. & 2016 Elsevier Ltd. All rights reserved.
1.
Introduction
polymethyl methacrylate cement. However, PMMA cement has several disadvantages such as generation of heat during
In surgical applications, such as bone grafting, bone cement
curing that causes necrosis of the surrounding tissue (Zhang
is used as a coating between the bone and the implantin
et al., 2014), toxicity of the monomer, embrittlement due to
order to ensure proper fixation. Polymethylmethacrylate (PMMA) bone cements are mostly used in such applications
aging and low biocompatibility at the cement–bone interface. Biocements formed from calcium/phosphate (Calcium
and consist of two primary components: (i) a powder con-
Phosphate Cements-CPCs) are produced by mixing water or
sisting of copolymers based on the substance polymethyl
aqueous phosphate solution with a solid calcium phosphate
methacrylate and (ii) a liquid monomer, methylmethacrylate
phase, resulting in a malleable paste (Heinemann et al., 2013).
(MMA). These two components are mixed and form a
They have been attracting great attention as an alternate
n
Corresponding author. Tel.: þ30 210 4142319. E-mail address: [email protected] (D. Karalekas).
http://dx.doi.org/10.1016/j.jmbbm.2015.12.041 1751-6161/& 2016 Elsevier Ltd. All rights reserved.
196
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
solution because they offer advantages such as high biocompatibility, bioactivity, osteoconduction and osteogenesis (Andriotis et al., 2010). These characteristics allow their use as a bioactive fixation onto metallic implants or to replace missing or damaged bones in several occasions of bone defects as well as they can also act as drug eluting carriers such as antibiotics, anti-inflammatory etc. due to their intrinsic porosity (Zhang et al., 2014; Bose and Tarafder, 2012). On the other hand, several limitations, such as poor mechanical properties and their inherent brittleness, have to be addressed as they limit their use to low load bearing places (Zhang et al., 2014) and the necessary clinical requirements are not met. In the case of self-setting CPCs, the hardening process takes place in an isotonic solution that acts as hardening liquid (Barinov and Komlev, 2011) and it has been proven that during this stage microstructural changes are occurred (Ginebra et al., 1997). Moreover, it has been demonstrated that these changes improve bone cements' mechanical behavior (Ginebra et al., 1997; Bimis and Karalekas, 2015), since the mechanical properties of a material are determined by its microstructure (Zhang et al., 2014). It is also reported in the literature that a more thorough investigation of the CPCs kinetics is imperative if to be used as drug delivery materials (Ginebra et al., 2012). FBG sensors exhibit several advantages such as fast response, high sensitivity, signal integrity, long-term stability, immunity against electromagnetic radiation, insensitivity to radio frequency interference and good corrosion resistance. In terms of performances, they present multifunctionality (allow measurement of parameters including strain, temperature, pressure, load, acceleration, vibration, etc.) and are easily multiplexed in a serial manner along a single optical fiber (Hill and Meltz, 1997; Morey et al., 1992). In addition, their light weight and small dimensions make them possible to be effectively embedded within host materials. For that reason, fiber optic sensors are considered one of the most recent and promising methods for monitoring of process-induced residual strains. Many works involving FBG sensors for investigating fabrication induced shrinkage strains and hygrothermal strains in polymeric and composite materials have been reported (Sun et al., 2014; Lai et al., 2010; Karalekas et al., 2009; Lai et al., 2012; Ramezani-Dana et al., 2014). In recent years, fiber Bragg grating sensors have been used in biomedical applications and to study polymerization shrinkage and water sorption of different biomaterials (Carvalho et al., 2006; Anttila et al., 2008; Schizas and Karalekas, 2011; Al-Fakih et al., 2012; Roriz et al., 2014). In a previous work by Bimis et al. (Bimis and Karalekas, 2015), an optical sensor was embedded in a self-setting calcium phosphate bone cement cylindrical specimen to investigate if the hardening induced strains could be recorded from the FBG response. According to the reported results a noticeable change in measured strains may be due to microstructural changes occurring during immersion in the hardening liquid. However, no clear change patterns were observed leading to the conclusion that future investigations should be carried out in a better controlled experimental conditions accompanied by SEM and XRD based examination. In the present study, a more extensive and thorough investigation was carried out. The magnitude of the developing
strains was initially investigated during the hardening stage of the examined biocement and then, after drying, when exposed into a liquid/humid environment. The hardening induced strains derived from the Bragg wavelength evolutions were combined with microstructural observations from Scanning Electron Microscopy (SEM) imaging performed to observe any bone cements' morphological structure changes occurring at different reaction times of the hardening stage and to assist in the understanding of the FBG results. Finally, the FBG sensor was used to monitor the development of the hygroscopic strains within the bone cement and to determine the material’s coefficient of moisture expansion (CME). It is noted, to the best of the authors' knowledge, that there are no similar works that study the hygroscopic response of a CPC in a humid environment.
2.
Materials and testing methods
2.1.
Materials and specimens
For the purposes of this study, a-TCP powder was mixed with a disodium hydrogen phosphate (Na2HPO4) aqueous solution, resulting in a malleable paste. Most of the cement paste was casted into an aluminum mould that had a cylindrical cavity of 12 mm diameter and 40 mm length. The mould was specially designed in order to allow the proper placement of a 0.125 mm in diameter optical sensor, through the specimens' longitudinal axis and the Bragg grating to be centrally located (Karalekas et al., 2009; Lai et al., 2012). Overall, two cylindrical samples were created with an embedded FBG sensor of 1 mm grating length and were labeled as Sample A and B, respectively. The specimens were let in the mould to solidify for four days and then were recovered and immersed in a bath filled with Ringer solution, for 40 days in total, in order to start the hardening stage. The bath was placed in an environmental chamber that had a stable temperature of 23 1C. At all times, the embedded fiber Bragg grating was interrogated using a MicronOpticss SM125 optical sensing interrogator that recorded the peak wavelength from the Bragg grating every 24 h. The remaining cement paste was casted into prismatic 3D printed moulds, made from ABS plastic, that had dimensions of 6 6 12 mm3 (width height length). The created specimens followed the same casting process as the cylindrical samples and then were immersed in separate baths filled with Ringer solution. The surrounding environment of the baths was also kept at a stable temperature of 23 1C. However, during hardening stage and at different time periods, prismatic specimens were recovered in couples, each time, from the isotonic solution and dried in an oven at 100 1C for two hours in order to stop any reaction in progress. These time periods concerned the first hours of immersion and specifically 48, 72, 96, 144, 192 and 240 h since hardening period commenced. Based on previous studies, it has been demonstrated (Ginebra et al., 1997; Komlev et al., 2012) that the microstructural changes take place and evolve within 12– 15 days of immersion in aqueous media. After the recovery process was completed those prismatic specimens were used for SEM investigation. Eventually more prismatic specimens
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
recovered at 72, 120, 168, 264 and 432 h after their immersion in the hardening liquid, followed the same drying process and then were used for XRD characterization of the material. All solids were characterized using a Siemens/Bruker D5000 X-ray diffraction with CuKα radiation. Morphological characterization was performed on gold coated specimens using a Field Emission Scanning Electron Microscope (FE-SEM, LEO SUPRA 35VP). The specimens with the embedded FBG sensor stayed immersed for, at least, 15 days in order to let the hardening process, and the full evolution of the microstructure, to be completed (Ginebra et al., 1997) and until the recorded peak wavelength was stabilized. Afterwards, they were recovered, dried in an oven at 30 1C in order to subtract the absorbed liquid media from the material's pores and then left at ambient temperature to cool down. Subsequently, each one was exposed in a different type of wet environment under a stable temperature of 23 1C: (a) a liquid one by using a container filled with Ringer solution (b) a humid one by using an environmental chamber with settable values of relative humidity (RH). In case (a), the FBG instrumented sample was immersed into the liquid media until it was fully saturated. At the same time, an identical control specimen without an embedded FBG sensor followed the same immersion procedure in order to record the sample's liquid uptake. The peak wavelength and weight gain were recorded simultaneously every 1 min during the first 13 min since immersion commenced and from then on every 1440 min (24 h). In case (b), the initial set value of RH was 25% and was increased by 5% each time, when equilibrium between the sample and the surrounding environment was achieved, and up to a maximum value of RH (80%) reached by the environmental chamber. At the end of each RH increment, the sample was weighed and the peak wavelength from the optical fiber was recorded.
2.2.
FBG sensing principle and strain measurements
The basic functional principle of a Bragg sensor is the refractive index modulation of a standard optical fiber's core, whose fundamental purpose is to reflect a narrow wavelength band of a broadband light source. For a single mode and uniform FBG, the reflected wavelength is centered on the Bragg wavelength and directly related to the period of the induced index modulation Λ and the mean effective refractive index neff, through the Bragg condition: λΒ0 ¼2neffΛ. In the case when the FBG is subjected to a homogeneous axial strain,εz , and uniform temperature change ΔΤ, the difference in Bragg wavelength , obtained from the peak shift of the spectra, before and after loading is: ΔλΒ ¼ 1 pe ϵz þ af þ ξ ΔΤ λB0
ð1Þ
where λB0 is the reference reflected peak wavelength(before loading), λB is the reflected peak wavelength after loading, pe is a grating gage factor measured experimentally, af is the CTE of the glass fiber and ξ is the thermo-optic constant. It is noted that εz includes all axially applied strains to the fiber, due to mechanical, thermal expansion mismatch, shrinkage due to solidification of the surrounding matrix, swelling due
197
to moisture absorption etc. (Karalekas et al., 2009; Lai et al., 2012). In the case where the embedded sensor and the host biocement material are not subjected to any temperature variations, the resulted hardening strains (εhs ) can be calculated by further simplifying Eq. (1) to the following form: ΔλΒ ¼ 1 pe ϵz ¼ 1 pe ðϵhs Þ λB0
ð2Þ
where, the considered reference value (λB0) corresponds to the one of an unembedded optical fiber at ambient temperature (ΔΤ ¼0). After hardening completion, in order to calculate the developed hygroscopic strains due to moisture uptake Eq. (2) takes the following form: ! ΔW ΔλΒ0 ð3Þ ¼ 1 pe ϵz ¼ 1 pe ϵhygro ¼ 1 pe β Wref λΒ0 where, εhygro are the hygroscopic strains due to liquid/moisture absorption and β is the bone cement's coefficient of moisture expansion (CME). In this case, as reference wavelength (λB0) is considered the reflected peak Bragg wavelength value recorded at the end of the hardening stage using Eq. 2, just before subjecting the specimen to hygroscopic ageing.
2.3.
Weight gain measurements
The weight gain of the hardened samples during their exposure in liquid and humid environment was expressed as follows (Karalekas et al., 2009): W¼
WðtÞ Wref ΔW x100 ¼ ð%Þ Wref Wref
ð4Þ
where, Wref is the specimens' weight at dry state and W(t) their weight at time t. A digital balance of 10 3 g resolution was used for measuring Wref and W(t). In the case of the liquid environment, each time the control specimen was recovered from the liquid media and before it was weighed, a gentle shake was applied to it in order to remove the excessive amount of liquid on the sample's surface.
3.
Results
3.1.
Induced strains during hardening stage
In Fig. 1, the induced strains developed during the hardening stage of the FBG instrumented samples are presented. As shown, the strain values start from 372 10 6 (με) for Sample A and 344 10 6(με) for Sample B after 24 h of specimens' immersion in the Ringer solution. After 30–40 days of immersion their values were declined to 71 10 6 (με) and 79 10 6 (με), respectively. Moreover, two distinct linear regions, of different slope each, are also observed. More specifically, in Sample Α a linear trend of the strain readings is evident until the first 7 days of immersion. Calculating the slope of the obtained trend line, by fitting least squares statistical model, a value of φ1Α ¼ þ24.48 με/day was found with r2 ¼ 0.9806. Upon completion of the 7th day and until the 30th day of immersion, the resulted strain values exhibited a new linear trend having a different slope compared to the previous one. The new obtained trend line
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journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
had a value of φ2Α ¼ þ3.92 με/day with r2 ¼ 0.981. Afterwards, 25 days in the Ringer solution and remained stable until the
a slope value of φ2Β ¼þ3.64 με/day with r2 ¼ 0.947 up to the 25th day of immersion. Then, the strain values were leveled off at 79 10 6 (με) for the rest of the immersion time.
end of the immersion period. Accordingly, in Sample Β the obtained strain values
3.2.
the strain magnitudes were stabilized at 71 10
6
(με) after
Scanning Electron Microscopy investigation
exhibited similar slope trends compared to those of Sample A. The value of the first slope was calculated to be 2
φ1Β ¼þ25.22 με/day with r ¼ 0.974. However, as seen in Fig. 1 this linear behavior is maintained until the 5th day of immersion where changed to the second linear curve having
Fig. 1 – Induced strains during hardening stage of biocement specimens.
In Fig. 2 some representative SEM images from the examined samples at different reaction times are presented. By comparing them one concludes that microstructural changes took place when the bone cement was immersed into the hardening liquid. Initially, after 48 h of immersion commencement a-TCP particles exhibit a sandy texture and on these particles an early creation of a layer composed of small crystal structures can be distinguished as seen in Fig. 2a. After 72 h (3 days) and 96 h (4 days) of immersion significantly larger crystal structures were observed having either leaf-like or needle-like shapes, leading to the conclusion that growth of these structures was taking place (Fig. 2b and c). In the next presented image (Fig. 2d), referring to 144 h (6 days) of immersion, an entanglement of the previously observed structures having uniform size is observed. However, there are also areas on cement's surface where the size of the crystal structures varies. This finding is also verified by (Fig. 3a) where a higher resolution was applied, suggesting that the growth of these structures was still in progress. After 192 h (8 days) of immersion, as seen in (Fig. 2e), the created layer on a-TCP particles has now become an entangled
Fig. 2 – SEM images in various reaction times: (a) 48 h (b) 72 h (c) 96 h (d) 144 h (e) 192 h and (f) 240 h.
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
199
Fig. 3 – SEM images using higher magnification at selected reaction times: (a) at 144 h and (b) at 240 h.
Fig. 5 – Evolution of hygroscopic strains, in liquid environment, during the first 13 min of immersion.
Fig. 4 – XRD patterns of (a) JCPDS card for hydroxyapatite 9-0432, (b) JCPDS card for β-TCP, (c) JCPDS card for α-TCP, (d) starting powder (e) cement before immersion in Ringer solution (f) cement after immersion in Ringer solution for 3, 5, 7, 11 and 18 days. Asterisk denotes the disappearance of α-TCP peak after 7 days. network of crystals and it is enough dense so that particles' initial geometry has altered. Finally, after 240 h (10 days) of immersion, in (Fig. 2f), the crystals' network appears to be more compact, clearly supported from the image of a higher magnification presented in (Fig. 3b). It is seen that a thick layer of a complex crystals network has been created occupying the existing voids between a-TCP particles.
3.3.
X-ray diffraction characterization
Fig. 4 depicts the XRD patterns of the: (i) standard powders (αTCP, β-TCP, hydroxyapatite), (ii) α-TCP powder that was used in this study, as well as the examined cement specimens at different hardening times. The specimens are labeled with a
number and a letter, where the number corresponds to the time period, in days, the specimen stayed in the hardening liquid while the letters designate a sequential numbering. The XRD diffractogram of the powder used in this work besides α-TCP peaks exhibits the characteristic peaks of βTCP. The presence of β-TCP polymorph is common when quenching is not fast enough or when the reactant components contain impurities which promote the nucleation of βTCP (Ginebra et al., 1997). In the case of the cement specimen remained in hardening liquid for 24 h, it is clear that new peaks are emerged in the 2θ:30–351 indicating the formation of apatite. Moreover, in the following days the peaks of apatite correspond to (211), (112) and (300) reflections are overlapped, due to peak broadening. Finally, residual α-TCP was detected for the last time in the specimens recovered after 7 days of immersion in the hardening liquid.
3.4.
Hygroscopic strains measurement
3.4.1.
Liquid environment
In Figs. 5 and 6 the developed hygroscopic strains (εhygro) and the weight gain (WL) as a function of sample's immersion time in a liquid environment, are presented. Since the experiment was conducted twice, the obtained results were numbered as (I) and (II), respectively. Both times, the sample's weight gain exhibited two distinct stages: (a) a rapid liquid uptake during the first two minutes of immersion and (b) further evolution of liquid absorption with a lower rate in the following days. As seen in Fig. 5 after seven days of immersion the weight measurements were practically
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journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
Fig. 6 – Evolution of hygroscopic strains, in liquid environment, until sample's saturation. Fig. 8 – Hygroscopic strains as a function of sample's humid content. Fig. 9 the variation of weight change vs. relative humidity is also presented.
4.
Fig. 7 – Hygroscopic strains as a function of sample's liquid content. stabilized indicating that saturation levels were reached. The steep increase of hygroscopic strains observed in Fig. 5 is replotted for a shorter immersion time in Fig. 6. For both experimental runs, designated as εhygroI and εhygroII, a rapid strain evolution takes place reaching values of þ110.2 10 6 (με) and þ84.7 10 6 (με) within the first two minutes of immersion into the Ringer solution. The developed hygroscopic strains are also presented as a function of sample's weight gain in Fig. 7 where the data can be approximated by two straight lines. These slopes can also determine the coefficients of moisture expansion of the studied samples. The first slope is exhibited when the sample's weight gain is between 0r WL ð%Þo29 and its value was calculated to be β(1Ι) ¼ 3.75 10 6/%w/w (r2 ¼ 0.971) and β(1ΙΙ) ¼2.74 10 6/%w/w (r2 ¼ 0.9059) for the two experimental trials (I and II), respectively. The second slope is appearing for a liquid uptake range of 29r WL ð%Þo37, where the corresponding values were β(2Ι) ¼16.2 10 6/%w/w (r2 ¼0.9856) and β(2ΙΙ) ¼11.3 10 6/%w/w (r2 ¼ 0.9929).
3.4.2.
Humid environment
In Fig. 8 the developed hygroscopic strains as a function of sample’s moisture content, designated as WH, in a humid environment are presented. The strains values exhibit a variation trend that was found to have a slope value of β¼3.98 10 4/%w/w for a moisture uptake range of . In
Discussion
The recorded strain values during the hardening stage of the studied bone cement are of low magnitude where the highest compressive strains, of 370 10 6 (με), are developed during the beginning of the hardening process when immersed in a bath of a Ringer solution. After 30 days of immersion in the hardening solution a much smaller compressive strain, namely 70 10 6 (με), is obtained after following two distinct linear paths of strain relief. The obtained hardening strain levels are in accordance with the ones initially reported in (Bimis and Karalekas, 2015) where range values between 300 10 6 (με) and 100 10 6 (με) are presented for the same type of bone cement. The findings from the SEM investigation demonstrated that microstructural changes took place during the followed hardening stage, as it is also supported by several studies (Ginebra et al., 1997, 2012; Komlev et al., 2012) regarding selfsetting biocements. It is noted that the initial crystal growth, observed after 2, 3 and 4 days from immersion commencement, coincided with an initial linear behavior of the FBGbased recorded strain data, leading to the conclusion that the change in strain readings over time could be attributed to the formation of the observed crystal structures. After 6 days (144 h) of immersion, a clear entanglement of these structures was observed for the first time from the SEM images. In the following days, the entanglement became more complex as it is depicted from the captured images at 8 and 10 days (192 and 240 h) of immersion. The entanglement of the crystal structures that resulted in a complex network, appears to be the cause that strain values exhibited a new, of different slope, linear pattern until the 30th day of immersion. Afterwards, and until the end of the immersion period, stabilization of the strain values at around 100 microstrains (με), indicated that no additional microstructural changes took place. Ginebra et al. (1997) and Komlev et al. (2012) have conducted SEM investigations in self-setting calcium phosphate
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
bone cements during hardening stage. They have reported that the observed microstructural changes were completed after 12–15 days of samples' immersion into the hardening liquid instead of 25 days obtained in the present study. However, Zhang et al. (2014) reports that various parameters such as smaller particle size, higher setting temperature and low liquid-to-powder ratio can affect kinetics and as a consequence CPCs' setting time. Considering that the aforementioned parameters were not the same with the ones of the present study, such differences in the time period where microstructural changes take place are expectable. As far as XRD characterization is concerned, the overlap of the apatite peaks that correspond to (211), (112) and (300) reflections suggests reduction of crystal size and decrease of crystallinity. This is probably due to the presence of carbonate ions in the apatite structure or to the formation of calcium deficient hydroxyapatite because of a-TCP conversion (Yao et al., 2004), which is directly related with the crystal structure growth. In addition, the fact that a-TCP was detected for the last time after 7 days, since hardening period started, suggests that a-TCP conversion was nearly completed around this time period. The evolution of the hygroscopic strains and samples' liquid uptake exhibit the same behavior, as it is observed from Figs. 5 and 6 that concern the liquid environment. It is noted that the differences in the recorded strain values can be possibly attributed to the fatigue of the interface between the FBG sensor and the bone cement material, due to the successive drying-swelling process that the sample had to undergo to repeat the immersion experiments. As mentioned before, in Fig. 7 the calculated slopes β(1Ι),β(1ΙΙ) and β(2Ι), β(2ΙΙ) correspond to the coefficient of moisture expansion (CME) of the biocement under investigation. The experimental data led to the calculation of two distinct values for β. It was also evident that β increased considerably after a critical value of water gain ( 29%) was reached. All measured CME values are summarized in Table 1. In Fig. 8 a linear change of the hygro strain readings in relation to the measured weight gain is exhibited for the case of the humid environment. The calculated coefficient of moisture expansion was found to be two orders of magnitude higher than the ones obtained for the case of the liquid immersed specimens. This surprising and interesting finding needs to be investigated further since it may be due to the presence of different and complex diffusion mechanisms (Yunping et al., 1994) and specimens' fabrication parameters such as pore size, interconnectivity of pores, tortuosity, and in general to the different way with which the pores are filled in a wet environment. In Fig. 9 a linear variation of weight change to relative humidity is presented for the examined RH interval (25–80%). A similar linear behavior up to 90% RH is reported by Shinsaku Tada et al. work (Shinsaku and Table 1 – CPC coefficient of moisture expansion (CME). 0r WL o29 (%) Experiment I Experiment II
6
β(1Ι) ¼3.75 10 /% w/w β(1ΙΙ) ¼2.74 10 6/% w/w
29 r WL r 37 (%) 6
β(2Ι) ¼ 7.43 10 /% w/w β(2ΙΙ) ¼ 4.26 10 6/% w/w
201
Fig. 9 – Hygroscopic strains, in humid environment, as a function of relative humidity. Kazumasa, 2005) for a cement based material while a sharp increase of its slope was observed for 90–100% RH.
5.
Conclusions
In this study, FBG sensors were embedded in CPC self-setting samples in order to investigate the developed strains during hardening stage. The obtained results from the FBG recordings revealed small compressive strains. Their values decreased for several days, exhibiting two distinct slopes of strain relief, until they were stabilized. The FBG-based recordings were coupled with a SEM investigation where images were captured at specific time intervals of specimen immersion into the hardening liquid. The SEM findings have shown that microstructural changes, similar to those reported in the literature, occur at two stages characterized by: (a) the development of plate-like and needle-like crystal structures on the bone cement' surface and (b) the further growth of crystal structures that result in an entanglement between them and finally in a complex crystal network. The growth of the crystal structures and the creation of the entangled crystal network affected the embedded optical sensor leading to strain evolution patterns exhibiting two distinct slopes during the samples' immersion period. Additionally, after completion of the hardening stage the samples integrating the FBG sensor were dried and used to obtain the axial hygroscopic expansion of the examined CPC material. The results demonstrate that the measured hygroscopic strains exhibited the same behavior as the specimen's liquid gain and that the material’s absorption process can be monitored in an effective way leading to the conclusion that FBGs can be used to investigate CPCs kinetics. The combination of the hygroscopic strains to the knowledge of the moisture uptake enabled the identification of two distinct macroscopic coefficients of moisture expansion of the studied samples.
Acknowledgments This research has been co-financed by the European Union (European Social Fund – ESF) and Greek National Funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework
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journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
(NSRF) – Research Funding Program: THALES, and within the context of MIS 379380 research project.
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Komlev, V.S., Fadeeva, I.V., Barinov, S.M., Rau, J.V., Fosca, M., Gurin, A.N., Gurin, N.A., 2012. Phase development during setting and hardening of a bone cement based on α-tricalcium and octacalcium phosphates. J. Biomater. Appl. 26, 1051–1068. Lai, M., Friedrich, K., Botsis, J., Burkhart, T., 2010. Evaluation of residual strains in epoxy with different nano/micro-fillers using embedded fiber Bragg grating sensor. Compos. Sci. Technol. 70, 2168–2175. Lai, M., Botsis, J., Cugnoni, J., Coric, D., 2012. An experimental– numerical study of moisture absorption in an epoxy. Compos. A: Appl. Sci. Manuf. 43, 1053–1060. Morey, W.W., Dunphy, J.R., Meltz, G., 1992. Multiplexing fiber Bragg grating sensors. In: Proceedings of SPIE 1586. Boston. Ramezani-Dana, H., Casari, P., Perronnet, A., Fre´our, S., Jacquemin, F., Lupi, C., 2014. Hygroscopic strain measurements by fibre Bragg gratings sensors in organic matrix composites – application to monitoring of a composite structure. Composite A 58, 76–82. Roriz, P., Carvalho, L., Fraza˜o, O., Santos, J.L., Simo˜es, J.A., 2014. From conventional sensors to fibre optic sensors for strain and force measurements in biomechanics applications: a review. J. Biomech. 47, 1251–1261. Sun, Y., Wang, Y., Kim, Y., Han, B., 2014. Dual-configuration fiber Bragg grating sensor technique to measure coefficients of thermal expansion and hygroscopic swelling. Exp. Mech. 54, 593–603. Schizas, C., Karalekas, D., 2011. Mechanical characteristics of an Ormocomps biocompatible hybrid photopolymer. J. Mech. Behav. Biomed. Mater. 4, 99–106. Shinsaku, T., Kazumasa, W., 2005. Dynamic determination of sorption isotherm of cement based materials. Cem. Concr. Res. 35, 2271–2277. Yao, F., LeGeros, J.P., LeGeros, R.Z., 2004. Simultaneous incorporation of carbonate and fluoride in synthetic apatites: effect on crystallographic and physico-chemical properties. Acta Biomater. 5, 2169–2177. Yunping, X., Zdeneˇk, P.B., Hamlin, M.J., 1994. Moisture diffusion in cementitious materials adsorption isotherms. Adv. Cem. Based Mater. 1, 248–257. Zhang, J., Liu, W., Schnitzler, V., Tancret, F., Bouler, J.M., 2014. Calcium phosphate cements for bone substitution: chemistry, handling and mechanical properties. Acta Biomater. 10, 1035–1049.
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Research paper
Monitoring of hardening and hygroscopic induced strains in a calcium phosphate bone cement using FBG sensor A. Bimisa, D. Karalekasa,n, N. Bouropoulosb, D. Mouzakisc, S. Zaoutsosc a
Laboratory of Advanced Manufacturing Technologies and Testing, University of Piraeus, 18534 Piraeus, Greece Department of Materials Science, University of Patras, 26504 Patras, Greece c Department of Mechanical Engineering, Technological Educational Institute of Thessaly, 41110 Larissa, Greece b
art i cle i nfo
ab st rac t
Article history:
This study initially deals with the investigation of the induced strains during hardening
Received 5 October 2015
stage of a self-setting calcium phosphate bone cement using fiber-Bragg grating (FBG)
Received in revised form
optical sensors. A complementary Scanning Electron Microscopy (SEM) investigation was
23 December 2015
also conducted at different time intervals of the hardening period and its findings were
Accepted 25 December 2015
related to the FBG recordings. From the obtained results, it is demonstrated that the FBG
Available online 6 January 2016
response is affected by the microstructural changes taking place when the bone cement is
Keywords:
immersed into the hardening liquid media. Subsequently, the FBG sensor was used to
Calcium phosphate cement
monitor the absorption process and hygroscopic response of the hardened and dried
Fiber Bragg grating sensor
biocement when exposed to a liquid/humid environment. From the FBG-based calculated
Hygroscopic strains
hygric strains as a function of moisture concentration, the coefficient of moisture
Coefficient of moisture expansion
expansion (CME) of the examined bone cement was obtained, exhibiting two distinct linear regions. & 2016 Elsevier Ltd. All rights reserved.
1.
Introduction
polymethyl methacrylate cement. However, PMMA cement has several disadvantages such as generation of heat during
In surgical applications, such as bone grafting, bone cement
curing that causes necrosis of the surrounding tissue (Zhang
is used as a coating between the bone and the implantin
et al., 2014), toxicity of the monomer, embrittlement due to
order to ensure proper fixation. Polymethylmethacrylate (PMMA) bone cements are mostly used in such applications
aging and low biocompatibility at the cement–bone interface. Biocements formed from calcium/phosphate (Calcium
and consist of two primary components: (i) a powder con-
Phosphate Cements-CPCs) are produced by mixing water or
sisting of copolymers based on the substance polymethyl
aqueous phosphate solution with a solid calcium phosphate
methacrylate and (ii) a liquid monomer, methylmethacrylate
phase, resulting in a malleable paste (Heinemann et al., 2013).
(MMA). These two components are mixed and form a
They have been attracting great attention as an alternate
n
Corresponding author. Tel.: þ30 210 4142319. E-mail address: [email protected] (D. Karalekas).
http://dx.doi.org/10.1016/j.jmbbm.2015.12.041 1751-6161/& 2016 Elsevier Ltd. All rights reserved.
196
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
solution because they offer advantages such as high biocompatibility, bioactivity, osteoconduction and osteogenesis (Andriotis et al., 2010). These characteristics allow their use as a bioactive fixation onto metallic implants or to replace missing or damaged bones in several occasions of bone defects as well as they can also act as drug eluting carriers such as antibiotics, anti-inflammatory etc. due to their intrinsic porosity (Zhang et al., 2014; Bose and Tarafder, 2012). On the other hand, several limitations, such as poor mechanical properties and their inherent brittleness, have to be addressed as they limit their use to low load bearing places (Zhang et al., 2014) and the necessary clinical requirements are not met. In the case of self-setting CPCs, the hardening process takes place in an isotonic solution that acts as hardening liquid (Barinov and Komlev, 2011) and it has been proven that during this stage microstructural changes are occurred (Ginebra et al., 1997). Moreover, it has been demonstrated that these changes improve bone cements' mechanical behavior (Ginebra et al., 1997; Bimis and Karalekas, 2015), since the mechanical properties of a material are determined by its microstructure (Zhang et al., 2014). It is also reported in the literature that a more thorough investigation of the CPCs kinetics is imperative if to be used as drug delivery materials (Ginebra et al., 2012). FBG sensors exhibit several advantages such as fast response, high sensitivity, signal integrity, long-term stability, immunity against electromagnetic radiation, insensitivity to radio frequency interference and good corrosion resistance. In terms of performances, they present multifunctionality (allow measurement of parameters including strain, temperature, pressure, load, acceleration, vibration, etc.) and are easily multiplexed in a serial manner along a single optical fiber (Hill and Meltz, 1997; Morey et al., 1992). In addition, their light weight and small dimensions make them possible to be effectively embedded within host materials. For that reason, fiber optic sensors are considered one of the most recent and promising methods for monitoring of process-induced residual strains. Many works involving FBG sensors for investigating fabrication induced shrinkage strains and hygrothermal strains in polymeric and composite materials have been reported (Sun et al., 2014; Lai et al., 2010; Karalekas et al., 2009; Lai et al., 2012; Ramezani-Dana et al., 2014). In recent years, fiber Bragg grating sensors have been used in biomedical applications and to study polymerization shrinkage and water sorption of different biomaterials (Carvalho et al., 2006; Anttila et al., 2008; Schizas and Karalekas, 2011; Al-Fakih et al., 2012; Roriz et al., 2014). In a previous work by Bimis et al. (Bimis and Karalekas, 2015), an optical sensor was embedded in a self-setting calcium phosphate bone cement cylindrical specimen to investigate if the hardening induced strains could be recorded from the FBG response. According to the reported results a noticeable change in measured strains may be due to microstructural changes occurring during immersion in the hardening liquid. However, no clear change patterns were observed leading to the conclusion that future investigations should be carried out in a better controlled experimental conditions accompanied by SEM and XRD based examination. In the present study, a more extensive and thorough investigation was carried out. The magnitude of the developing
strains was initially investigated during the hardening stage of the examined biocement and then, after drying, when exposed into a liquid/humid environment. The hardening induced strains derived from the Bragg wavelength evolutions were combined with microstructural observations from Scanning Electron Microscopy (SEM) imaging performed to observe any bone cements' morphological structure changes occurring at different reaction times of the hardening stage and to assist in the understanding of the FBG results. Finally, the FBG sensor was used to monitor the development of the hygroscopic strains within the bone cement and to determine the material’s coefficient of moisture expansion (CME). It is noted, to the best of the authors' knowledge, that there are no similar works that study the hygroscopic response of a CPC in a humid environment.
2.
Materials and testing methods
2.1.
Materials and specimens
For the purposes of this study, a-TCP powder was mixed with a disodium hydrogen phosphate (Na2HPO4) aqueous solution, resulting in a malleable paste. Most of the cement paste was casted into an aluminum mould that had a cylindrical cavity of 12 mm diameter and 40 mm length. The mould was specially designed in order to allow the proper placement of a 0.125 mm in diameter optical sensor, through the specimens' longitudinal axis and the Bragg grating to be centrally located (Karalekas et al., 2009; Lai et al., 2012). Overall, two cylindrical samples were created with an embedded FBG sensor of 1 mm grating length and were labeled as Sample A and B, respectively. The specimens were let in the mould to solidify for four days and then were recovered and immersed in a bath filled with Ringer solution, for 40 days in total, in order to start the hardening stage. The bath was placed in an environmental chamber that had a stable temperature of 23 1C. At all times, the embedded fiber Bragg grating was interrogated using a MicronOpticss SM125 optical sensing interrogator that recorded the peak wavelength from the Bragg grating every 24 h. The remaining cement paste was casted into prismatic 3D printed moulds, made from ABS plastic, that had dimensions of 6 6 12 mm3 (width height length). The created specimens followed the same casting process as the cylindrical samples and then were immersed in separate baths filled with Ringer solution. The surrounding environment of the baths was also kept at a stable temperature of 23 1C. However, during hardening stage and at different time periods, prismatic specimens were recovered in couples, each time, from the isotonic solution and dried in an oven at 100 1C for two hours in order to stop any reaction in progress. These time periods concerned the first hours of immersion and specifically 48, 72, 96, 144, 192 and 240 h since hardening period commenced. Based on previous studies, it has been demonstrated (Ginebra et al., 1997; Komlev et al., 2012) that the microstructural changes take place and evolve within 12– 15 days of immersion in aqueous media. After the recovery process was completed those prismatic specimens were used for SEM investigation. Eventually more prismatic specimens
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
recovered at 72, 120, 168, 264 and 432 h after their immersion in the hardening liquid, followed the same drying process and then were used for XRD characterization of the material. All solids were characterized using a Siemens/Bruker D5000 X-ray diffraction with CuKα radiation. Morphological characterization was performed on gold coated specimens using a Field Emission Scanning Electron Microscope (FE-SEM, LEO SUPRA 35VP). The specimens with the embedded FBG sensor stayed immersed for, at least, 15 days in order to let the hardening process, and the full evolution of the microstructure, to be completed (Ginebra et al., 1997) and until the recorded peak wavelength was stabilized. Afterwards, they were recovered, dried in an oven at 30 1C in order to subtract the absorbed liquid media from the material's pores and then left at ambient temperature to cool down. Subsequently, each one was exposed in a different type of wet environment under a stable temperature of 23 1C: (a) a liquid one by using a container filled with Ringer solution (b) a humid one by using an environmental chamber with settable values of relative humidity (RH). In case (a), the FBG instrumented sample was immersed into the liquid media until it was fully saturated. At the same time, an identical control specimen without an embedded FBG sensor followed the same immersion procedure in order to record the sample's liquid uptake. The peak wavelength and weight gain were recorded simultaneously every 1 min during the first 13 min since immersion commenced and from then on every 1440 min (24 h). In case (b), the initial set value of RH was 25% and was increased by 5% each time, when equilibrium between the sample and the surrounding environment was achieved, and up to a maximum value of RH (80%) reached by the environmental chamber. At the end of each RH increment, the sample was weighed and the peak wavelength from the optical fiber was recorded.
2.2.
FBG sensing principle and strain measurements
The basic functional principle of a Bragg sensor is the refractive index modulation of a standard optical fiber's core, whose fundamental purpose is to reflect a narrow wavelength band of a broadband light source. For a single mode and uniform FBG, the reflected wavelength is centered on the Bragg wavelength and directly related to the period of the induced index modulation Λ and the mean effective refractive index neff, through the Bragg condition: λΒ0 ¼2neffΛ. In the case when the FBG is subjected to a homogeneous axial strain,εz , and uniform temperature change ΔΤ, the difference in Bragg wavelength , obtained from the peak shift of the spectra, before and after loading is: ΔλΒ ¼ 1 pe ϵz þ af þ ξ ΔΤ λB0
ð1Þ
where λB0 is the reference reflected peak wavelength(before loading), λB is the reflected peak wavelength after loading, pe is a grating gage factor measured experimentally, af is the CTE of the glass fiber and ξ is the thermo-optic constant. It is noted that εz includes all axially applied strains to the fiber, due to mechanical, thermal expansion mismatch, shrinkage due to solidification of the surrounding matrix, swelling due
197
to moisture absorption etc. (Karalekas et al., 2009; Lai et al., 2012). In the case where the embedded sensor and the host biocement material are not subjected to any temperature variations, the resulted hardening strains (εhs ) can be calculated by further simplifying Eq. (1) to the following form: ΔλΒ ¼ 1 pe ϵz ¼ 1 pe ðϵhs Þ λB0
ð2Þ
where, the considered reference value (λB0) corresponds to the one of an unembedded optical fiber at ambient temperature (ΔΤ ¼0). After hardening completion, in order to calculate the developed hygroscopic strains due to moisture uptake Eq. (2) takes the following form: ! ΔW ΔλΒ0 ð3Þ ¼ 1 pe ϵz ¼ 1 pe ϵhygro ¼ 1 pe β Wref λΒ0 where, εhygro are the hygroscopic strains due to liquid/moisture absorption and β is the bone cement's coefficient of moisture expansion (CME). In this case, as reference wavelength (λB0) is considered the reflected peak Bragg wavelength value recorded at the end of the hardening stage using Eq. 2, just before subjecting the specimen to hygroscopic ageing.
2.3.
Weight gain measurements
The weight gain of the hardened samples during their exposure in liquid and humid environment was expressed as follows (Karalekas et al., 2009): W¼
WðtÞ Wref ΔW x100 ¼ ð%Þ Wref Wref
ð4Þ
where, Wref is the specimens' weight at dry state and W(t) their weight at time t. A digital balance of 10 3 g resolution was used for measuring Wref and W(t). In the case of the liquid environment, each time the control specimen was recovered from the liquid media and before it was weighed, a gentle shake was applied to it in order to remove the excessive amount of liquid on the sample's surface.
3.
Results
3.1.
Induced strains during hardening stage
In Fig. 1, the induced strains developed during the hardening stage of the FBG instrumented samples are presented. As shown, the strain values start from 372 10 6 (με) for Sample A and 344 10 6(με) for Sample B after 24 h of specimens' immersion in the Ringer solution. After 30–40 days of immersion their values were declined to 71 10 6 (με) and 79 10 6 (με), respectively. Moreover, two distinct linear regions, of different slope each, are also observed. More specifically, in Sample Α a linear trend of the strain readings is evident until the first 7 days of immersion. Calculating the slope of the obtained trend line, by fitting least squares statistical model, a value of φ1Α ¼ þ24.48 με/day was found with r2 ¼ 0.9806. Upon completion of the 7th day and until the 30th day of immersion, the resulted strain values exhibited a new linear trend having a different slope compared to the previous one. The new obtained trend line
198
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
had a value of φ2Α ¼ þ3.92 με/day with r2 ¼ 0.981. Afterwards, 25 days in the Ringer solution and remained stable until the
a slope value of φ2Β ¼þ3.64 με/day with r2 ¼ 0.947 up to the 25th day of immersion. Then, the strain values were leveled off at 79 10 6 (με) for the rest of the immersion time.
end of the immersion period. Accordingly, in Sample Β the obtained strain values
3.2.
the strain magnitudes were stabilized at 71 10
6
(με) after
Scanning Electron Microscopy investigation
exhibited similar slope trends compared to those of Sample A. The value of the first slope was calculated to be 2
φ1Β ¼þ25.22 με/day with r ¼ 0.974. However, as seen in Fig. 1 this linear behavior is maintained until the 5th day of immersion where changed to the second linear curve having
Fig. 1 – Induced strains during hardening stage of biocement specimens.
In Fig. 2 some representative SEM images from the examined samples at different reaction times are presented. By comparing them one concludes that microstructural changes took place when the bone cement was immersed into the hardening liquid. Initially, after 48 h of immersion commencement a-TCP particles exhibit a sandy texture and on these particles an early creation of a layer composed of small crystal structures can be distinguished as seen in Fig. 2a. After 72 h (3 days) and 96 h (4 days) of immersion significantly larger crystal structures were observed having either leaf-like or needle-like shapes, leading to the conclusion that growth of these structures was taking place (Fig. 2b and c). In the next presented image (Fig. 2d), referring to 144 h (6 days) of immersion, an entanglement of the previously observed structures having uniform size is observed. However, there are also areas on cement's surface where the size of the crystal structures varies. This finding is also verified by (Fig. 3a) where a higher resolution was applied, suggesting that the growth of these structures was still in progress. After 192 h (8 days) of immersion, as seen in (Fig. 2e), the created layer on a-TCP particles has now become an entangled
Fig. 2 – SEM images in various reaction times: (a) 48 h (b) 72 h (c) 96 h (d) 144 h (e) 192 h and (f) 240 h.
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
199
Fig. 3 – SEM images using higher magnification at selected reaction times: (a) at 144 h and (b) at 240 h.
Fig. 5 – Evolution of hygroscopic strains, in liquid environment, during the first 13 min of immersion.
Fig. 4 – XRD patterns of (a) JCPDS card for hydroxyapatite 9-0432, (b) JCPDS card for β-TCP, (c) JCPDS card for α-TCP, (d) starting powder (e) cement before immersion in Ringer solution (f) cement after immersion in Ringer solution for 3, 5, 7, 11 and 18 days. Asterisk denotes the disappearance of α-TCP peak after 7 days. network of crystals and it is enough dense so that particles' initial geometry has altered. Finally, after 240 h (10 days) of immersion, in (Fig. 2f), the crystals' network appears to be more compact, clearly supported from the image of a higher magnification presented in (Fig. 3b). It is seen that a thick layer of a complex crystals network has been created occupying the existing voids between a-TCP particles.
3.3.
X-ray diffraction characterization
Fig. 4 depicts the XRD patterns of the: (i) standard powders (αTCP, β-TCP, hydroxyapatite), (ii) α-TCP powder that was used in this study, as well as the examined cement specimens at different hardening times. The specimens are labeled with a
number and a letter, where the number corresponds to the time period, in days, the specimen stayed in the hardening liquid while the letters designate a sequential numbering. The XRD diffractogram of the powder used in this work besides α-TCP peaks exhibits the characteristic peaks of βTCP. The presence of β-TCP polymorph is common when quenching is not fast enough or when the reactant components contain impurities which promote the nucleation of βTCP (Ginebra et al., 1997). In the case of the cement specimen remained in hardening liquid for 24 h, it is clear that new peaks are emerged in the 2θ:30–351 indicating the formation of apatite. Moreover, in the following days the peaks of apatite correspond to (211), (112) and (300) reflections are overlapped, due to peak broadening. Finally, residual α-TCP was detected for the last time in the specimens recovered after 7 days of immersion in the hardening liquid.
3.4.
Hygroscopic strains measurement
3.4.1.
Liquid environment
In Figs. 5 and 6 the developed hygroscopic strains (εhygro) and the weight gain (WL) as a function of sample's immersion time in a liquid environment, are presented. Since the experiment was conducted twice, the obtained results were numbered as (I) and (II), respectively. Both times, the sample's weight gain exhibited two distinct stages: (a) a rapid liquid uptake during the first two minutes of immersion and (b) further evolution of liquid absorption with a lower rate in the following days. As seen in Fig. 5 after seven days of immersion the weight measurements were practically
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journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
Fig. 6 – Evolution of hygroscopic strains, in liquid environment, until sample's saturation. Fig. 8 – Hygroscopic strains as a function of sample's humid content. Fig. 9 the variation of weight change vs. relative humidity is also presented.
4.
Fig. 7 – Hygroscopic strains as a function of sample's liquid content. stabilized indicating that saturation levels were reached. The steep increase of hygroscopic strains observed in Fig. 5 is replotted for a shorter immersion time in Fig. 6. For both experimental runs, designated as εhygroI and εhygroII, a rapid strain evolution takes place reaching values of þ110.2 10 6 (με) and þ84.7 10 6 (με) within the first two minutes of immersion into the Ringer solution. The developed hygroscopic strains are also presented as a function of sample's weight gain in Fig. 7 where the data can be approximated by two straight lines. These slopes can also determine the coefficients of moisture expansion of the studied samples. The first slope is exhibited when the sample's weight gain is between 0r WL ð%Þo29 and its value was calculated to be β(1Ι) ¼ 3.75 10 6/%w/w (r2 ¼ 0.971) and β(1ΙΙ) ¼2.74 10 6/%w/w (r2 ¼ 0.9059) for the two experimental trials (I and II), respectively. The second slope is appearing for a liquid uptake range of 29r WL ð%Þo37, where the corresponding values were β(2Ι) ¼16.2 10 6/%w/w (r2 ¼0.9856) and β(2ΙΙ) ¼11.3 10 6/%w/w (r2 ¼ 0.9929).
3.4.2.
Humid environment
In Fig. 8 the developed hygroscopic strains as a function of sample’s moisture content, designated as WH, in a humid environment are presented. The strains values exhibit a variation trend that was found to have a slope value of β¼3.98 10 4/%w/w for a moisture uptake range of . In
Discussion
The recorded strain values during the hardening stage of the studied bone cement are of low magnitude where the highest compressive strains, of 370 10 6 (με), are developed during the beginning of the hardening process when immersed in a bath of a Ringer solution. After 30 days of immersion in the hardening solution a much smaller compressive strain, namely 70 10 6 (με), is obtained after following two distinct linear paths of strain relief. The obtained hardening strain levels are in accordance with the ones initially reported in (Bimis and Karalekas, 2015) where range values between 300 10 6 (με) and 100 10 6 (με) are presented for the same type of bone cement. The findings from the SEM investigation demonstrated that microstructural changes took place during the followed hardening stage, as it is also supported by several studies (Ginebra et al., 1997, 2012; Komlev et al., 2012) regarding selfsetting biocements. It is noted that the initial crystal growth, observed after 2, 3 and 4 days from immersion commencement, coincided with an initial linear behavior of the FBGbased recorded strain data, leading to the conclusion that the change in strain readings over time could be attributed to the formation of the observed crystal structures. After 6 days (144 h) of immersion, a clear entanglement of these structures was observed for the first time from the SEM images. In the following days, the entanglement became more complex as it is depicted from the captured images at 8 and 10 days (192 and 240 h) of immersion. The entanglement of the crystal structures that resulted in a complex network, appears to be the cause that strain values exhibited a new, of different slope, linear pattern until the 30th day of immersion. Afterwards, and until the end of the immersion period, stabilization of the strain values at around 100 microstrains (με), indicated that no additional microstructural changes took place. Ginebra et al. (1997) and Komlev et al. (2012) have conducted SEM investigations in self-setting calcium phosphate
journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
bone cements during hardening stage. They have reported that the observed microstructural changes were completed after 12–15 days of samples' immersion into the hardening liquid instead of 25 days obtained in the present study. However, Zhang et al. (2014) reports that various parameters such as smaller particle size, higher setting temperature and low liquid-to-powder ratio can affect kinetics and as a consequence CPCs' setting time. Considering that the aforementioned parameters were not the same with the ones of the present study, such differences in the time period where microstructural changes take place are expectable. As far as XRD characterization is concerned, the overlap of the apatite peaks that correspond to (211), (112) and (300) reflections suggests reduction of crystal size and decrease of crystallinity. This is probably due to the presence of carbonate ions in the apatite structure or to the formation of calcium deficient hydroxyapatite because of a-TCP conversion (Yao et al., 2004), which is directly related with the crystal structure growth. In addition, the fact that a-TCP was detected for the last time after 7 days, since hardening period started, suggests that a-TCP conversion was nearly completed around this time period. The evolution of the hygroscopic strains and samples' liquid uptake exhibit the same behavior, as it is observed from Figs. 5 and 6 that concern the liquid environment. It is noted that the differences in the recorded strain values can be possibly attributed to the fatigue of the interface between the FBG sensor and the bone cement material, due to the successive drying-swelling process that the sample had to undergo to repeat the immersion experiments. As mentioned before, in Fig. 7 the calculated slopes β(1Ι),β(1ΙΙ) and β(2Ι), β(2ΙΙ) correspond to the coefficient of moisture expansion (CME) of the biocement under investigation. The experimental data led to the calculation of two distinct values for β. It was also evident that β increased considerably after a critical value of water gain ( 29%) was reached. All measured CME values are summarized in Table 1. In Fig. 8 a linear change of the hygro strain readings in relation to the measured weight gain is exhibited for the case of the humid environment. The calculated coefficient of moisture expansion was found to be two orders of magnitude higher than the ones obtained for the case of the liquid immersed specimens. This surprising and interesting finding needs to be investigated further since it may be due to the presence of different and complex diffusion mechanisms (Yunping et al., 1994) and specimens' fabrication parameters such as pore size, interconnectivity of pores, tortuosity, and in general to the different way with which the pores are filled in a wet environment. In Fig. 9 a linear variation of weight change to relative humidity is presented for the examined RH interval (25–80%). A similar linear behavior up to 90% RH is reported by Shinsaku Tada et al. work (Shinsaku and Table 1 – CPC coefficient of moisture expansion (CME). 0r WL o29 (%) Experiment I Experiment II
6
β(1Ι) ¼3.75 10 /% w/w β(1ΙΙ) ¼2.74 10 6/% w/w
29 r WL r 37 (%) 6
β(2Ι) ¼ 7.43 10 /% w/w β(2ΙΙ) ¼ 4.26 10 6/% w/w
201
Fig. 9 – Hygroscopic strains, in humid environment, as a function of relative humidity. Kazumasa, 2005) for a cement based material while a sharp increase of its slope was observed for 90–100% RH.
5.
Conclusions
In this study, FBG sensors were embedded in CPC self-setting samples in order to investigate the developed strains during hardening stage. The obtained results from the FBG recordings revealed small compressive strains. Their values decreased for several days, exhibiting two distinct slopes of strain relief, until they were stabilized. The FBG-based recordings were coupled with a SEM investigation where images were captured at specific time intervals of specimen immersion into the hardening liquid. The SEM findings have shown that microstructural changes, similar to those reported in the literature, occur at two stages characterized by: (a) the development of plate-like and needle-like crystal structures on the bone cement' surface and (b) the further growth of crystal structures that result in an entanglement between them and finally in a complex crystal network. The growth of the crystal structures and the creation of the entangled crystal network affected the embedded optical sensor leading to strain evolution patterns exhibiting two distinct slopes during the samples' immersion period. Additionally, after completion of the hardening stage the samples integrating the FBG sensor were dried and used to obtain the axial hygroscopic expansion of the examined CPC material. The results demonstrate that the measured hygroscopic strains exhibited the same behavior as the specimen's liquid gain and that the material’s absorption process can be monitored in an effective way leading to the conclusion that FBGs can be used to investigate CPCs kinetics. The combination of the hygroscopic strains to the knowledge of the moisture uptake enabled the identification of two distinct macroscopic coefficients of moisture expansion of the studied samples.
Acknowledgments This research has been co-financed by the European Union (European Social Fund – ESF) and Greek National Funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework
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journal of the mechanical behavior of biomedical materials 60 (2016) 195 –202
(NSRF) – Research Funding Program: THALES, and within the context of MIS 379380 research project.
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