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Hindawi Publishing CorporationISRN BiomaterialsVolume 2013, Article ID 750720, 10 pageshttp://dx.doi.org/10.5402/2013/750720

Research ArticleEffects of Heat Treatment on theMechanical and DegradationProperties of 3D-Printed Calcium-Sulphate-Based Scaffolds

Zuoxin Zhou,1 Christina A. Mitchell,2 Fraser J. Buchanan,1 and Nicholas J. Dunne1

1 School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AH, UK2 School of Medicine, Dentistry and Biomedical Sciences, Queen’s University Belfast, Grosvenor Road, Belfast BT12 6BP, UK

Correspondence should be addressed to Nicholas J. Dunne; [email protected]

Received 20 August 2012; Accepted 23 September 2012

Academic Editors: F. Feyerabend, C. Galli, D. Letourneur, and X. Wang

Copyright © 2013 Zuoxin Zhou et al. is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ree-dimensional printing (3DP) has been employed to fabricate scaffolds with advantages of fully controlled geometries andreproducibility. In this study, the scaffold structure design was established through investigating the minimum feature size andpowder size distribution. It was then fabricated from the 3DP plaster-based powders (CaSO4⋅1/2H2O). Scaffolds produced fromthis material demonstrated low mechanical properties and a rapid degradation rate. is study investigated the effects of heattreatment on the mechanical and in vitro degradation properties of the CaSO4 scaffolds. e occurrence of dehydration during theheating cycle offered moderate improvements in the mechanical and degradation properties. By using a heat treatment protocolof 200○C for 30min, compressive strength increased from 0.36 ± 0.13MPa (pre-heat-treated) to 2.49 ± 0.42MPa (heat-treated).Heat-treated scaffolds retained their structure and compressive properties for up to two days in a tris-buffered solution, whileuntreated scaffolds completely disintegrated within a few minutes. Despite the moderate improvements observed in this study, theheat-treated CaSO4 scaffolds did not demonstrate mechanical and degradation properties commensurate with the requirementsfor bone-tissue-engineering applications.

1. Introduction

Bone defects larger than a critical size cannot be healed bynormal bone remodelling processes and thus require bonesubstitution. e autogra, which is recognised as the “goldstandard” for bone repair, has been widely used for decades.However, it still has noted drawbacks, including risk ofdisease transmission and limited availability compared toever-increasing surgical demand [1]. In the United States, forexample, there are annually more than 0.5 million surgicalprocedures which are related to bone repair [2].

One of the breakthroughs in bone tissue engineering wasthe development of 3D scaffolds that replace and restore thelost tissues.ey serve as a template to allow cell seeding andcarry cells to the desired site. Despite the initial success in thedevelopment of 3D scaffolds, researchers now face a greaterchallenge in repairing injured bone in load-bearing sites [3].In order to maintain the function of load-bearing bones, thescaffold needs to exhibit appropriate mechanical properties.

ese properties are highly dependent on scaffold designand geometry. A general consensus for the optimal bone-tissue-engineered scaffold design is tomimic the architecture,mechanical, and biochemical properties of natural bone [4].However, it is difficult to control the geometry of scaffoldsusing traditional scaffold-manufacturing techniques, such assolvent casting/particulate leaching, thermal-induced phaseseparation, and sponge replication. Instead of making well-designed scaffolds, traditional techniques are more likely toproduce scaffolds with random structures [5].

Behind this driving force, much attention has beenrecently drawn on solid free-forming (SFF) techniques.Unlike the traditional scaffold-manufacturing methods,SFF techniques are capable of creating scaffolds that arepredesigned by using 3D CAD/CAM (Computer-AidedDesign/Computer-Aided Manufacturing). It is possible tomake scaffolds with fully controlled geometries, and, con-sequently, predetermined properties. Another advantage ofusing SFF techniques is the high reproducibility, which

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exhibit great potential for clinical applications. Further-more a computer tomography scan of a defect site gener-ates computer data, from which scaffolds can be custom-manufactured in a reproducible manner and accurately rep-resent the defect shape [6].

e 3Dprinting (3DP) technique is one of themost inves-tigated SFF techniques in manufacturing scaffolds. 3D mod-els are printed from bottom to top in the powder bed. Plasterof Paris or calcium sulphate hemihydrate (CaSO4⋅1/2H2O)was one of the �rst materials to be used for 3DP. It canbe wetted by commercially formulated binder (98% contentwater), and then forms a gypsum paste (CaSO4⋅2H2O) byactivating self-hydration [7] though a chemical reaction (1)forming a gypsum paste (CaSO4⋅2H2O). e CaSO4-basedpowder is one of the few materials that are commerciallyavailable in the 3DP manufacturing. CaSO4 based materialshave previously been used to �ll the bone defects in non-load-bearing applications [8–10]. It has proved itself to be an effec-tive bone void �ller in both animal [11] and human studies[12–14]. CaSO4 implants have been shown not to increasethe extent of the in�ammation reaction a�er implantation[11]; they have the capability to provide a framework forosteoblast attachment and are readily resorbed by osteoclasts[15]. Currently, CaSO4-based materials are also used as anadditive for incorporation into bioceramics and biopolymersas a �ller component. Additionally the incorporation ofCaSO4 can assist in tailoring the degradation properties,increasing dimensional stability, and reducing cost [16].However, CaSO4-based biomaterials do not demonstratesufficientmechanical properties for the repair of load-bearingbone defects and also, due to its fast degradation rate, CaSO4quickly loses the bulk of its shape and mechanical propertiesin vivo. ese drawbacks inhibit the use of this material forbone augmentation following disease or trauma:

CaSO4 ⋅12H2O +

32H2O⟶ CaSO4 ⋅ 2H2O (1)

Dehydration of CaSO4 through heat treatment is themostcommon method to improve its properties. Lowmunkonget al. [17] reported that heat treatment could insolubilisegypsum block, thereby maintaining the structure whenimmersed in water. e mechanism of CaSO4 dehydrationhas been intensively reported [18, 19]. Water molecules inhydrous CaSO4 can be easily extracted during heat treatment.ree anhydrous species can be detected when CaSO4 issub�ected to different heat treatment regimes: the �rst anhy-drite, 𝛾𝛾-CaSO4, is formed between 130○C and 200○C. It iscalled “soluble anhydrite” because the 𝛾𝛾-CaSO4 retains highreactivity and it is able to rehydrate back to hemihydrateCaSO4 [19]. With the temperature further increasing, theCaSO4 material will continue its transformation to 𝛽𝛽-CaSO4and 𝛼𝛼-CaSO4 until the absolute melting temperature (∼1450○C) [19]. e anhydrite obtained at high temperaturehas lower reactivity than the 𝛾𝛾-CaSO4.

e purpose of this study was to develop CaSO4 scaffoldsusing the 3DP technique. Improvement in the mechanicalproperties and degradation pro�le of the resultant CaSO4scaffolds was investigated by utilising heat treatments atvarious conditions. e in�uence of heating temperature

on the dehydration process of CaSO4 was analysed usingthermal gravimetric analysis (TGA) and X-ray diffraction(XRD).

2. Materials andMethods

2.1. Materials. Calcium sulphate hemihydrate powder (ZP102, Z Corporation, UK) and water-based binder (ZB 7,Z Corporation, UK) were purchased and used in a 3Dprinter (Zcorp 310, Z Corporation, UK). e particle sizeand particle size distribution of 3DP CaSO4 powders weredetermined using a two laser Sympatec HELOS/BF ParticleSizer (Sympatec Limited, UK). e powder was scannedin triplicate to obtain the average of 𝐷𝐷10, 𝐷𝐷50, and 𝐷𝐷90values, which represent 10%, 50%, and 90% of the material,respectively, to have lower particle size than that value.

2.2. Scaffold Design and Manufacture. A porous cylinderstructure (diameter = 18.0mm and height = 13.2mm)(Figure 1) was designed. e pore channels were 100%interconnected, and they branched orthogonally to give 3Dporosity. e porous structure was supported by a peripheralsleeve. 3DP specimens with three different pore and strutsizes (0.8, 1.2, and 1.6mm) were initially manufactured todetermine the minimum feature size that could be producedconsistently.

e scaffold design with minimum size features wasconverted to an STL �le and imported to the 3D printer. e3D structure was sliced into 2D layers with layer thicknessof 0.1mm. During the 3DP process, the feed area was �rst�lled with calcium-sulphate hemihydrate powder and theroller spread a powder layer from the feed area to the buildarea (Figure 2). e print head deposited binder dropletsselectively within the build area. A�er the �rst layer was com-pletely built up, the roller returned to the feed area and thenspread another powder layer to the build area.is procedurewas repeated continuously and it took approximately 40minto construct the complete scaffold. e unbound powderswithin the structure acted temporarily as a support to thesurrounding bound powders. e un-bound powders werethen removed using compressed air, and the scaffolds driedat 73○C for 1 h.

2.3. Heat Treatment Protocol. Heat treatment of the hydrousCaSO4 was carried out in a furnace (BCF 11/8, EliteermalSystem Lt, UK). e 3DP scaffolds were heat treated atvarious temperatures ranging from 150○C to the Tammanntemperature (861○C). e Tammann temperature refers tothe sintering temperature, which is determined as half of theabsolute melting temperature. It has been reported that theTammann temperature of CaSO4 is 861○C [19]. However,when the heating temperature was increased above 250○C,the CaSO4 scaffold underwent signi�cant colour change(Figure 3). is colour change was not evident for the CaSO4scaffolds heat-treated at 861○C; however, large deformationof the scaffold structure was observed.

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Sleeve

1.2 mm

(a) (b)

F 1: (a) Schematic diagram showing 3D CAD scaffold design and (b) examples of the 3D printed CaSO4⋅2H2O scaffold. Scale bar =1 cm.

Roller

(a)

Print head

(b)

(c) (d)

F 2: Schematic diagram of the 3DP process. (a) e roller spreads one layer of powder from the feed area to the build area; (b) printhead selectively injects binder droplet on the powder bed; (c) aer printing a layer, the roller returns to the feed area; (d) powder in the feedarea is raised while that in the build area is lowered. e roller then spreads another layer of powder.

Temperatures of 150○C, 200○C, and 250○Cwere thereforeselected for further investigation. All heating processes com-menced at room temperature at a heating rate of 10○C/min.Two different dwell times at the target temperature (30minand 1 h) were also investigated. e effects of heatingtemperature and dwell time on the CaSO4 dehydration

process were then evaluated. 3DP scaffolds that had beenheat treated at various temperatures were crushed to pow-der form using a pestle and mortar for subsequent X-raydiffraction (XRD) analysis carried out using a Philips X’ PertPRO diffractometer (PANalytical UK, Cambridge, UK). eresults were analysed using the Phillips X’ Pert High Score

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Untreated 150◦C 200◦C 250◦C 500◦C 861◦C

F 3: Pictorial representation of 3D printed CaSO4-based scaffolds following different heat treatment protocols. e peripheral sleeveon parts (500○C and 861○C) was removed showing evidence of organic decomposition and large deformation. Scale bar = 1 cm.

Soware (PANalytical Ltd, UK). Mass loss of CaSO4 materialfollowing heat treatment was also determined using thermalgravimetric analysis (TGA). CaSO4 powder was placed inthe NETZSCH STA 449 C Jupiter apparatus (NETZSCH-GeratebauGmbH,Germany) and heated from25○C to 300○Cat a rate of 10○C/min, which corresponded to the full heattreatment process that scaffolds were subjected to.

Blocks of the same dimensions as the scaffolds (diameter= 18.0mm and height = 13.2mm) were manufactured using3DP. ey were then subjected to the same heat treatmentas the scaffolds. Mass loss of both blocks and scaffolds wasmeasured and then compared with the theoretical mass lossof complete dehydration from hydrous CaSO4 to anhydrite.

2.4. Compression Testing. Compressive properties of the scaf-folds were measured using a universal materials test system(EZ50, Lloyds Instruments, UK) with a 5 kN load cell and ata rate of displacement of 0.5mm/min. A total of four scaffoldswere tested for each condition. A specimen was determinedto have failed when the load in the post-peak region reducedto 80% of the peak load. e compressive strength wasde�ned as the maximum load recorded, divided by the initialcross-sectional area of the scaffold.e compressivemoduluswas determined by measuring the maximum slope of theelastic region of stress-versus-strain curve immediately fromthe toe-in region. Simpson’s Rule was used to calculate thecompressive toughness, which was denoted as the area undercompressive stress-versus-strain curve up to the point offailure.

2.5. Degradation Properties. e in vitro degradationproperties of the CaSO4 scaffolds before and aer heattreatment (i.e., sintering temperature of 200○C anddwell time of 30min) were investigated. Each scaffoldwas weighed and then immersed in pH 7.4 tris-HClbuffered solution (37○C, 100mL). Tris has a full nameof tris(hydroxymethyl)aminomethane and is a chemicalcompound that is regularly used as a buffer due to itslow cost and ability to maintain pH level between 7 and9 via the absorbance of counter ions (+H and −OH). Ateach predetermined time point (1, 2, 4, and 7 days), threescaffolds of each group were removed from the bufferedsolution. Dimensional changes were measured immediately

on removal. Scaffolds were rinsed with deionised water anddried in a 37○C oven for 48 h. Subsequently, the dry masswas measured to calculate the mass change before and aerimmersion in buffered solution. e compressive propertiesof each scaffold were determined as per Section 2.4.

2.6. Statistical Analysis. Data collected from all the experi-mental tests was evaluated for statistical signi�cance usinga one-way Analysis of Variance (ANOVA) followed by apost hoc Tukey’s HSD test for the comparison between eachgroup. A value of 𝑃𝑃 < 0.05 was considered to be signi�cantlydifferent. Data that was approximately normally distributedwas decided on basis of normal probability tests. Tests wereconducted using Minitab 14 student soware (Minitab, Inc.,USA) and SPSS 13.0 soware (SPSS, USA).

3. Results and Discussion

3.1. Scaffold Design and 3DP Powder Size. �ell-de�nedscaffolds were produced using the 3DP technique for eachof the pore and strut size con�gurations tested (i.e., 0.8,1.2 and 1.6mm). Aer manufacture, each of the scaffolddesigns possessed sufficient green strength to withstand theair-gun pressure during removal of the unbound powder.However, it was difficult to remove all the unbound powder.However, it was difficult to remove all the un-bound powderfrom the scaffold with the smallest feature size of 0.8mm.erefore, the minimum feature size that was chosen for thescaffold design was 1.2mm. Relationship between optimumscaffold pore size and cell activity has always been a con�ictissue in the literature [20]. Big pores (>0.5mm) favourfast vascularisation but also decrease speci�c surface arealimiting cell attachment [21]. is represents a potential lim-itation of 3DP technology due to the difficulty in removingunbound powder from the small cavities within the scaffoldfollowing manufacture. Plasma treatment of the powderparticles may offer a potential solution to enhance powder�owability [22]. Depowdering could also be more effectiveif the powder is preheated to remove its moisture effects.Powder demonstrating a relatively low particle size has theadvantage of being easily removed, but it has the propensity toagglomerate in the powder bed [22]. e commercial ZP102powder was processed to an appropriate powder particle

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Average result (µm)

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F 4: Particle size and particle size distribution of CaSO4⋅1/2H2O powder. ∗q3lg/% is the unit standing for logarithm of percentage oftotal particles.

10 20 30 40 50 60

2θ

Untreated

150◦C

200◦C

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F 5: XRD patterns for untreated CaSO4⋅2H2O and CaSO4, fol-lowing heat treatment at different temperatures. ▽ = CaSO4⋅2H2O,⇓ = CaSO4, ◻ = CaCO3, ⚬ = CaC2O4⋅H2O.

size (𝐷𝐷10 = 28.1 μm, 𝐷𝐷50 = 67.8 μm, Figure 4) for 3DP,therefore avoiding agglomeration. Additionally this powderparticle size limited the minimum feature size, especiallyin the case of manufacturing complex cellular structure inthis study. Powder particle size also has an in�uence on thelayer thickness that can be achieved. in powder layersare preferable as a relatively higher level resolution can beachieved, but it is also suggested that layers should be thickerthan the largest particle size of the powder. Taking intoconsiderations all the necessary factors, 100 μm was chosenas the layer thickness for this study as powder particles beingused had a𝐷𝐷90 = 101.8 μm.

90

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25 50 75 100 125 150 175 200 225 250 275 300

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F 6: TGA curve of the CaSO4⋅1/2H2Opowder. stage (1) (25○Cto ∼95○C): early dehydration to lose zeolitic water; stage (2) (∼95○Cto ∼200○C): transformation from CaSO4⋅2H2O to 𝛾𝛾-CaSO4; stage(3) (∼200○C to ∼250○C): no phase transformation; stage (4) furtherdehydration.

3.2. Effects of Heat Treatment on CaSO4 Dehydration. ecommercially available 3DP powder consisted of mainlycalcium sulphate semihydrate and small amounts of water-soluble organic additives, which assist in binding the powderparticles during printing. e presence of these additiveshas the potential to limit the temperature which may beapplied to the scaffolds during heat treatment, as decompo-sition may occur. e colour change of the scaffolds, whichwas evident aer heat treatment suggested the presence oforganic components in the commercial ZP102 powder usedin this study. Previous studies have used maltodextrin andsugar to bind the 3DP plaster powder during 3DP (Plaster

6 ISRN Biomaterials

0

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Block Scaffold

Mas

s lo

ss (

%)

†

F 7: Percentage of mass loss (Mean ± SD) of 3DP blocks andscaffolds following heat treatment at 200○C for 30min. e upperline represents the theoretical mass loss of a complete conversionfrom CaSO4⋅2H2O to CaSO4. e lower line represents that of acomplete conversion from CaSO4⋅1/2H2O to CaSO4. †𝑃𝑃 > 0.05.

Powder V2, online, 26 May 2012), which can result in the�nal printed samples undergoing signi�cant decompositionwhen heated above 250○C. However, the exact additivesused in commercial 3DP powder remain unknown due tocommercial secrecy. Some researchers have suggested thecolour change observed aer heat treatment may be relatedto heated glycerine, which has been detected in commercialbinder formulae [17]. In this study, the organic componentsunderwent signi�cant decomposition when the scaffoldswere exposed to heat treatment temperatures greater than250○C. In order to reduce the extent of organic decomposi-tion, the temperature used during the heat treatment processwas not raised above 250○C in this study.

XRD analysis of the scaffolds prior to heat treatmentshowed peaks characteristic of dihydrate CaSO4 (Figure5). is showed that during the 3DP process the hemi-hydrate CaSO4 plaster powders reacted with the water-based binder and converted to the dihydrate species.Following heat treatment at 150○C, a substantial por-tion of the dihydrate CaSO4 peaks disappeared. How-ever, the dehydration process was not complete at 150○C,which was indicated by the presence of some attenu-ated peaks of dihydrate CaSO4. All characteristic peaksfor dihydrate CaSO4 disappeared when the temperaturewas increased to 200○C, which was indicative of fullextraction of water molecules from hydrous CaSO4 pow-der. No distinct differences were observed between theXRD patterns for the powders heat treated at 200○C and250○C.

e CaSO4 dehydration initiated from the start of theheating cycle, as shown by TGA (Figure 6). It was observedduring the �rst stage of dehydration, that the rate of massloss was relatively low. is early dehydration was dueto the loss of loosely held zeolitic water before the laterloss of lattice water [23]. is corresponded with otherstudies in which CaSO4 dehydration was slow below 95○Cand then accelerated between 95○C and 170○C [24]. Inthis study, a signi�cant dehydration process was observedduring the same temperature range (95○C–170○C). e

change in mass related to the conversion of CaSO4⋅2H2Oto 𝛽𝛽-CaSO4⋅1/2H2O and 𝛽𝛽-CaSO4⋅1/2H2O to 𝛾𝛾-CaSO4 [23].e �rst anhydrous species obtained during the heatingcycle was 𝛾𝛾-CaSO4. e complete formation of 𝛾𝛾-CaSO4was evident at 196.9○C. e TGA curve started to plateauat this temperature. e TGA results supported the XRDpatterns, which showed that the complete dehydration of theCaSO4 was obtained at approximately 200○C. No signi�canttransformation was found during the third stage (200○C to250○C), as only a 0.3% mass loss was recorded. Conversionfrom 𝛾𝛾-CaSO4 to other anhydrous species started fromapproximately 250○C. Mass loss above 250○C could also berelated to the decomposition of organic additives in thecommercial 3DP powder.

XRD and TGA showed the CaSO4 dehydration wascompleted at approximately 200○C. Following heat treatmentat 200○C for 30min, mass loss of 3DP blocks was 11.4% of theoriginal mass (Figure 7). e theoretical mass loss of a com-plete conversion from CaSO4⋅1/2H2O and CaSO4⋅2H2O toCaSO4 is approximately 5% and 20%, respectively.erefore,CaSO4⋅1/2H2O powder was partially wetted by the water-based binder during the 3DP process. 3DP parts should havea chemical formula of CaSO4⋅nH2O (0.5 < 𝑛𝑛 < 2). However,it was difficult to calculate a precise value of n, due to asmall portion of additives in the commercial CaSO4⋅1/2H2Opowder. It has been reported that impurities may have asigni�cant effect on the mass loss value [25]. is result wasadaptive to 3DP scaffolds, because there was no signi�cantdifference in the mass loss between blocks and scaffolds (𝑃𝑃-value > 0.05).

3.3. Effects of Heat Treatment on Compressive Properties.CaSO4⋅nH2O scaffolds not subjected to heat treatmentdemonstrated low compressive strength, compressive modu-lus, and toughness (Figure 8).e compressive strength (0.36± 0.13MPa), for example, was signi�cantly lower than thatreported for cancellous bone (4–12MPa) [3]. It was reportedthat tensile strength, elongation, and notched impact strengthwere decreasedwhenP�Awas highly �lledwith gypsum [16].

Following heat treatment, the compressive properties ofthe CaSO4 scaffolds dramatically increased. e compressivestrength increased signi�cantly from 0.36 ± 0.13MPa (pre-heat treated) to 2.49 ± 0.42MPa (heat treated at 200○C for30min), and the compressive modulus was also signi�cantlyincreased from 4.98 ± 1.17MPa (pre-heat treated) to 28.81± 3.07MPa (heat treated at 200○C for 30min) (𝑃𝑃 value <0.05). Improvements in compressive properties indicated theanhydrous CaSO4 showed greater mechanical performancewhen compared to its hydrous counterpart. As a resultof dehydration, the �rst anhydrite was converted between90○C and 200○C, which contributed to the improvement inmechanical properties.

Observing the compressive stress-versus-strain curves(Figure 9), scaffolds initially underwent elastic displacementfollowed by permanent plastic deformation that was causedby the generation of failed struts and microcracks withinthe periphery sleeve of the scaffold. Heat treatment of thescaffolds at 200○C resulted in an increase in compressive

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F 8: Compressive strength (a), compressive modulus (b), and compressive toughness (c) (Mean ± SD) for CaSO4⋅2H2O and CaSO4scaffolds following different heat treatment protocols. ∗𝑃𝑃 < 0.05; ∗∗𝑃𝑃 < 0.001.

modulus, and the plastic region was extended, suggestinghigher toughness. Conversely the scaffolds that underwentheat treatment at 250○C demonstrated a lower level of plasticdeformation and typically failed shortly aer reaching thepeak load. erefore, a strong correlation was obtainedbetween the degree of dehydration and the mechanical prop-erties. e greatest improvements in compressive strength(692%), compressivemodulus (579%), and toughness (700%)were all obtained when scaffolds were heated at 200○Cfor 30min. XRD and TGA characterisation showed thatapproximately 200○Cwas the temperature point whenCaSO4reached complete dehydration. However, when the heattreatment temperature was increased from 200○C to 250○C,a downward trend in compressive properties was observed.It is postulated that the reduction in compressive proper-ties was due in part to organic additive decomposition attemperatures of ≥250○C. e 3DP scaffolds heat treated at250○C demonstrated a signi�cant reduction in toughnessand failed in a more brittle manner than scaffolds heattreated at lower temperatures (Figure 8). e duration ofthe heat treatment cycle also played an important role. In

comparison to a heat treatment cycle of 1 h, heat treatment for30min showed superior compressive properties. erefore,the degree of organic decomposition was both tempera-ture and time dependent. erefore, overheating the CaSO4should be prevented when deciding the optimum heatingtemperature and dwell time. Highest compressive propertiesfor CaSO4 scaffolds were achieved when a heat treatmentprotocol of 200○C for 30min was followed.

3.4. Effects of Heat Treatment on Degradation Properties.Immersion of the CaSO4⋅nH2O scaffold in tris-bufferedsolution resulted in complete dissolution in less than 10min(Figure 10). Previous studies showed that CaSO4⋅nH2O hasa rapid solubility rate [17]. Due to the rapid degradation ofthe pre-heat-treated CaSO4⋅nH2O scaffolds, the compressiveproperties of this group could not be determined.

e degradation properties of heat-treated scaffolds(200○C for 30min) were also determined. XRD and TGAcharacterisation showed the gypsum completely lost itswater molecules and converted to the anhydrate species at

8 ISRN Biomaterials

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F 9: Compressive stress-versus-stain curves of CaSO4⋅2H2Oand CaSO4 scaffolds following heat treatment at different tempera-tures. Heat treatment cycle was 0.5min.

Untreated

(<10 min) (Day 1) (Day 2) (Day 4)

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F 10: Pictorial representation of CaSO4-based scaffolds beforeand aer in vitro degradation at different time points. NoteCaSO4⋅1/2H2O scaffold disintegrated rapidly in vitro, as shown onthe top le. Scale bar = 1 cm.

approximately 200○C. However, this dehydration process isreversible, that is, rehydration can occur.

Gypsum (CaSO4⋅2H2O) = Hemihydrate (CaSO4⋅0.5H2O) = Soluble Anhydrite (CaSO4) [18].

It was observed that heat-treated scaffolds swelled whenimmersed in tris-buffered solution (Figure 10). e averageheight increased by 25.0% (Day 1) and 28.8% (Day 2), andthe average diameter increased by 17.2% (Day 1) and 21.7%(Day 2) (Figure 11). Dimensional expansion of the scaffoldswas related to the reconversion of soluble anhydrite tohydrous CaSO4. e material increased in mass throughoutthe rehydration process, as con�rmed by marginal increasein the dry mass for Day 1 (0.8%) and Day 2 (0.9%) (Fig-ure 11), thereby making it difficult to determine the rateof degradation. e scaffolds subjected to heat treatmentmaintained a large portion of their structure during the �rsttwo days of degradation (Figure 10). Due to this rehydrationprocess, it was difficult to determine the degradation rateof the scaffolds. Despite the mass increase, mechanicalproperties were decreased. e heat-treated CaSO4 scaffolds

demonstrated compressive properties on Day 2, which weregreater than half the level recorded at Day 0 (Figure 12).is was a signi�cant improvement when compared to thescaffolds that were not subjected to a heat treatment regime(𝑃𝑃 value < 0.05).

Heat-treated CaSO4 scaffolds underwent in vitro degra-dation from outer surface inwards, causing greater deforma-tion of the peripheral sleeve of the scaffold structure. OnDay 2, small notches were observed on the outer sleeve ofthe CaSO4 scaffolds, suggesting that the dissolution processhad started. Subsequently, the heat-treated CaSO4 scaffoldsshowed a signi�cant reduction in structural integrity betweenDay 2 and Day 4. Consequently, large variations and reduc-tions in the dimensional, mass and compressive propertieswere recorded at Day 4. Between Day 4 and Day 7, the CaSO4scaffolds underwent further dissolution and degradation,making it very difficult to obtain any quantitative data beyondDay 4.

3.5. Limitation and Future Work. Despite the moderateimprovements in both compressive and in vitro degradationproperties of 3DP CaSO4-based scaffolds that have beenrecorded in this study, limitations have also been noted.Firstly, both the compressive and in vitro degradation proper-ties did not meet the requirements for scaffolds used in bonetissue engineering. It has been suggested that mechanicalproperties of bone scaffolds shouldmimic those of cancellousbone, demonstrating a compressive strength of between 4 and12MPa [3]. e degradation rate of the scaffolds should alsocoincide with the rate of new bone formation, which typicallytakes several months to complete for bone tissue. erefore,until these properties can be substantially improved byusing alternative strategies, further research efforts in thesame direction should be pursued with caution. Potentialalternative approaches may include the incorporation of abioresorbable polymer (polylactic acid or polycaprolactone)with the CaSO4 matrix in�ltration within the structure orcoating of the surface. Secondly, the commercial 3DP plasterpowder used contains a relatively small amount of organicadditive, which may pose a biocompatibility issue since thecomposition of this material remains unknown. Future stud-ies should consider using more pure CaSO4 material. Sincethe absence of additive may reduce the reactivity betweenpowder and binder, the incorporation of a FDA-approvalmaterial (e.g., maltodextrin) into pure CaSO4 may prove aviable alternative. Finally, the application of a water-solublepolymer may be worth considering as a potential bindingagent, as they can be burned away at high temperature.

4. Conclusion

is study has shown that the application of a suitable heattreatment protocol can improve the mechanical and in vitrodegradation properties of 3DP CaSO4-based scaffolds. eaugmented properties were a consequence of the dehydrationprocess that occurred during heat-treatment. Notwithstand-ing this fact, the heat treated CaSO4 scaffolds demon-strated moderate improvements in compressive properties

ISRN Biomaterials 9

12

14

16

18

20

22

24

12

13

14

15

16

17

18

19

20

D0 D1 D2 D4

Dia

met

er (

mm

)

Hei

ght

(mm

)

Height

Diameter

Time in tris-HCl buffer solution (day)

(a)

0.85

0.9

0.95

1

1.05

1.1

1.15

Mas

s ch

ange

D1 D2 D4


(b)

F 11: Dimensional changes (a) and mass changes (b) (Mean ± SD) of heat-treated CaSO4 scaffolds aer 1, 2, and 4 days immersion intris-buffered solution. Mass change: dry mass aer degradation/dry mass before degradation.

0

0.5

1

1.5

2

2.5

3

3.5

Co

mp

ress

ive

stre

ngt

h (

MP

a)

∗∗

∗

∗

D0 D1 D2 D4


F 12: Compressive strength (Mean ± SD) of heat-treatedCaSO4 scaffolds aer 0, 1, 2, and 4 days immersion in tris-bufferedsolution. ∗𝑃𝑃 < 0.05; ∗∗𝑃𝑃 < 0.001.

and degradation rate that are considered not appropriate forbone-tissue-engineering applications. Overall, the �ndingsof this study highlighted not only the potential possibilityof using 3DP CaSO4-based scaffolds but also the perceivedlimitations. Alternative strategies have been suggested in aneffort to improve the manufacture and applicability of 3DPCaSO4-based bone scaffolds.

Disclosure

e authors of this paper do not have direct �nancialrelationships or agreements with the commercial companiesdetailed in this paper. erefore, no con�ict of interests forany of the authors exists regarding the content of this paper.

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