bioengineering Article Effects of Sterilization Cycles on PEEK for Medical Device Application Amit Kumar ID , Wai Teng Yap, Soo Leong Foo and Teck Kheng Lee * College Central, Institute of Technical Education, 2 Ang Mo Kio Drive, Singapore 567720, Singapore; [email protected] (A.K.); [email protected] (W.T.Y.); [email protected] (S.L.F.) * Correspondence: [email protected]; Tel.: +65-6580 5007 Received: 19 January 2018; Accepted: 14 February 2018; Published: 21 February 2018 Abstract: The effects of the sterilization process have been studied on medical grade thermoplastic polyetheretherketone (PEEK). For a reusable medical device, material reliability is an important parameter to decide its lifetime, as it will be subjected to the continuous steam sterilization process. A spring nature, clip component was selected out of a newly designed medical device (patented) to perform this reliability study. This clip component was sterilized for a predetermined number of cycles (2, 4, 6, 8, 10, 20 ... 100) at 121 ◦C for 30 min. A significant decrease of ~20% in the compression force of the spring was observed after 30 cycles, and a ~6% decrease in the lateral dimension of the clip was observed after 50 cycles. No further significant change in the compression force or dimension was observed for the subsequent sterilization cycles. Vickers hardness and differential scanning calorimetry (DSC) techniques were used to characterize the effects of sterilization. DSC results exhibited no significant change in the degree of cure and melting behavior of PEEK before and after the sterilization. Hardness measurement exhibited an increase of ~49% in hardness after just 20 cycles. When an unsterilized sample was heated for repetitive cycles without the presence of moisture (121 ◦C, 10 and 20 cycles), only ~7% of the maximum change in hardness was observed. Keywords: polyetheretherketone; PEEK; DSC; Vickers hardness; medical grade; medical device 1. Introduction In the late 1990s, polyetheretherketone (PEEK) was initially developed as a high-performance thermoplastic to replace metal-based orthopedic [1,2] and trauma [3,4] implants. Invibio Ltd. (Thornton Cleveleys, United Kingdom) first offered PEEK commercially as a biomaterial for implant applications in 1998, and since then research on PEEK as a biomaterial has enhanced significantly [5]. For implant applications, many PEEK composite materials have been developed, which have been studied for their behavior under mechanical impact, biotribology, friction, dynamic, damage, and fracture [6–10]. A thermomechanical study performed on PEEK composite [11,12] reported the changes in crystallinity, macroscopic decoloration, large deformation in impact, high strain rate, and heating-induced deformation. As all implants were subjected to body temperature, which is significantly below the glass transition temperature of PEEK, no significant change in elastic properties of PEEK was observed [13]. However, it was reported that the yielding and plastic flow behavior of PEEK was affected at physiological temperatures [12,14]. For a medical device, which will be used in surgery, the steam sterilization process is mandatory to properly disinfect the device before its next use. The influence of the sterilization process on the micromechanical properties of carbon fiber-reinforced PEEK has been studied [15] for bone implant applications. It was reported that after 3 steam sterilization cycles, there was no significant change in elastic modulus, hardness, or coefficient of friction of carbon composite PEEK. The influences of thermal cycling (temperature range between +60 ◦C and −60 ◦C, 750 cycles) on the fatigue behavior Bioengineering 2018, 5, 18; doi:10.3390/bioengineering5010018 www.mdpi.com/journal/bioengineering Bioengineering 2017, 4, x FOR PEER REVIEW 2 of 10 Bioengineering 2018, 5, 18 2 of 10 of carbon/PEEK laminates were also studied [16]. No significant change in tensile properties was ofobserved, carbon/PEEK but a decrease laminates of were 25% alsoin fatigue studied streng [16].th No was significant observed. change In another in tensile study, properties the fatigue was observed,performance but of a decreasePEEK was of 25%investigated in fatigue under strength sterilization was observed. and thermal In another aging study, cycles the and fatigue no performancesignificant change of PEEK in fatigue was investigated performance under of PEEK sterilization was found and thermal [17]. Most aging of these cycles studies and no performed significant changeon PEEK in fatigueare basically performance for implant of PEEK applications, was found low [17 ].temperature Most of these thermal studies cy performedcling fatigue, on PEEKor to areinvestigate basically the for implantbehavior applications, after only lowa few temperature cycles of thermal sterilization. cycling However, fatigue, or PEEK to investigate should thebe behaviorinvestigated after for only a higher a few number cycles of of sterilization. sterilization However,cycles in order PEEK to should test its bereliability investigated in surgical for a higherdevice numberapplications. of sterilization cycles in order to test its reliability in surgical device applications. InIn thisthis work, thethe thermal reliabilityreliability ofof medicalmedical gradegrade PEEKPEEK (natural)(natural) waswas investigatedinvestigated forfor itsits useuse inin surgicalsurgical devicedevice applications.applications. Although there ar aree many available medical grade plastic materials, such asas PTFEPTFE (polytetrafluoroethylene),(polytetrafluoroethylene), PEEK,PEEK, PCPC (polycarbonate),(polycarbonate), PSPS (polysulfone),(polysulfone), and PVDFPVDF (polyvinylidene(polyvinylidene fluoride),fluoride), PEEK presents the bestbest combinationcombination of mechanicalmechanical (stiffness, hardness, and wear)wear) andand thermalthermal (high(high glassglass transitiontransition andand meltingmelting temperature)temperature) properties required for thethe dimensional and thermal stability of a medicalmedical devicedevice [[18].18]. An importantimportant reason toto choosechoose naturalnatural PEEK isis itsits radiolucent radiolucent property property [13 [13],], which which makes makes it transparent it transparent to radiography. to radiography. If any If surgical any surgical device isdevice fabricated is fabricated using natural using natural PEEK, thenPEEK, the then real-time the real-time monitoring monitoring of surgery of surgery is possible, is possible, as X-ray as can X- clearlyray can see clearly through see through this material. this material. 2. Materials and Methods InIn thisthis work,work, two two different different medical medical grade grade PEEK PEEK materials materials (SUSTAPEEK (SUSTAPEEK MG MG NAT NAT 1000X 1000X'-12 ϕ mm-12 andmm 'and-50 mmϕ-50 from mm Roechling,from Roechling, Germany Germany and TECAPEEK and TECAPEEK MT natural MT rod,natural'-50mm rod, fromϕ-50mm Ensinger, from Germany)Ensinger, Germany) were used were to fabricate used to samplesfabricate forsamples thermal for reliability thermal reliability studies. Bothstudies. materials Both materials were of medicalwere of grademedical natural grade PEEK, natural having PEEK, nearly having similar nearly mechanical similar mechanical and thermal and properties, thermal properties, except the waterexcept absorption the water absorption value which value was which 0.2% forwas Roechling 0.2% for Roechling PEEK and PEEK 0.02% and for Ensinger0.02% for PEEKEnsinger in 24 PEEK h at 23in 24◦C[ h19 at, 2023]. °C [19,20]. 2.1. Specimen Preparation Figure1 1aa shows shows a a picturepicture of of thethe spring-typespring-type clip clip component component part part of of a amedical medicaldevice device[ [21],21], which isis manufacturedmanufactured from from Roechling Roechling PEEK PEEK using using the the machining machining process. process. Figure Figure1b shows1b shows the imagethe image of test of samplestest samples used used for the for hardness the hardness and differential and differential scanning scanning calorimetry calorimetry (DSC) characterization,(DSC) characterization, which arewhich also are manufactured also manufactured by the machining by the machining process using process PEEK using materials PEEK procured materials from procured Roechling from and Ensinger.Roechling Fifteenand Ensinger. samples, Fifteen each 20samples,× 10 × each4 mm 20 in × size,10 × were4 mm fabricated in size, were using fabricated two different using sizes two ofdifferent cylindrical sizes rod, of cylindrical with 12 mm rod, and with 50 mm 12 diameters,mm and 50 to investigatemm diameters, the effectto in ofvestigate primary the processing effect of conditionsprimary processing on the thermal conditions cyclic on performance the thermal of cyclic PEEK performance during sterilization. of PEEK during sterilization. Figure 1. (a) Spring-type clip component of a medical device and (b) fabricated test specimens for Figure 1. (a) Spring-type clip component of a medical device and (b) fabricated test specimens for hardness and differential scanning calorimetry (DSC) characterization. hardness and differential scanning calorimetry (DSC) characterization. 2.2. Autoclave Sterilization 2.2. Autoclave Sterilization A pressure cooker tester (PC-242HS, HIRAYAMA, Tokyo, Japan) autoclave was used to perform A pressure cooker tester (PC-242HS, HIRAYAMA, Tokyo, Japan) autoclave was used to perform the sterilization test on the clip component and test samples. Samples were heated up to 121 °C for the sterilization