Effects of the mold temperature on the mechanical properties and crystallinity of hydroxyapatite whisker‐reinforced polyetheretherketone scaffolds

Abstract

Porous and bioactive polyetheretherketone (PEEK) scaffolds have potential to replace metallic scaffolds for biologic fixation of permanent implants adjacent to trabecular bone, such as interbody spinal fusion devices. The objective of this study was to investigate the effects of the mold temperature and PEEK powder on the mechanical properties and crystallinity of hydroxyapatite (HA) whisker-reinforced PEEK scaffolds prepared using compression molding and porogen leaching. Scaffolds were prepared at mold temperatures ranging 340-390°C with a 50 or 10 μm PEEK powder, 75 vol% porosity, and 20 vol% HA whiskers. Scaffold mechanical properties were evaluated in unconfined, uniaxial compression and the PEEK matrix crystallinity was measured using specular reflectance Fourier transform infrared spectroscopy. Increased mold temperature resulted in increased compressive modulus, yield strength, and yield strain, reaching a plateau at ~370°C. HA reinforcements were observed to be segregated between PEEK particles, which inhibited PEEK particle coalescence during compression molding at temperatures less than 365°C but also ensured that bioactive HA reinforcements were exposed on scaffold strut surfaces. Increased mold temperature also resulted in decreased PEEK crystallinity, particularly for scaffolds molded at greater than 375°C. The PEEK powder size exhibited relatively minor effects on the scaffold mechanical properties and PEEK crystallinity. Therefore, the results of this study suggested that HA-reinforced PEEK scaffolds should be compression molded at 370-375°C. The apparent compressive modulus, yield strength, and yield strain for scaffolds molded at 370-375°C was 75-92 MPa, 2.0-2.2 MPa, and 2.5-3.6%, respectively, which was within the range exhibited by human vertebral trabecular bone.

Characterization of New PEEK/HA Composites with 3D HA Network Fabricated by Extrusion Freeforming

“…Based on the reported positive effects of using high temperatures on both shear viscosity of PEEK and mechanical properties of the resulting compression molded parts [53,54] with no adverse effect on biocompatibility in vivo [54], a mold temperature of 400 °C was selected in this study. While mold temperatures of 400 °C guarantee minimum viscosity, caution must be exercised when working at temperature in excess of 380 °C as there is a risk of thermal oxidation [55]. Potential for oxidation can be decreased by a reduction in dwell time spent at the target temperature; such optimization of dwell time is crucial.…”

Fused Filament Fabrication of PEEK: A Review of Process-Structure-Property Relationships

“…During layer-by-layer manufacturing, residual stress is accumulated, leading to warping and interlayer delamination [78,79]. These problems are more important for additive manufacturing of semi-crystalline polymers with a high melting temperature such as PEEK and can significantly affect dimensional accuracy and mechanical properties [80][81][82]. In fact, the degree of crystalline perfection of PEEK is considerably influenced by the rate of cooling and thermal gradient during crystallisation from melt [83].…”

A novel bioactive PEEK/HA composite with controlled 3D interconnected HA network

“…Conrad et al [26] demonstrated that this increased mould temperature may also increase the compressive modulus, yield strength and strain of the traditional PEEK/HA composites. Working temperatures greater than 380℃ have been shown to reduce crystallinity, which is known to be beneficial for ductility and toughness [27,28] , with no adverse effect on biocompatibility in vivo [27] . Given the positive effects described on both shear viscosity and mechanical properties of the resulting PEEK samples, a mould temperature of 400℃ was selected for this study.…”