Abstract
3D-printed bone scaffolds hold great promise for the individualized treatment of critical-size bone defects. Among the resorbable polymers available for use as 3D-printable scaffold materials, poly(ε-caprolactone) (PCL) has many benefits. However, its relatively low stiffness and lack of bioactivity limit its use in load-bearing bone scaffolds. This study tests the hypothesis that surface-oxidized cellulose nanocrystals (SO-CNCs), decorated with carboxyl groups, can act as multi-functional scaffold additives that (1) improve the mechanical properties of PCL and (2) induce biomineral formation upon PCL resorption. To this end, an in vitro biomineralization study was performed to assess the ability of SO-CNCs to induce the formation of calcium phosphate minerals. In addition, PCL nanocomposites containing different amounts of SO-CNCs (1, 2, 3, 5, and 10 wt%) were prepared using melt compounding extrusion and characterized in terms of Young's modulus, ultimate tensile strength, crystallinity, thermal transitions, and water contact angle. Neither sulfuric acid-hydrolyzed CNCs (SH-CNCs) nor SO-CNCs were toxic to MC3T3 preosteoblasts during a 24 h exposure at concentrations ranging from 0.25 to 3.0 mg/mL. SO-CNCs were more effective at inducing mineral formation than SH-CNCs in simulated body fluid (1x). An SO-CNC content of 10 wt% in the PCL matrix caused a more than 2-fold increase in Young's modulus (stiffness) and a more than 60% increase in ultimate tensile strength. The matrix glass transition and melting temperatures were not affected by the SO-CNCs but the crystallization temperature increased by about 5.5°C upon addition of 10 wt% SO-CNCs, the matrix crystallinity decreased from about 43 to about 40%, and the water contact angle decreased from 87 to 82.6°. The abilities of SO-CNCs to induce calcium phosphate mineral formation and increase the Young's modulus of PCL render them attractive for applications as multi-functional nanoscale additives in PCL-based bone scaffolds.
Highlights
Bone is one of a few organs in the body that have the ability to self-regenerate without scar tissue formation following injury
The SO-CNCs had no effect on PCL glass transition and melting temperature but increased the crystallization temperature and hydrophilicity of the nanocomposites
The effectiveness of SO-CNCs to enhance the mechanical properties of PCL and induce the deposition of a calcium-phosphate layer on their surface makes them interesting multi-functional additives for resorbable polymer-based bone scaffolds
Summary
Bone is one of a few organs in the body that have the ability to self-regenerate without scar tissue formation following injury. Critical-sized defects typically need to be repaired with bone grafts, such as autografts, allografts, or xenografts, or with metal or ceramic implants. Bioactive Cellulose Nanocrystal Nanocomposites have several limitations including donor site morbidity, donor bone supply shortage, infection, corrosion, stress shielding, and secondary surgery. Compared to ceramics and metals, bioresorbable polymers have several inherent benefits for bone scaffold applications, including 3D printability and biodegradability. Their inferior strength, stiffness, and bioactivity present critical challenges that need to be addressed. A mechanical mismatch between the scaffold and surrounding bone tissue could result in bone resorption through a stress shielding effect or failure of the scaffolds at the defect sites (Liu et al, 2006; Sultana, 2013)
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