Pilot Study: Evaluation of Shape Memory Polymer Use in Orthodontic Archwires

Objectives: The objective of this pilot study was to evaluate the mechanical properties of shape memory polymer (SMP) for use in orthodontic archwires. This pilot data was also used to theoretically predict the effect that Cellulose Nanocrystals (CNC) would have on material properties.
Methods: A biocompatible polyurethane-based SMP was extruded through a die measuring 0.019inX0.025in (0.48mmX0.64mm) to replicate a common size orthodontic archwire. After extrusion, SMP wire was cut into 10 separate 4in segments (101.6mm) and a micrometer was used to measure the cross-sectional dimension of each wire at 10 different locations. The cross-sectional measurements were used to calculate the average cross-sectional area for each individual wire, allowing for engineering stress calculations. Sample wires were then individually placed into a uniaxial tensile tester at a temperature of 37°C and displaced until the point of failure. Elongation of SMP wire in response to the force applied was recorded, which was then used to mathematically determine stress and strain, respectively. Stress versus strain curves were subsequently plotted and utilized to determine the modulus of elasticity for each wire. A sample size of n=10 wires was tested. Rule-of-mixtures calculations were then used to determine theoretical values for elastic moduli at CNC volume fractions of 0.02, 0.03, 0.04 and 0.05.
Results: The average modulus of elasticity for each wire was 1156.372MPa and ranged from 718.32MPa-2166.5MPA (SD= 473.649MPa). According to calculations, incorporating CNC into wire samples at volume fractions 0.02, 0.03, 0.04, and 0.05 increased the modulus of elasticity to 1573.021MPa (SD= 662.893MPa), 1786.943MPa (SD=740.170MPa), 2004.771MPA (SD=818.55MPa), and 2226.561MPa (SD=898.119MPa), respectively.
Conclusions: Results from this study create optimism for the use of polyurethane-based archwires in orthodontic practice. Moreover, the incorporation of CNC particles into archwires could improve their mechanical properties. Future studies will focus on the effect of implementing CNC particles into orthodontic archwires.