Christopher M. Yakacki

Modeling the relaxation mechanisms of amorphous shape memory polymers.

In this progress report, we review two common approaches to constitutive modeling of thermally activated shape memory polymers, then focus on a recent thermoviscoelastic model that incorporates the time-dependent effects of structural and stress relaxation mechanisms of amorphous networks. An extension of the model is presented that incorporates the effects of multiple discrete structural and stress relaxation processes to more accurately describe the time-dependent behavior. In addition, a procedure is developed to determine the model parameters from standard thermomechanical experiments. The thermoviscoelastic model was applied to simulate the unconstrained recovery response of a family of (meth)acrylate-based networks with different weight fractions of the crosslinking agent. Results showed significant improvement in predicting the temperature-dependent strain recovery response.

Shape-Memory Polymers for Biomedical Applications

Shape-memory polymers (SMPs) are a class of mechanically functional “smart” materials that have generated substantial interest for biomedical applications. SMPs offer the ability to promote minimally invasive surgery, provide structural support, exert stabilizing forces, elute therapeutic agents, and biodegrade. This review focuses on several areas of biomedicine including vascular, orthopedic, and neuronal applications with respect to the progress and potential for SMPs to improve the standard of treatment in these areas. Fundamental studies on proposed biomedical SMP systems are discussed with regards to biodegradability, tailorability, sterilization, and biocompatibility. Lastly, a proposed research and development pathway for SMP-based biomedical devices is proposed based on trends in the recent literature.

Tailorable and programmable liquid-crystalline elastomers using a two-stage thiol–acrylate reaction

This study introduces an unexplored method to synthesize and program liquid-crystalline elastomers (LCEs) based on a two-stage thiol–acrylate Michael addition and photopolymerization (TAMAP) reaction. This methodology can be used to program permanently-aligned monodomain samples capable of “hands-free” shape switching as well as offer spatio-temporal control over liquid-crystalline behaviour. LCE networks were shown to have a cytocompatible response at both stages of the reaction.

Pullout strength of suture anchors: Effect of mechanical properties of trabecular bone

This study investigated the relationships between trabecular microstructure and elastic modulus, compressive strength, and suture anchor pullout strength. Twelve fresh-frozen humeri underwent mechanical testing followed by micro-computed tomography (microCT). Either compression testing of cylindrical bone samples or pullout testing using an Arthrex 5mm Corkscrew was performed in synthetic sawbone or at specific locations in the humerus such as the greater tuberosity, lesser tuberosity, and humeral head. Synthetic sawbone underwent identical mechanical testing and microCT analysis. Bone volume fraction (BVF), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), and connectivity density were compared against modulus, compressive strength, and pullout strength in both materials. In cadaveric bone, modulus showed correlations to all of the microstructural properties, while compressive and pullout strength were only correlated to BVF, SMI, and TbSp. The microstructure of synthetic bone differed from cadaveric bone as SMI and TbTh showed little variation across the densities tested. Therefore, SMI and TbTh were the only microstructural properties that did not show correlations to the mechanical properties tested in synthetic bone. This study helps identify key microstructure-property relationships in cadaveric and synthetic bone as well as illustrate the similarities and differences between cadaveric and synthetic bone as biomechanical test materials.

The effect of the trabecular microstructure on the pullout strength of suture anchors

This study investigates how the microstructural properties of trabecular bone affect suture anchor performance. Seven fresh-frozen humeri were tested for pullout strength with a 5mm Arthrex Corkscrew in the greater tuberosity, lesser tuberosity, and humeral head. Micro-computed tomography analysis was performed in the three regions of interest directly adjacent to individual pullout experiments. The morphometric properties of bone mineral density (BMD), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbS), trabecular number (TbN), and connectivity density were compared against suture anchor pullout strength. BMD (r=0.64), SMI (r=-0.81), and TbTh (r=0.71) showed linear correlations to the pullout strength of the suture anchor with p-values<0.0001. A predictive model was developed to explain the variances in the individual BMD, SMI, and TbTh correlations. The multi-variant model of pullout strength showed a stronger relationship (r=0.86) compared to the individual experimental results. This study helps confirm BMD is a major influence on the pullout strength of suture anchors, but also illustrates the importance of local microstructure in pullout resistance of suture anchors.

Compression forces of internal and external ankle fixation devices with simulated bone resorption.

Background: Internal and external fixation techniques have been developed to provide rigidity and stability to a fusion site such as in tibiotalocalcaneal (TTC) arthrodesis. Compression of the fusion site plays an integral role in primary bone healing, though little work has been done to quantify the compressive force values of ankle fixation devices. Materials and Methods: Using synthetic and cadaveric bone models, a Newdeal/Integra PantaNail and DePuy VersaNail were tested as compressive intramedullary (IM) nails while an Encore True/Lok and an Ace-Fischer frame were tested as external fixators. Results: The PantaNail experienced maximum compressive loads of 1898 and 1255 N in synthetic and cadaveric constructs, respectively. The VersaNail experienced max compressive loads 388 N during installation. All IM nails tested experienced decreased compressive loads after removal of the external guide and instrumentation. The external fixators were loaded to approximately 1200 N in both synthetic and cadaveric constructs. The decrease in compressive load was recorded as a function of simulated fusion site bone resorption for all devices. The IM nails experienced a 90% reduction in load with less than 1 mm of resorption, while the external fixators held 50% load for over 4 mm of resorption. These data were verified using a simple constitutive model of IM nails and external fixators. Conclusion: Intramedullary nails are capable of generating compression, however, are unable to provide sustained compression for any considerable amount of resorption. External fixators are inherently capable of applying and sustaining greater amounts of compression. Clinical Relevance: Surgeons who perform TTC arthrodesis procedures should be aware of a devices ability to generate and sustain compression with respect to bone resorption.

Cytotoxicity and thermomechanical behavior of biomedical shape-memory polymer networks post-sterilization.

Shape-memory polymers (SMPs) are being increasingly proposed for use in biomedical devices. This paper investigates the cytotoxicity, surface characteristics and thermomechanics of two acrylate-based SMP networks as a function of sterilization using a minimal essential media elution test, FTIR-ATR and dynamic mechanical analysis (DMA). Networks sterilized by low-temperature plasma elicited a cytotoxic response and are shown to completely destroy the cell monolayer. FTIR-ATR analysis showed evidence of surface oxidation with an increase and broadening of the absorbance peak from approximately 3500 to 3100 cm(-1), which is associated with an increase in hydroxyl groups. DMA revealed small, but statistically significant, differences in reduction of the glass transition temperatures of both networks when sterilized with gamma irradiation. One network showed an increase in rubbery modulus, which is an indication of crosslink density, after gamma irradiation. Lastly, practical sterilization concerns of SMP devices are discussed in light of the different methods.

Effect of crosslinking and long-term storage on the shape-memory behavior of (meth)acrylate-based shape-memory polymers

This work highlights the free- and fixed-strain shape-memory response of amorphous (meth)acrylate-based shape-memory polymers as the level of crosslinking is varied from uncrosslinked to highly crosslinked (corresponding to a decrease in failure strains and overall increase in rubbery moduli and failure stresses). The effect of long-term storage on the free-strain shape-memory response is also considered. Tensile deformation levels during the shape-memory cycle are 90% of failure strain values to determine the full extent of free- and fixed-strain recovery behavior. All materials demonstrate full shape-recovery under free-strain conditions (material is unconstrained during recovery); however, total recoverable strains increase with decreasing crosslinking level, with uncrosslinked and lightly crosslinked materials recovering strains on the order of 3–10× that of moderately and highly crosslinked materials. In contrast, under fixed-strain conditions (material is fully constrained in the fixed shape during recovery), the magnitude of recovery stress generation increases with increasing crosslinking level, with highly crosslinked materials demonstrating recovery stress levels 4–20× that of lightly crosslinked and uncrosslinked materials. The ability to produce recovery stresses on par with those reached during deformation also increases with crosslinking level. Stored-shape fixation and free-strain recovery levels remain stable after long-term storage in the deformed temporary state at 20 °C; however, recovery onset temperatures increase (by up to 9 °C) with storage time spanning ∼1 year, as do rates of free-strain recovery (by up to 9×), due to physical aging. Results indicate that aging could potentially be used as a method for shape-memory response optimization.

Shape-Memory and Shape-Changing Polymers

This article is a perspective that introduces several review articles in the field of shape-memory and shape-changing polymers. It is intended to emphasize the versatility of these materials as active, smart polymers as well as highlight the potential impact of these materials. The biomedical device industry commonly attracts these materials; however, examples for non-biomedical applications, such as origami structures and their recent attention are also discussed.

Anterior cruciate ligament fixation: Is radial force a predictor of the pullout strength of soft-tissue interference devices?

BACKGROUNDnIn anterior cruciate ligament (ACL) reconstruction, an interference device achieves soft-tissue graft fixation by radially compressing the graft against the bone.nnnPURPOSEnThe objective of this study was to measure the radial force generated by different interference devices and evaluate the effect of this radial force on the pullout strength of graft-device constructs.nnnSTUDY DESIGNnControlled laboratory study.nnnMETHODSnA resultant force (F(R)) was used as a representative measure of the total radial force generated. Bovine tendons were fixated in either synthetic bone or porcine tibia using one of following devices: (1) RCI titanium screw, (2) PEEK screw, (3) IntraFix sheath-and-screw device, and (4) ExoShape sheath-and-insert device. F(R) was measured while each device was inserted into synthetic bone mounted on a test machine (n=5 for each device). In a subsequent test series, graft-device constructs were loaded to failure at 50mm/min. The pullout strength was measured as the ultimate load before failure (n=10 for each device).nnnRESULTSnThe F(R) values generated during insertion into synthetic bone were 777 ± 86N, 865 ± 140N, 1313 ± 198N, and 1780 ± 255N for the RCI screw, PEEK screw, IntraFix, and ExoShape, respectively. The pullout strengths in synthetic bone for the RCI screw, PEEK screw, IntraFix and ExoShape were 883 ± 125N, 716 ± 249N, 1147 ± 142N, and 1233 ± 190N, respectively.nnnCONCLUSIONSnThese results suggest that the F(R) generated during interference fixation affects the pullout strength with sheath-based devices providing superior F(R) compared with interference screws. The use of synthetic bone was validated by comparing the pullout strengths to those when tested in porcine tibia.nnnCLINICAL RELEVANCEnThese results could be valuable to a surgeon when determining the best fixation device to use in the clinical setting.