Liquid crystal elastomer shell actuators with negative order parameter

Liquid crystals (LCs) are nonsolids with long-range orientational order, described by a scalar order parameter ⟨P2⟩=1/2⟨3cos2β−1⟩. Despite the vast set of existing LC materials, one-third of the order parameter value range, −1/2< 〈P2〉 < 0, has until now been inaccessible. Here, we present the first material with negative LC order parameter in its ground state, in the form of elastomeric shells. The optical and actuation characteristics are opposite to those of conventional LC elastomers (LCEs). This novel class of anti-ordered elastomers gives access to the previously secluded range of liquid crystallinity with 〈P2〉 < 0, providing new challenges for soft matter physics and adding a complementary type of LCE actuator that is attractive for applications in, e.g., soft robotic

Patient-Specific Finite-Element Analysis of Three Intramedullary Nails for Tibiotalocalcaneal Fusion

Category: Ankle, Hindfoot Introduction/Purpose: Tibiotalocalcaneal (TTC) arthrodesis is a salvage procedure for patients with severe osteoarthritis and other degenerative ankle conditions. Oftentimes, an intramedullary (IM) nail is implanted across the joints and then fixed with tibial and calcaneal screws. Maintaining compression and load sharing are both largely desired to promote fusion via primary bone healing; however, compression can be lost due to small amounts of bone resorption and IM nails are now being made from carbon-fiber epoxy to minimize stress shielding. To date, no one has been able to directly characterize or compare the specific amount of these parameters across nails in a single model. The purpose of this study is to compare influence of nail design and materials for compressive and load-sharing properties using a patient-specific finite-element model. Methods: A titanium nail, a pseudoelastic nickel-titanium nail, and carbon fiber-epoxy nail were investigated for (1) load sharing between the nail body and tibia under gait loading and (2) compression loss as a function of resorption in the talus. A patient- specific model of the ankle, both in geometry and material properties, was generated from a quantitative computed tomography (QCT) scan of a healthy leg. The models were segmented and meshed using SCANIP and exported into ABAQUS for finite- element analysis. Compression in the nickel-titanium nail was simulated by pre-stretching the pseudoelastic compressive element. Conversely, compression in the titanium and carbon-fiber nails were generated by giving the nail jacket an orthotropic contraction coefficient in the model. After compression was set, each nail was subjected to an applied gait load that peaked at 1121 N. Resorption was simulated using a thin compressible layer of bone in the talus and decreasing the modulus and Poisson’s ratio. Results: Surprisingly, the carbon-fiber nail showed similar stress shielding to the titanium nail, with 72% and 77% of the stress being transferred through the devices instead of the ankle, respectively. Even though carbon fiber-epoxy has a significantly lower modulus than titanium (75 GPa vs 110 GPa), the overall stiffness of the nails was still much greater than that of bone (~30,000 N/mm vs. ~44,000 N/mm vs. ~3,000 N/mm, respectively). The pseudoelastic nail only shielded 32% of the stress values by comparison. For the titanium and carbon-fiber nails, over 85% of the initial compression provided by the nail drops with 0.10 mm of resorption. The pseudoelastic nail maintained 90% of its initial compression after 0.10 mm of resorption. Conclusion: IM nail design and materials played a significant role in maintaining compression and load sharing. The pseudoelastic nail had the lowest degree of stress-shielding (32%) and maintained compression for over 0.10 mm of simulated resorption. Constant compression and the avoidance of “resorption gapping” is paramount to drive primary bone healing in joint fusions due to lack of periosteal/endosteal anatomy crossing the fusion site, thus impairing the ability for secondary bone healing (callus healing). This model allows for direct comparison between devices and can be used pre-operatively to predict patient-specific performance and help aid in device selection for TTC fusion

Characterization of poly(para-phenylene)-MWCNT solvent-cast composites

Poly(para-phenylene) (PPP) is one of the strongest and stiffest thermoplastic polymers due to its aromatic backbone structure. However, because of this chemistry, this also means that typical processing techniques require high temperatures and pressures to allow for formability. This study demonstrates that, unlike similar aromatic thermoplastics, PPP has the unique ability to be solvent-cast using conventional solvents, which allows for facile fabrication of thin films and coatings under ambient conditions. The purpose of this research was to investigate the properties of solvent-cast PPP, which is not currently available in literature. In addition, through the solvent-casting technique, composite materials can be created by combining PPP with multi-walled carbon nanotubes (MWCNTs) in attempts to enhance structural properties and electrical conductivity. A method was developed for solvent-casting of PPP through chloroform evaporation and subsequent methanol soaking, resulting in homogenous average thicknesses of 0.10 ± 0.04 mm. Mechanical testing of solvent-cast PPP resulted in an elastic modulus of 4.2 ± 0.2 GPa with 13 ± 2.3% strain-to-failure. The addition of MWCNT reinforcement increased ultimate tensile strength at the expense of ductility. Composites maintained a yielding response up to 6 vol.% of MWCNTs, which also corresponded to the largest strength values observed. Ultimate tensile strength increased from 96 MPa from the matrix to a maximum of 121 MPa. Electrical conductivity of the composites increased from 4.5 × 10−6 to 1.02 × 10−3 S/cm from 3 to 20 vol.% MWCNTs, although values plateau at 5 vol.%

Geometry of cassowary bone daggers

Geometry of cassowary bone daggers accessioned in the Hood Museum of Art, Dartmouth Colleg

Results of finite element analysis (FEA) - Imax direction

Results of finite element analysis (FEA) - Imax directio

Geometry of human bone daggers

Geometry of human bone daggers accessioned in the Hood Museum of Art, Dartmouth Colleg

Results of finite element analysis (FEA) - Imin direction

Results of finite element analysis (FEA) - Imin directio