Reginald F. Hamilton

Molecular tandem repeat strategy for elucidating mechanical properties of high-strength proteins

Significance Squid have teeth-like structural [squid ring teeth (SRT)] proteins inside their suckers, which have segmented semicrystalline morphology with repetitive amorphous and crystalline domains. These proteins have high elastic modulus and toughness. However, a clear relationship between molecular structure and mechanical properties of this material remains elusive. To investigate the genetic basis of material properties in SRT sequences, we developed a new approach for the design and production of structural proteins. We show that the toughness and flexibility of these synthetic SRT mimics increase as a function of molecular weight, whereas the elastic modulus and yield strength remain unchanged. These results suggest that artificial proteins produced by our approach can help to illuminate the genetic basis of protein material behavior in SRT. Many globular and structural proteins have repetitions in their sequences or structures. However, a clear relationship between these repeats and their contribution to the mechanical properties remains elusive. We propose a new approach for the design and production of synthetic polypeptides that comprise one or more tandem copies of a single unit with distinct amorphous and ordered regions. Our designed sequences are based on a structural protein produced in squid suction cups that has a segmented copolymer structure with amorphous and crystalline domains. We produced segmented polypeptides with varying repeat number, while keeping the lengths and compositions of the amorphous and crystalline regions fixed. We showed that mechanical properties of these synthetic proteins could be tuned by modulating their molecular weights. Specifically, the toughness and extensibility of synthetic polypeptides increase as a function of the number of tandem repeats. This result suggests that the repetitions in native squid proteins could have a genetic advantage for increased toughness and flexibility.

Structure and interfacial analysis of nanoscale TiNi thin film prepared by biased target ion beam deposition

Ultrathin, 65 nm thick, TiNi alloy films were fabricated by cosputtering Ti and Ni targets using the recently developed biased target ion beam deposition technique. Preheating the substrate by exposure to a low energy ion source resulted in as-deposited films with a pure B2 atomic crystal structure containing no secondary crystal structures or precipitates. Continuous films were produced with a smooth surface and minimal substrate/film interfacial diffusion. The diffusion layer was a small ratio of film thickness, which is a prerequisite for the B2 phase to undergo the martensitic transformation in ultrathin films.

Feasibility of self-pre-stressing concrete members using shape memory alloys

Shape memory alloys are a class of smart materials that recover apparent plastic deformation (∼6%–8% strain) after heating, thus “remembering” the original shape. This shape memory effect can be exploited for self-post-tensioning applications, and NiTi-based shape memory alloys are promising as shape memory effect is possible at elevated temperatures amenable to practical application compared to conventional NiTi. This study investigates the feasibility of self-post-tensioned concrete elements by activating the shape memory effect of NiTiNb, a class of wide-hysteresis shape memory alloys, using the heat of hydration of grout. First, the microstructure characterization of the NiTiNb wide-hysteresis shape memory alloys is discussed. Then, the tensile stress-induced martensitic transformations in NiTiNb shape memory alloy tendons are studied. Next, the temperature increase due to the heat of hydration of four commercially available grouts is investigated. Pull-out tests are also conducted to investigate the bond between the grout and shape memory alloy bar. Results show that the increase in temperature due to hydration heat can provide significant strain recovery during a free recovery experiment, while the same temperature increase only partially activates the shape memory alloys during a constrained recovery.

Shape memory and martensite deformation response of Ni2MnGa

The purpose of this paper is twofold: to establish the magnitude of detwinning stress levels of non-modulated martensite in Ni53Mn25Ga22 as a function of heat treatments utilizing single crystals, and to study the shape memory strains from constant-load temperature cycling experiments for aged and unaged conditions. The maximum transformation strains of 5.2% are consistent with the theoretical predictions based on energy minimization theory. The results exhibit a remarkable narrowing of the thermal hysteresis (as low as 2 ◦ C) with increasing applied stress and a considerable two-way shape memory effect. Using microscopy, it is shown that aging produces a finer martensitic plate structure with an accompanying increase in detwinning stress compared to the unaged case.

Magnetic shape memory in Ni2MnGa as influenced by applied stress

The authors report on the stress-strain and strain-temperature behaviors of a Ni2MnGa alloy. Depending on the applied stress during cooling from austenite, different levels of transformation strain were observed culminating in the formation of a single variant of martensite and transformation strain of 4%. The results underscore an optimum stress level that maximizes shape memory strains. At higher stress levels, plasticity curtails the level of transformation strain and the concomitant magnetic shape memory strains. Similarly, the results uncover an optimum bias stress to maximize magnetic induced shape memory, producing strains exceeding 5% with reversible strains of 3%.

Narrow thermal hysteresis of NiTi shape memory alloy thin films with submicrometer thickness

NiTi shape memory alloy (SMA) thin films were fabricated using biased target ion beam deposition (BTIBD), which is a new technique for fabricating submicrometer-thick SMA thin films, and the capacity to exhibit shape memory behavior was investigated. The thermally induced shape memory effect (SME) was studied using the wafer curvature method to report the stress-temperature response. The films exhibited the SME in a temperature range above room temperature and a narrow thermal hysteresis with respect to previous reports. To confirm the underlying phase transformation, in situ x-ray diffraction was carried out in the corresponding phase transformation temperature range. The B2 to R-phase martensitic transformation occurs, and the R-phase transformation is stable with respect to the expected conversion to the B19′ martensite phase. The narrow hysteresis and stable R-phase are rationalized in terms of the unique properties of the BTIBD technique.

Crystallization and microstructure evolution of nanoscale NiTi thin films prepared by biased target ion beam deposition

NiTi alloy thin films of nanoscale thickness were fabricated using a novel technique known as biased target ion beam deposition (BTIBD). Ni-poor/Ti-rich, near equiatomic NiTi, and Ni-rich film composition ranges were investigated in the as-deposited condition. Heat treatment was necessary to crystallize the otherwise amorphous as-deposited films. Crystallization and microstructure evolution were contrasted with those for nanoscale thickness films fabricated using the more common magnetron sputtering technique. For each composition range, the as-deposited magnetron sputtered films exhibit a columnar-void morphology. In situ transmission electron microscopy heating results show that crystallization requires the morphology to merge, which produces small grains. Larger grains are formed in crystallized BTIBD films, which are attributed to increased adatom mobility facilitated by independent control of low energy ions. This work postulates that enhanced mobility in BTIBD eliminates the columnar-void morphology...

Crystallization of nanoscale NiTi alloy thin films using rapid thermal annealing

This work utilizes short time heat treatments of submicrometer-thickness NiTi alloy films fabricated using biased target ion beam deposition and investigates crystallization. Films were fabricated on Si substrates, and thicknesses were about 150 nm, which were much less than conventional thicknesses on the order of micrometers. To understand the composition dependence, Ni concentrations were varied such that alloys ranged from Ti-rich to near-equiatomic. Rapid thermal annealing was used for the heat treatment and temperatures ranged from 465 up to 540 °C for 10 min. X-ray diffraction measurements for each of the NiTi alloy compositions revealed that the crystallization temperature was equivalent (∼490 °C) and the B2 austenitic atomic crystal structure existed. Evolutions of surface morphologies, measured using atomic force microscopy, as a function of heat treatment temperature confirmed the composition independence of the crystallization temperature. To investigate the structure using transmission electr...

Target Shape Optimization of Functionally Graded Shape Memory Alloy Compliant Mechanism

Nickel Titanium (NiTi) shape memory alloys (SMAs) exhibit shape memory and/or superelastic properties, enabling them to demonstrate multifunctionality by engineering microstructural and compositional gradients at selected locations. This paper focuses on the design optimization of NiTi compliant mechanisms resulting in single-piece structures with functionally graded properties, based on user-defined target shape matching approach. The compositionally graded zones within the structures will exhibit an on demand superelastic effect (SE) response, exploiting the tailored mechanical behavior of the structure. The functional grading has been approximated by allowing the geometry and the superelastic properties of each zone to vary. The superelastic phenomenon has been taken into consideration using a standard nonlinear SMA material model, focusing only on 2 regions of interest: the linear region of higher Young’s modulus of elasticity and the superelastic region with significantly lower Young’s modulus of elasticity. Due to an outside load, the graded zones reach the critical stress at different stages based on their composition, position and geometry, allowing the structure morphing. This concept has been used to optimize the structures’ geometry and mechanical properties to match a user-defined target shape structure. A multi-objective evolutionary algorithm (NSGA II - Non-dominated Sorting Genetic Algorithm) for constrained optimization of the structure’s mechanical properties and geometry has been developed and implemented.Copyright

Biomimicry of the Manduca Sexta Forewing Using SRT Protein Complex for FWMAV Development

A new thermoplastic protein complex, Squid Ring Teeth SRT, has been adapted for use in the artificial reconstruction of a Manduca sexta wing. The SRT protein complex exhibits consistent material properties over a wide range of temperatures 25i¾?C to 196i¾?C and retains it mechanical integrity across a large frequency spectrum 0.1 Hz to 150 Hz. Insect-inspired wings comprised of SRT can therefore be reliable and robust, which are essential characteristics for flapping wing MAVs FWMAV. The preliminary results in this paper suggest that a thorough analysis of an SRT-based wing be conducted using load cell, optical digitization, and PIV techniques. With these results, we believe it will be possible to accurately mimic the M. sexta wing in order to pave the way for next generation FWMAV development.