Orientation Of Helical Axes Research Articles

Stimuli-Driven Control of the Helical Axis of Self-Organized Soft Helical Superstructures.

Supramolecular and macromolecular functional helical superstructures are ubiquitous in nature and display an impressive catalog of intriguing and elegant properties and performances. In materials science, self-organized soft helical superstructures, i.e., cholesteric liquid crystals (CLCs), serve as model systems toward the understanding of morphology- and orientation-dependent properties of supramolecular dynamic helical architectures and their potential for technological applications. Moreover, most of the fascinating device applications of CLCs are primarily determined by different orientations of the helical axis. Here, the control of the helical axis orientation of CLCs and its dynamic switching in two and three dimensions using different external stimuli are summarized. Electric-field-, magnetic-field-, and light-irradiation-driven orientation control and reorientation of the helical axis of CLCs are described and highlighted. Different techniques and strategies developed to achieve a uniform lying helix structure are explored. Helical axis control in recently developed heliconical cholesteric systems is examined. The control of the helical axis orientation in spherical geometries such as microdroplets and microshells fabricated from these enticing photonic fluids is also explored. Future challenges and opportunities in this exciting area involving anisotropic chiral liquids are then discussed.

Periodic Planar‐Homeotropic Anchoring Realized by Photoalignment for Stabilization of Chiral Superstructures

AbstractSelf‐organization of chiral superstructures in liquid crystal (LC) materials is a powerful tool in developing functional devices based on soft matter. The control and manipulation of the helical axis orientation in chiral nematic liquid crystals are currently attracting significant attention, but are hindered by limited control over the anchoring at the interfaces. The development of new alignment techniques with local control over polar and azimuthal anchoring can open up a wide range of possibilities. Here a new photoalignment process based on a mixture of photosensitive azo compounds and homeotropic alignment materials is developed and demonstrated. The alignment of the LC director near the surface depends on both intensity and polarization of the UV illumination. By two‐beam interference UV illumination, periodic stripes of planar and homeotropic anchoring are realized. In addition it is demonstrated that the stripes of planar and homeotropic anchoring stabilize and align cholesteric fingers in cells with thickness smaller than the pitch. The 2D director configuration in the cholesteric fingers is identified by matching the experimental observations with numerical simulations based on finite element Q‐tensor calculations.

Determination of consistent patterns of range of motion in the ankle joint with a computed tomography stress-test

Background Measuring the range of motion of the ankle joint can assist in accurate diagnosis of ankle laxity. A computed tomography-based stress-test (3D CT stress-test) was used that determines the three-dimensional position and orientation of tibial, calcaneal and talar bones. The goal was to establish a quantitative database of the normal ranges of motion of the talocrural and subtalar joints. A clinical case on suspected subtalar instability demonstrated the relevance the proposed method. Methods The range of motion was measured for the ankle joints in vivo for 20 subjects using the 3D CT stress-test. Motion of the tibia and calcaneus relative to the talus for eight extreme foot positions were described by helical parameters. Findings High consistency for finite helical axis orientation ( n) and rotation ( θ) was shown for: talocrural extreme dorsiflexion to extreme plantarflexion (root mean square direction deviation ( η) 5.3° and θ: SD 11.0°), talorucral and subtalar extreme combined eversion–dorsiflexion to combined inversion–plantarflexion ( η: 6.7°, θ: SD 9.0° and η:6.3°, θ: SD 5.1°), and subtalar extreme inversion to extreme eversion ( η: 6.4°, θ: SD 5.9°). Nearly all dorsi – and plantarflexion occurs in the talocrural joint ( θ: mean 63.3° (SD 11°)). The inversion and internal rotation components for extreme eversion to inversion were approximately three times larger for the subtalar joint ( θ: mean 22.9° and 29.1°) than for the talocrural joint ( θ: mean 8.8° and 10.7°). Comparison of the ranges of motion of the pathologic ankle joint with the healthy subjects showed an increased inversion and axial rotation in the talocrural joint instead of in the suspected subtalar joint. Interpretation The proposed diagnostic technique and the acquired database of helical parameters of ankle joint ranges of motion are suitable to apply in clinical cases.

Ankle and subtalar kinematics measured with intracortical pins during the stance phase of walking.

The absence of external landmarks on the talus has rendered the description of ankle and subtalar joint kinematics difficult. Abnormal motion at these joints has, however, been implied in the etiology of an array of lower extremity overuse injuries. Intracortical pins were inserted under local anesthesia in the tibia, talus, and calcaneus with external marker clusters traced by a video motion analysis system. Kinematic data were collected during walking trials on a flat surface for three subjects. Gait pattern was monitored by comparison of ground reaction force curves during stance phase with and without the pins inserted. Results were presented in terms of helical axis orientation for both joints and the component rotations about these axes. Large intersubject differences were seen in both ankle and subtalar joint helical axis orientation. Maximum rotations over the complete stance phase for the ankle and subtalar joints respectively were: eversion/inversion, 6.3 degrees and 8.3 degrees; dorsiflexion/plantarflexion, 18.7 degrees and 3.7 degrees; and abduction/adduction, 5.0 degrees and 6.1 degrees. The majority of ankle eversion/inversion occurred at the subtalar joint; however, the ankle component cannot be ignored. Abduction/adduction range of motion at the subtalar joint was surprisingly high, indicating that this component motion during walking is not purely attributable to the ankle joint. Future research should include greater subject numbers in order to present more universally applicable results. The in vivo kinematics of the talus during weightbearing activity are poorly understood. The description of this motion may assist in the structuring of clinical rehabilitation and in the design and insertion of ankle joint prostheses.

A Study of the Dynamics of Reflection Color, Helical Axis Orientation, and Domain Size in Cholesteric Liquid Crystal Displays

AbstractThe relaxation from the field‐induced homeotropic state to the equilibrium planar state of cholesteric liquid crystals is investigated. By using an optical retro‐reflection technique, we have isolated the dynamics of the orientation and pitch of cholesteric helices, and through scattering and microscopy techniques, we have determined domain size as a function of time for various surface treatments. These three factors determine the viewing angle, color, and brightness of a display. An analysis of the significance of surface selection in cholesteric displays is presented.

White Reflective Displays from Polymer‐Stabilized Cholesteric Textures

AbstractWe report the optical and electro‐optical properties of a white reflective display from a chiral polymer stabilized cholesteric texture (C‐PSCT). The new PSCT system is prepared by photopolymerizing a small quantity of a chiral monomer in a cholesteric host. The chiral polymer network phase separates into micron‐size regions, which provide the additional chirality and influence the pitch and orientation of helical axes of a cholesteric liquid crystal. Therefore, a distribution of cholesteric helical axes Bragg reflects a white image.

Porcine Cerebroside Sulfate Activator (Saposin B) Secondary Structure: CD, FTIR, and NMR Studies

Cerebroside sulfate activator protein (CSAct or saposin B) is one of a group of heat stable, low-molecular-weight proteins that appear to share a common structural motif. These have been referred to as saposin-like proteins and are thought to share a multiple amphipathic helical barrel structure with a conserved pattern of disulfide linkages. Porcine kidney CSAct was prepared in high purity and consisted of three major glycosylated subforms. The protein was studied by physical–chemical methods and evaluated by various methods for structural prediction. All suggest that CSAct has high amounts of α-helical conformation and little if any β-sheet. Circular dichroism (CD) studies indicate 45–50% helical conformation depending on buffer and temperature. There was only a moderate loss in helical content with increasing temperature and no indication of thermal denaturation. Fourier transform infrared spectroscopy (FTIR) measurements on deuterium hydrated self-films also indicated a predominantly helical structure. Helical axis orientation was investigated by both oriented CD and FTIR dichroism, which suggested that the helical axes were roughly parallel and oriented along the axis of the surface on which the self-films had been deposited. One-dimensional nuclear magnetic resonance spectra showed large chemical shift dispersion, indicating a defined tertiary structure with little variation between 6 and 85°C. NOESY spectra failed to show the strong NOE cross peaks expected for a highly helical conformation. This may indicate short-term conformational flexibility within the helices or molecular aggregation at the high protein concentrations employed. These observations are consistent with the 3-4-helix bundle motif suggested for saposin-like proteins by various predictive algorithms.

A Finite Helical Axis as a Landmark for Kinematic Reference of the Knee

Reference coordinates based on the finite helical axis for flexion of the knee from 0 to 90 deg are proposed. Six degree-of-freedom tracking allows the use of such a helical axis as a kinematic landmark for knee motion representation. Data from five human subjects in vivo are presented as a path of finite helical axes for flexion of the knee from 20 to 80 deg. The finite helical axis rotates by an average of 11.4 deg, the centrode translates an average of 19.8 mm, and the total axial translation averages 0.1 mm during flexion from 20 to 80 deg. Error due to the transducer was measured on a fixed-pivot pendulum and found to be 1.0 deg and 1.9 mm rms for the helical axis orientation and position, respectively, and 0.1 mm for the axial translation. Reproducibility and soft tissue effects on the measurements were repeatable to 4.0 deg and 2.7 mm rms in orientation and position, respectively, and 0.1 mm for the axial translations. Soft tissue errors averaged 4.9 deg and 3.6 mm in position and orientation, and 0.3 mm in the axial translations.

Comment on ‘‘Nuclear magnetic resonance spectroscopy in cholesteric liquid crystals. I. Orientational order parameter measurements’’

NMR spectra in cholesteric liquid crystals cannot be analyzed in the same way as in nematic liquid crystals, due to the random orientation of helical axes in the sample and due to the effects of diffusion along the helical axes. This comment points out that the authors of a previous article did in fact analyze their NMR spectra without taking these two considerations into account.