Precise Position Control of Holonomic Inchworm Robot Using Four Optical Encoders.

In this study, an XYθ position sensor is designed/proposed to realize the precise control of the XYθ position of a holonomic inchworm robot in the centimeter to submicrometer range using four optical encoders. The sensor was designed to be sufficiently compact for mounting on a centimeter-sized robot for closed-loop control. To simultaneously measure the XYθ displacements, we designed an integrated two-degrees-of-freedom scale for the four encoders. We also derived a calibration equation to decrease the crosstalk errors among the XYθ axes. To investigate the feasibility of this approach, we placed the scale as a measurement target for a holonomic robot. We demonstrated closed-loop sequence control of a star-shaped trajectory for multiple-step motion in the centimeter to micrometer range. We also demonstrated simultaneous three-axis proportional-integral-derivative control for one-step motion in the micrometer to sub-micrometer range. The close-up trajectories were examined to determine the detailed behavior with sub-micrometer and sub-millidegree resolutions in the MHz measurement cycle. This study is an important step toward wide-range flexible control of precise holonomic robots for various applications in which multiple tools work precisely within the limited space of instruments and microscopes.

Minimum amino acid residues of an α-helical peptide leading to lipid nanodisc formation.

Nanodiscs are a relatively new class of nanoparticles composed of amphiphilic α-helical scaffold peptides and a phospholipid bilayer, and find potential applications in various fields. In order to identify the minimum number of amino acid residues of an amphiphilic α-helical peptide that leads to nanodisc formation, seven peptides differing in lengths (22-, 18-, 14-, 12-, 10-, 8-, and 6-mers) that mimic and modify the C-terminal domain of apoA-I (residues 220-241) were synthesized. At a concentration of 0.3 mM, the 6- and 8-mer peptides did not present any surface activity. In case of the 10-mer peptide, the aqueous surface tension initially decreased and reached a constant value of 51.9 mN/m with the 14-, 18-, and 22-mer peptides. Moreover, upon mixing the surface-active peptides (14-, 18-, and 22-mers) with dipalmitoylphosphatidylcholine (DMPC) liposomes (2.5:1, peptide : DMPC), the turbid DMPC liposome solution rapidly became transparent. Further analysis of this solution by negative-stain transmission electron microscopy (NS-TEM) indicated the presence of disk-like nanostructures. The average diameter of the nanodiscs formed was 9.5 ± 2.7 nm for the 22-mer, 8.1 ± 2.7 nm for the 18-mer, and 25.5 ± 8.5 nm for the 14-mer peptides. These results clearly demonstrate that the surface properties of peptides play a critical role in nanodisc formation. Furthermore, the minimum length of an amphiphilic peptide from the C-terminal of apoA-I protein that can lead to nanodisc formation is 14 amino acid residues.

Surfactant-like properties of an amphiphilic α-helical peptide leading to lipid nanodisc formation.

Nanodiscs are self-assembled discoidal nanoparticles composed of amphiphilic α-helical scaffold proteins or peptides that wrap themselves around the circumference of a lipid bilayer in a beltlike manner. In this study, an amphiphilic helical peptide that mimics helix 10 of human apoA-I was newly synthesized by solid phase peptide synthesis using Fmoc chemistry, and its physicochemical properties, including surface tension, self-association, and solubilization abilities, were evaluated and related directly to nanodisc formation. The synthesized peptide having hydrophobic and hydrophilic faces behaves like a general surfactant, affording a critical association concentration (CAC) of 2.7 × 10(-5) M and a γCAC of 51.2 mN m(-1) in aqueous solution. Interestingly, only a peptide solution above its CAC was able to microsolubilize L-α-dimyristoylphosphatidylcholine (DMPC) vesicles, and lipid nanodiscs with an average diameter of 9.5 ± 2.7 nm were observed by dynamic light scattering and negative stain transmission electron microscopy. Moreover, the ζ potentials of the lipid nanodiscs were measured for the first time as a function of pH, and the values changed from positive (20 mV) to negative (-30 mV). In particular, nanodisc solutions at acidic pH 4 (20 mV) or basic pH 9 (-20 mV) were found to be stable for more than 6 months as a result of the electrostatic repulsion between the particles.