Self-Propelled Metal–Polymer Hybrid Micromachines with Bending and Rotational Motions

Two self-propelled micromachines were fabricated with gold/platinum micromotors that exhibit simple translational motion in a fuel solution. In each one, two micromotors were connected with a joint of polymer tube formed by stacking cationic poly­(allylamine hydrochloride) (PAH) and anionic poly­(acrylic acid) (PAA) using a layer-by-layer technique. A bent structure was created by making one longitudinal side of the joint more swellable with alkaline treatment. The joint containing fewer PAA/PAH bilayers was flexible and allowed a larger range of Brownian angular fluctuation. In the fuel solution, bending and stable rotation were observed for the micromotors tethered with soft and rigid angled joints, respectively. The radius and angular velocity of the rotation depended on the angle of the joint. Such tethered micromotors can be used to realize sophisticated micro/nanomachines for microscale surgery and drug delivery

Falling test apparatus.

<p>(a) Custom-made apparatus consisting of drop object, guide pipe, and polyethylene sheet. (b) Temporal acceleration profiles of the object dropped from different heights (0.1−1.0 m) measured by the commercial accelerometer.</p

Schematics of the fluidic based impact sensor.

<p>(a) Schematic overview of the basic structure of the sensor. (b, c) Working principle of the sensor based on which impacts are detected depending on different combinations of immiscible working fluids.</p

Fluorescence micrographs showing two successive rounds of vesicle fusion (mixing), reaction, and budding/fission (aliquoting).

<p>The 5-µm scale bar in panel (a) also applies to panel (b) and to panels (e)–(h), whereas the 10-µm scale bar in panel (c) also applies to panel (d).</p

Stochasticity in Gene Expression in a Cell-Sized Compartment

The gene expression in a clonal cell population fluctuates significantly, and its relevance to various cellular functions is under intensive debate. A fundamental question is whether the fluctuation is a consequence of the complexity and redundancy in living cells or an inevitable attribute of the minute microreactor nature of cells. To answer this question, we constructed an artificial cell, which consists of only necessary components for the gene expression (<i>in vitro</i> transcription and translation system) and its boundary as a microreactor (cell-sized lipid vesicle), and investigated the gene expression noise. The variation in the expression of two fluorescent proteins was decomposed into the components that were correlated and uncorrelated between the two proteins using a method similar to the one used by Elowitz and co-workers to analyze the expression noise in <i>E. coli</i>. The observed fluctuation was compared with a theoretical model that expresses the amplitude of noise as a function of the average number of intermediate molecules and products. With the assumption that the transcripts are partly active, the theoretical model was able to well describe the noise in the artificial system. Furthermore, the same measurement for <i>E. coli</i> cells harboring an identical plasmid revealed that the <i>E. coli</i> exhibited a similar level of expression noise. Our results demonstrated that the level of fluctuation found in bacterial cells is mostly an intrinsic property that arises even in a primitive form of the cell

Result of the impact sensors with different neck diameter <i>d</i>_neck.

<p>(a) Fraction of responded sensors versus peak acceleration values with <i>d</i>_neck = 1.5−3.0 mm. Insets show typical images of responded states after droplet falls. Blue dashed lines represent the fitted curves based on the use of the sigmoidal function. (b) Plot showing the dependence of <i>g</i>_th on <i>d</i>_neck.</p

State diagram of the response of the sensor as a function of <i>Ar</i> and <i>Bo</i>.

<p>Filled symbols represent sensors that “responded,” while open symbols represent sensors that “did not respond.” Circle: water, cross: ethanol 10%, star: ethanol 40%, square: sucrose 45 wt%, diamond: sucrose 65 wt%, pentagon: sucrose 70 wt%, triangle: glycerol 60 wt%, inverse triangle: glycerol 90 wt%, and hexagon: glycerol. Straight and dashed lines represent the results obtained with <i>d</i>_neck = 3.0 mm and 2.0 mm, respectively.</p

Deformation Modes of Giant Unilamellar Vesicles Encapsulating Biopolymers

The shapes of giant unilamellar vesicles (GUVs) enclosing polymer molecules at relatively high concentration, used as a model cytoplasm, significantly differ from those containing only small molecules. Here, we investigated the effects of the molecular weights and concentrations of polymers such as polyethylene glycol (PEG), bovine serum albumin (BSA), and DNA on the morphology of GUVs deflated by osmotic pressure. Although small PEG (MW < 1000) does not alter the mode of shape transformation even at >10% (w/w), PEG with MW > 6000 induces budding and pearling transformation at above 1% (w/w). Larger PEG frequently induced small buddings and tubulation from the membrane of mother GUVs. A similar trend was observed with BSA, indicating that the effect is irrelevant to the chemical nature of polymers. More surprisingly, long strands of DNA (>10⁵ bp) enclosed in GUVs induced budding transformation at concentrations as low as 0.01–0.1% (w/w). We expect that this molecular size dependency arises mainly from the depletion volume effect. Our results showed that curving, budding, and tubulation of lipid membranes, which are ubiquitous in living cells, can result from simple cell-mimics consisting of the membrane and cytosolic macromolecules, but without specific shape-determining proteins

Result of the impact sensor with different oil phase.

<p>(a) Fraction of responded sensors versus peak acceleration tested using different oil viscosities. (b) Comparison of <i>g</i>_th for fluorocarbon oils with different viscosities.</p

Programmed Transport and Release of Cells by Self-Propelled Micromotors

Autonomous transport and release of bacterial cells by self-propelled micromotors were achieved. The motors consisted of zinc and platinum hemispheres formed on polystyrene beads and moved as a result of simultaneous redox reactions occurring on both metal ends. The highly negative redox potential of zinc enabled the selection of a wide variety of organic redox compounds as fuels, such as methanol and <i>p</i>-benzoquinone. The movement of motors was observed in solutions of fuels. To realize autonomous capture, transport, and release of cargo, a self-assembled monolayer (SAM) was formed on the platinum part of the motor. This SAM could be desorbed by coupling the reaction with the dissolution of zinc, which could also be controlled by adjusting the concentration of Zn²⁺ ions. <i>Escherichia coli</i> (<i>E. coli</i>) cells were captured by the motor (due to hydrophobic interactions), transported, and released following SAM desorption at the mixed potential