Sen Wai Kwok

Camouflage and Display for Soft Machines

Mechanical Chameleon A wide range of animals can adapt their color patterns as a means of camouflage or otherwise changing their appearance. This is accomplished through changes in coloration, contrast, patterning, or shape. Morin et al. (p. 828) show at a basic level that some of these features can be added as microfluidic layers attached to mobile, flexible, soft machines. By pumping different fluids through the channels, the robots were able to change their coloration or overall contrast and could thus blend into the background of the surface they were lying upon. Conversely, by pumping through fluids of different temperature, the infrared profile of the robot could be changed without changing its visible coloration. Soft robots with microfluidic channels in a skin layer show camouflaging abilities. Synthetic systems cannot easily mimic the color-changing abilities of animals such as cephalopods. Soft machines—machines fabricated from soft polymers and flexible reinforcing sheets—are rapidly increasing in functionality. This manuscript describes simple microfluidic networks that can change the color, contrast, pattern, apparent shape, luminescence, and surface temperature of soft machines for camouflage and display. The color of these microfluidic networks can be changed simultaneously in the visible and infrared—a capability that organisms do not have. These strategies begin to imitate the functions, although not the anatomies, of color-changing animals.

Rhodium-catalyzed enantioselective cyclopropanation of olefins with N-sulfonyl 1,2,3-triazoles.

N-Sulfonyl 1,2,3-triazoles readily form rhodium(II) azavinyl carbenes, which react with olefins to produce cyclopropanes with excellent diastereo- and enantioselectivity and in high yield.

Transannulation of 1-Sulfonyl-1,2,3-Triazoles with Heterocumulenes

Readily available 1-mesyl-1,2,3-triazoles are efficiently converted into a variety of imidazolones and thiazoles by Rh(II)-catalyzed denitrogenative reactions with isocyanates and isothiocyanates, respectively. The proposed triazole-diazoimine equilibrium results in the formation of highly reactive azavinyl metal-carbenes, which react with heterocumulenes causing an apparent swap of 1,2,3-triazole core for another heterocycle.

Transition Metal-Free Catalytic Synthesis of 1,5-Diaryl-1,2,3-Triazoles

1,5-Diarylsubstituted 1,2,3-triazoles are formed in high yield from aryl azides and terminal alkynes in DMSO in the presence of catalytic tetraalkylammonium hydroxide. The reaction is experimentally simple, does not require a transition-metal catalyst, and is not sensitive to atmospheric oxygen and moisture.

Magnetic Assembly of Soft Robots with Hard Components

and multi-modal locomotion [ 14,22 ] —can be realized through the use of microfl uidics in robotic design. Previously reported soft robots and machines have not been reconfi gurable: Their structures, once generated, were fi xed and not amenable to reversible changes that modify capabilities. In addition, devices that are composed of multiple materials and contain networks of channels perpendicular to each other vertically and horizontally can be exceedingly diffi cult to produce in single step; soft lithography—an effi cient technique for rapid prototyping and replication of planar and quasi-two-dimensional elastomeric structures—is insuffi cient to tackle all challenges associated with the fabrication of complex 3D microfl uidic networks in advanced systems. As such, methods that impart re-confi gurability, simplify the fabrication of actuators with complex designs, or facilitate the integration of non-elastomeric materials (e.g., metals, thermoplastics) and electronics (e.g. sensors and communication units) will greatly accelerate the development of soft robotics. Reversible modular assembly has been a widely used strategy for fabricating complex hard robots as it enables reconfi guration to suit new tasks, rapid testing of new designs, and easy repair and replacement of damaged parts. This strategy, together with advances in the fabrication and miniaturization of sensors, actuators, power sources, and units for control and communications, has led to the emergence of autonomous, compact hard robots capable of self-assembly [ 23 ] and self-reconfi guration. [ 24,25 ] Although inter-modular connections made from mechanical joinery (e.g., hooks-and-grooves interlock [ 23 ] ) are sturdy and reversible, they require either manual orientation and assembly, or the precise alignment for docking of the matching pieces; the latter necessitates the use of elaborate systems of sensors, with feedback and control, for the remote or automated assembly and disassembly of the components. [ 23 ] In contrast, magnetic connectors can self-align and assemble; [ 26–28 ]

Mid-Infrared Spectrometer Using Opto-Nanofluidic Slot-Waveguide for Label-Free On-Chip Chemical Sensing

A mid-infrared (mid-IR) spectrometer for label-free on-chip chemical sensing was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. Utilizing the large refractive index contrast (Δn ∼ 2) between the liquid core of the waveguide and the Si cladding, a broadband mid-IR lightwave can be efficiently guided and confined within a nanofluidic capillary (≤100 nm wide). The optical-field enhancement, together with the direct interaction between the probe light and the analyte, increased the sensitivity for chemical detection by 50 times when compared to evanescent-wave sensing. This spectrometer distinguished several common organic liquids (e.g., n-bromohexane, toluene, isopropanol) accurately and could determine the ratio of chemical species (e.g., acetonitrile and ethanol) at low concentration (<5 μL/mL) in a mixture through spectral scanning over their characteristic absorption peaks in the mid-IR regime. The combination of CMOS-compatible planar mid-IR microphotonics, and a high-throughput nanofluidic sensor system, provides a unique platform for chemical detection.

Using “Click-e-Bricks” to Make 3D Elastomeric Structures

Soft, 3D elastomeric structures and composite structures are easy to fabricate using click-e-bricks, and the internal architecture of these structures together with the capabilities built into the bricks themselves provide mechanical, optical, electrical, and fluidic functions.

REGIOSELECTIVE SYNTHESIS OF EITHER 1H- OR 2H-1,2,3- TRIAZOLES VIA MICHAEL ADDITION TO α,ß-UNSATURATED KETONES

The Michael reaction of NH-1,2,3-triazole (1) with α,β-unsaturated ketones was studied. 1H-1,2,3-triazolyl-ketones were selectively generated when 1 was combined neat with a variety of enones. The use of aprotic solvents with catalytic base gave the corresponding 2H-regioisomers. Together, these two protocols provide direct access to either the N1- or N2-substituted 1,3-triazolyl ketone regioisomers.

Designing Non‐charging Surfaces from Non‐conductive Polymers

Polymers that prevent the generation of static charge by contact electrification can be fabricated by copolymerizing an appropriate proportion of a molecule that has the tendency to charge positively, and a molecule that has the tendency to charge negatively, against a reference material. These non-conductive polymers resist charging by contact or rubbing, and prevent the adhesion of microscopic particles.

Mid-Infrared Opto-Nanofluidic Slot-Waveguide for Label-Free On-Chip Chemical Sensing

A mid-infrared sensor for label-free on-chip chemical detection was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. A sensitivity with 75 times improvement was achieved compared to conventional evanescent-wave sensing. Mid-infrared spectroscopy is a detection technique commonly used for identifying biochemicals and tracing of toxic molecules, and is free of target labels and sensor surface functionalization. The use of mid-IR spectrum circumvents the need for labeling the sample, because the characteristic wavelength of absorption by many functional groups present in chemical or biological molecules falls within this region of the spectrum. Herein, we present a new chip-scale optofluidic device that utilizes mid-IR techniques for label-free and surface functionalization-free chemical sensing. The optofluidic platform is built using CMOS processes, and is capable of accomplishing broad mid-IR spectral sensing. Fig. 1 schematically illustrates the structure of the mid-IR opto-nanofluidic device. The sensing element is a nanofluidic-channel slot-waveguide with its two ends connected to Si-SiO2-Si slot-waveguides. We embed the entire nanofluidic channel and part of the silicon-oxide slot-waveguides in the PDMS chamber. Upon filling the interior of the chamber with liquid analyte, the solution inside the nanofluidic channel converts the fluid-filled channel into a fluidic slot-waveguide. The mid-IR probe light, after passing through the nanofluidic channel, propagates into the second SiO2 slot-waveguide at the other end. The transmitted light is encoded with the absorption spectrum of the analyte in the fluid because the absorption of probe light by the analyte that fills the nanofluidic channel heavily modulates the intensity of the guided light at the characteristic absorption wavelengths. The enhancement of chemical sensitivity of our fluidic slot-waveguide is evaluated. Fig. 2 (a) compares the predicted optical-field profiles for propagating mid-IR (? = 3.3 μm) radiation within a rectangular-strip waveguide, to that of a nanofluidic slot waveguide. In the case of the rectangular strip-waveguide, the optical field is mainly retained inside the Si core and its penetration as an evanescent wave into the surrounding fluid is small. In the slot-waveguide the optical field is highly concentrated at the center of the fluidic channel and interacts strongly with the liquid inside the channel. Thus, even a slight change in the concentration of analyte will result in a significant modulation of intensity to the guided mid-IR that consequently boosts the sensitivity when sensing chemicals. From the plot in Fig. 2 (b), the enhancement-factor Sslot/Sstrip rises to 75 times as the slot-width narrows to d = 80 nm.