The 2017 Schoenborn Symposium Was Outstanding

1. Chan, C. M. Polymer Surface Modification and Characterization; Carl Hanser, GmbH & Co.: 1994 [/collapse]

1. Conway, J.M., J.V. Zurawski, L.L. Lee, S.E. Blumer-Schuette, R.M. Kelly. (2015) Lignocellulosic Biomass Degradation by the Extremely Thermophilic Genus Caldicellulosiruptor. In: Thermophilic Microorganisms. Fuli Li, editor. Caister Academic Press. pp 91-120.

2. Conway, J.M., W.S. Pierce, J.H. Le, J.H. Wright, G.W. Harper, A.L. Tucker, J.V. Zurawski, L.L. Lee, S. E. Blumer-Schuette, R.M. Kelly. (2016) Multi-Domain, Surface Layer Associated Glycoside Hydrolases Contribute to Plant Polysaccharide Degradation by Caldicellulosiruptor J. Biol. Chem. 291, 6732-6747.

3. Lipscomb, G.L., J. M. Conway, S.E. Blumer-Schuette, R.M. Kelly, and M.W.W. Adams. (2016) Highly Thermostable Kanamycin Resistance Marker Expands the Toolkit for Genetic Manipulation of Caldicellulosiruptor bescii. Appl. Environ. Microbiol. 82(14), 4421-4428. [/collapse]

1. Ying Liu, Y., Boyles, J., Genzer & Michael D. Dickey. Self-folding of polymer sheets using local light absorption. Soft Matter 8, 1-6 (2012).

2. Mailen, R.W., Liu, Y., Dickey, M.D., Zikry, M. & Genzer, J. Modelling of shape memory polymer sheets that self-fold in response to localized heating. Soft Matter 11, 7827-7834 (2015).[/collapse]

Background: Integrating liquid metals with other low modulus materials, such as elastomers and gels, imparts high electrical conductivity into a composite without altering its mechanical properties. Combined, these materials form ‘softer than skin’ systems that are well-suited for forming conformal interfaces and ultra-compliant soft electronics. This work characterizes the behavior of a eutectic alloy of gallium and indium (75% Ga, 25% In, by weight, ‘EGaIn’) in response to electric fields. The metal remains a liquid at room temperature (M.P., 15.5 ^oC) and has low toxicity. These fluidic metals are injectable into microfluidic systems, fibers, and capillary networks. Once injected, the metal remains in its place because of the adhesive nature of its thin native oxide. Preventing the oxide adhesion within microchannels enables reversible actuation of EGaIn. Actuating liquid metals may be useful for soft actuators, reconfigurable optical displays, frequency tunable antennas, and other opto-fluidic technologies.

Results: This work studies methods to reversibly move droplets of EGaIn through microchannels using low voltages. Pre-wetting the channels with an aqueous solution prior to injecting the metal prevents oxide adhesion; the water forms an interfacial ‘slip-layer’ between the metal and channel wall. Thereafter, an applied electric field (~10-20 V/m) actuates the liquid metal by establishing a gradient of surface tension; this effect is known as continuous electrowetting (CEW). Although CEW has been utilized before with mercury, which is toxic, the presence of a Ga oxide complicates CEW behavior. This work compares the electro – hydrodynamic behavior of EGaIn with and without the presence of an oxide ‘skin’. Optical microscopy and electrochemical measurements characterize the movement of EGaIn droplets under a variety of conditions. Specifically, we elucidate the influence of the electrolyte (i.e., composition, pH, and viscosity) on the metal-electrolyte interface. In addition, thin film interference characterizes the thickness and dynamics of the interfacial slip-layer of water.

Conclusions: Evaporation of water may cause the slip-layer to disappear and impede CEW. Therefore, this work explores novel microfabrication strategies to design interfaces that prevent oxide adhesion. These interfaces form alternative slip-layers, which also renders microfluidic channels with self- healing and self-cleaning abilities. In addition, these interfaces may be useful for the transport of chemical or biological samples within three-dimensional microfluidic devices and vascular networks using low voltages. [/collapse]

Background: Aromatic polyesters are one of the most important classes of polymers in textile and packaging industries due to their superior mechanical, optical, and processing properties. In order to comply with the low-VOC movement, such specialty polyesters were rendered water-dispersible by functionalizing the polymer backbone with ionic monomers. As a result of being partially soluble, these polyesters forms self-assembled nanoscale particles in water without the requirement of any additional stabilizer(s). These extremely small sized (~20 nm) nanoparticles are composed of hundreds of polymer molecules and offer various colloidal and morphological properties that are significantly different from the conventional emulsion polymerized latex particles. For instance, their nanometer dimensions can be very useful for developing nanocoatings with interesting optical properties.

Results: We fabricated nanofilms of brilliant colors using these polymer dispersions via facile convective deposition method. The microscale thickness of these nanofilms could be correlated to their macroscale optical properties via the constructive interference theory. Additionally, surface roughness modulation of these nanocoatings allowed us to obtain thin-film interference on both macro- and micro-level. Moreover, we observed coffee-ring effect as the addition of water droplets on such thin-films created multiple colorful ring-patterns where surfactants or electrolytes suppressed this effect.

Conclusions: Structural colors of purely physical origin are gaining a lot of interest in industries and academia due to numerous benefits over pigment-based colors, such as more vibrant color formation, resistance to photobleaching, and environment-friendly as no toxic chemical is required. This research is important in understanding the fundamental mechanism of film formation by these polymer nanoparticles. Moreover, the findings of our study open a new possibility and application of such environment-friendly waterborne dispersions in painting, photonic paper, and optical displays. [/collapse]

Background: Folding and bending are common phenomena found in nature, prominent in the formation of structures ranging from plants to proteins. By harnessing the potential of man-made materials, self-folding structures have applications in everything from biomedical engineering to transportation methods. Past research has focused on the ability to understand self-folding mechanisms of thermoplastics for use in robotic structures or simplistic shape formation [1-3]. Particular interest in self-bending materials has focused on the ability to control bimorph systems composed of hydrogels or elastomeric materials [4-5]. However, hydrogels and elastomers lack the strength needed for many practical applications. This work takes inspiration from nature to design thermoplastic structures that mimic natural systems through the generation and control of self-automated folds and global curvature. By increasing the complexity of possible designs, we can have a greater impact and generate final structures with a wider range of overall applications.

Results: We induce self-actuation into our materials by patterning pre-strained polystyrene films with ink from an inkjet printer. The polystyrene sheets are pre-strained to shrink by ~ 55% when heated above their activation temperature (T_a ~ T_g) which is ~ 103°C. By patterning inked regions along the surface of the material localized heating, and therefore shrinkage, is achieved via strain gradients through the thickness of the material [1]. The design of these inked regions (i.e., ink darkness and distribution) determines the direction of folding, onset actuation time, and final structure. These results are compared with finite element modeling as a predictive tool [6].

Conclusions: An indirect and direct mechanism of curvature were identified and systematically studied with our self-actuated polystyrene material. This global curvature control is useful for the production of positive and negative Gaussian curvature from planar polymer sheets. The degree of curvature is directly related to the ink distribution and darkness of each sample as well as the aspect ratio and geometry of the starting substrates. Experimental and computational results were quantitatively and qualitatively compared with excellent agreement for the production of biologically-inspired gripping devices with the ability to hold ~ 925x their own weight.

References:

1. Liu, Y., Boyles, J., Genzer, J., & Dickey, M. Self-folding of polymer sheets using local light absorption. Soft Matter. 8, 1-6 (2012).
2. Felton, S., al., Self-folding with shape memory composites. Soft Matter 9, 7688-7694 (2013).
3. Liu, Y., Mailen, R., Zhu, Y., Dickey, M., & Genzer, J. Simple geometric model to describe self-folding of polymer sheets. Rev. E 89, 1-8 (2014).
4. Eugonov, A., Korvink, J., Luchnikov, V. Polydimethylsiloxane bilayer films with an embedded spontaneoud curvature. Soft Matter 12, 45-52 (2016).
5. Abdullah, A., Braun, P., Hsia, K. Programmable shape transformation of elastic spherical domes. Soft Matter 12, 6184-6195 (2016).
6. Mailen, R., Liu, Y., Dickey, M., Zikry, M., Genzer, J. Modelling of shape memory polymer sheets that self-fold in response to localized heating. Soft Matter 11, 7827-7834 (2015). [/collapse]

Background: 3D printing is the process of joining materials to build objects from a computer-aided model (CAD) data, usually layer-upon-layer. Polymers are the most common materials to be printed today due to the simplicity of extruding them in molten form that quickly cools and hence solidifies. Although there is a great demand for printing conductive inks for electronics [1], current methods for 3D printing metals tend to be prohibitively expensive, and use energy-intensive lasers at sintering temperatures in excess of 800°C. Secondly, they need vacuum-like pressures to avoid oxidation while handling metal nanoparticles, leading to porosity in finished parts, low resolution and poor electrical conductivity, apart from having slow printing speeds. Finally, the operating procedures are impossible to integrate them with various polymeric, organic, soft and biological materials. Here, we present an alternate but simple approach that utilizes low melting point gallium-based alloys as complements to existing materials for 3D printing electronics allowing co-printing of these metals with polymers at room temperature.

Results: We have utilized these metals to build mechanically stable structures due to the formation of a thin oxide skin on the surface [2] despite having high surface tension (~10x water). The oxide skin is passivating, forms spontaneously in presence of air or dissolved oxygen and allows us to direct-write planar as well as free-standing, out-of-plane conductive microstructures down to a resolution of ~10 microns [3], on-demand using a shear-driven flow occurring at relatively low pressures (~10s of kPa). We exhibit the patterning of 3D multilayered microchannels with vasculature using these printed liquid metals acting as a sacrificial template at room-temperature [4], that can be employed in numerous lab-on-a-chip devices. We also demonstrate rapid prototyping of functional electronics such as flexible and stretchable antennas for radio-frequency defense communications, as well as consumer-based electronic devices like inductive power coils for wireless charging of smartphones, and wearable thermoelectric generators (TEGs) for energy-harvesting applications.

Conclusions: In summary, we show that a shear-driven flow dispensing approach for printing liquid metals can fabricate 2D & 3D microarchitectures. This mechanism validates that skin forming liquids can be molded into several shapes earlier prohibited by the weakening effects of gravity and surface tension, in order to print soft electronic devices at room temperature.

References:

1. Parekh, Dishit P., Denis Cormier, and Michael D. Dickey (2015). Multifunctional Printing: Incorporating Electronics into 3D Parts Made by Additive Manufacturing. Additive Manufacturing, CRC Press, 8: 215-258.

2. Michael D. Dickey (2014). Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS Appl. Mater. Interfaces., 6: 18369-18379.

3. Trlica, C., Dishit P. Parekh, Laza Panich, Collin Ladd, and Michael D. Dickey (2014). 3-D Printing of Liquid Metals for Stretchable and Flexible Conductors. Proceedings of SPIE, 9083: 1-10.

4. Parekh, Dishit P., Collin Ladd, Laza Panich, Khalil Moussa, and Michael D. Dickey (2016). 3D Printing of Liquid Metals as Fugitive Inks for Fabrication of 3D Microfluidic Channels. Chip, 16: 1812-1820. [/collapse]