Hu, Yuhang

Hu Y, Kim P, Aizenberg J. Harnessing structural instability and material instability in the hydrogel-actuated integrated responsive structures (HAIRS). Extreme Mechanics Letters. 2017;13 :84-90.Abstract

We describe the behavior of a temperature-responsive hydrogel actuated integrated responsive structure (HAIRS). The structure is constructed by embedding a rigid high-aspect-ratio post in a layer of poly(Nisopropylacrylamide) (PNIPAM) hydrogel which is bonded to a rigid substrate. As the hydrogel contracts, the post abruptly tilts. The HAIRS has demonstrated its broad applications in generating reversible micropattern formation, active optics, tunable wettability, and artificial homeostasis. To quantitatively describe and predict the system behavior, we construct an analytical model combining the structural instability, i.e. buckling of the post, and the material instability, i.e. the volume phase transition of PNIPAM hydrogel. The two instabilities of the system result in a large hysteresis in response to heating and cooling processes. Experimental results validate the predicted phenomenon of the abrupt tilting as temperature and large hysteresis in a heating-and-cooling cycle in the PNIPAM actuated HAIRS. Based on this model, we further discuss the influence of the material properties on the actuation of the structure.

Hu Y, You J-O, Aizenberg J. Micropatterned Hydrogel Surface with High-Aspect-Ratio Features for Cell Guidance and Tissue Growth. ACS Appl. Mater. Interfaces. 2016;8 (34) :21939-21945.Abstract

Surface topography has been introduced as a new tool to coordinate cell selection, growth, morphology, and differentiation. The materials explored so far for making such
structural surfaces are mostly rigid and impermeable. Hydrogel, on the other hand, was proved a better synthetic media for cell culture because of its biocompatibility, softness, and high permeability. Herein, we fabricated a poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogel substrate with high-aspectratio surface microfeatures. Such structural surface could effectively guide the orientation and shape of human mesenchymal stem cells (HMSCs). Notably, on the flat hydrogel surface, cells rounded up, whereas on the microplate patterned hydrogel surface, cells elongated and aligned along the direction parallel to the plates. The microplates were 2 μm thick, 20 μm tall, and 10−50 μm wide. The interplate spacing was 5−15 μm, and the intercolumn spacing was 5 μm. The elongation of cell body was more pronounced on the patterns with narrower interplate spacing and wider plates. The cells behaved like soft solid. The competition between surface energy and elastic energy defined the shape of the cells on the structured surfaces. The soft permeable hydrogel scaffold with surface structures was also demonstrated as being viable for longterm cell culture, and could be used to generate interconnected tissues with finely tuned cell morphology and alignment across a few centimeter sizes.

Hou X, Hu Y, Grinthal A, Khan M, Aizenberg J. Liquid-based gating mechanism with tunable multiphase selectivity and antifouling behaviour. Nature [Internet]. 2015;519 (7541) :70-73. Full TextAbstract
Living organisms make extensive use of micro- and nanometre-sized pores as gatekeepers for controlling the movement of fluids, vapours and solids between complex environments. The ability of such pores to coordinate multiphase transport, in a highly selective and subtly triggered fashion and without clogging, has inspired interest in synthetic gated pores for applications ranging from fluid processing to 3D printing and lab-on-chip systems. But although specific gating and transport behaviours have been realized by precisely tailoring pore surface chemistries and pore geometries, a single system capable of controlling complex, selective multiphase transport has remained a distant prospect, and fouling is nearly inevitable. Here we introduce a gating mechanism that uses a capillary-stabilized liquid as a reversible, reconfigurable gate that fills and seals pores in the closed state, and creates a non-fouling, liquid-lined pore in the open state. Theoretical modelling and experiments demonstrate that for each transport substance, the gating threshold—the pressure needed to open the pores—can be rationally tuned over a wide pressure range. This enables us to realize in one system differential response profiles for a variety of liquids and gases, even letting liquids flow through the pore while preventing gas from escaping. These capabilities allow us to dynamically modulate gas–liquid sorting in a microfluidic flow and to separate a three-phase air–water–oil mixture, with the liquid lining ensuring sustained antifouling behaviour. Because the liquid gating strategy enables efficient long-term operation and can be applied to a variety of pore structures and membrane materials, and to micro- as well as macroscale fluid systems, we expect it to prove useful in a wide range of applications.
Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. [Internet]. 2014;26 (4) :1622-1628. Publisher's VersionAbstract

Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the possible structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method for introducing anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol−gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure. Further, we prevent the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy and infrared spectroscopy studies clearly identify increasing degrees of sol−gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.

Yao X, Hu Y, Grinthal A, Wong T-S, Mahadevan L, Aizenberg J. Adaptive fluid-infused porous films with tunable transparency and wettability. Nature Materials [Internet]. 2013;12 :529-534. Full TextAbstract
Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported1-3. Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film's transparency and its ability to continuously manipulate various low-surface-tension droplets from free-sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.
Kim P, Hu Y, Alvarenga J, Kolle M, Suo Z, Aizenberg J. Rational Design of Mechano-Responsive Optical Materials by Fine Tuning the Evolution of Strain-Dependent Wrinkling Patterns. Adv. Optical Mater. [Internet]. 2013;1 (5) :381-388. Publisher's VersionAbstract
Rational design strategies for mechano‐responsive optical material systems are created by introducing a simple experimental system that can continuously vary the state of bi‐axial stress to induce various wrinkling patterns, including stripes, labyrinths, herringbones, and rarely observed checkerboards, that can dynamically tune the optical properties. In particular, a switching of two orthogonally oriented stripe wrinkle patterns from oxidized polydimethylsiloxane around the critical strain value is reported, as well as the coexistence of these wrinkles forming elusive checkerboard patterns, which are predicted only in previous simulations. These strain‐induced wrinkle patterns give rise to dynamic changes in optical transmittance and diffraction patterns. A theoretical description of the observed pattern formation is presented which accounts for the residual stress in the membrane and allows for the fine‐tuning of the window of switching of the orthogonal wrinkles. Applications of wrinkle‐induced changes in optical properties are demonstrated, including a mechanically responsive instantaneous privacy screen and a transparent sheet that reversibly reveals a message or graphic and dynamically switches the transmittance when stretched and released.
Zarzar LD, Liu Q, He X, Hu Y, Suo Z, Aizenberg J. Multifunctional Actuation Systems Responding to Chemical Gradients. Soft Matter. 2012;8 (32) :8289-8293. SoftMatter_Zarzar2012.pdf