Zi Liang Wu

Three-dimensional shape transformations of hydrogel sheets induced by small-scale modulation of internal stresses

Although Nature has always been a common source of inspiration in the development of artificial materials, only recently has the ability of man-made materials to produce complex three-dimensional (3D) structures from two-dimensional sheets been explored. Here we present a new approach to the self-shaping of soft matter that mimics fibrous plant tissues by exploiting small-scale variations in the internal stresses to form three-dimensional morphologies. We design single-layer hydrogel sheets with chemically distinct, fibre-like regions that exhibit differential shrinkage and elastic moduli under the application of external stimulus. Using a planar-to-helical three-dimensional shape transformation as an example, we explore the relation between the internal architecture of the sheets and their transition to cylindrical and conical helices with specific structural characteristics. The ability to engineer multiple three-dimensional shape transformations determined by small-scale patterns in a hydrogel sheet represents a promising step in the development of programmable soft matter.

Ultrathin tough double network hydrogels showing adjustable muscle-like isometric force generation triggered by solvent

Ultrathin double-network hydrogels, which have super-high toughness under micro-scale thickness (elastic elongation epsilon(b) > 1000%, tensile strength sigma(b) > 2 MPa and tearing energy G approximately 600 J m(-2)), and solvent-triggered fast and high isometric stress generation, were synthesized by coupling the salt-controlled swelling process and polymer chain pre-reinforced technique.

Control superstructure of rigid polyelectrolytes in oppositely charged hydrogels via programmed internal stress

Biomacromolecules usually form complex superstructures in natural biotissues, such as different alignments of collagen fibres in articular cartilages, for multifunctionalities. Inspired by nature, there are efforts towards developing multiscale ordered structures in hydrogels (recognized as one of the best candidates of soft biotissues). However, creating complex superstructures in gels are hardly realized because of the absence of effective approaches to control the localized molecular orientation. Here we introduce a method to create various superstructures of rigid polyanions in polycationic hydrogels. The control of localized orientation of rigid molecules, which are sensitive to the internal stress field of the gel, is achieved by tuning the swelling mismatch between masked and unmasked regions of the photolithographic patterned gel. Furthermore, we develop a double network structure to toughen the hydrogels with programmed superstructures, which deform reversibly under large strain. This work presents a promising pathway to develop superstructures in hydrogels and should shed light on designing biomimetic materials with intricate molecular alignments.

Stimuli-responsive topological change of microstructured surfaces and the resultant variations of wetting properties.

It is now well established that topological microstructures play a key role in the physical properties of surfaces. Stimulus-induced variations of topological microstructure should therefore lead to a change in the physical properties of microstructured responsive surfaces. In this paper, we demonstrate that roughness changes alter the wetting properties of responsive organic surfaces. Oriented nematic liquid crystalline elastomers (LCEs) are used to construct the microstructured surfaces via a replica molding technique. The topological microstructure of the surfaces covered with micropillars changes with temperature, due to the reversible contraction of the LCE pillars along the long axis at the nematic-to-isotropic phase transition. This is directly observed for the first time under environmental scanning electron microscopy (E-SEM). A high boiling point liquid, glycerol, is used to continuously monitor the contact angle change with temperature. The glycerol contact angle of the microstructured surfaces covered with small pillars decreases from 118° at room temperature to 80° at 140 °C, corresponding to a transition from Cassie state to Wenzel state.

Swelling-induced long-range ordered structure formation in polyelectrolyte hydrogel

A millimeter-scale periodic structure is created in a polyelectrolyte hydrogel by the rapid-heterogeneous swelling process, and is frozen by the polyion complexation of the polyelectrolyte network with the oppositely charged, semi-rigid polyelectrolyte. The hydrogel is synthesized from a cationic monomer, N-[3-(N,N-dimethylamino)propyl] acrylamide methyl chloride quaternary (DMAPAA-Q), in the presence of a small amount of the oppositely charged poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT) that has a semi-rigid nature. During the swelling process, surface creasing due to the large mismatching of swelling degree between the surface layer and the inner one of the poly DMAPAA-Q (PDMAPAA-Q) gel occurs, which induces highly oriented semi-rigid PBDT molecules along the tensile direction of the crease pattern. To accompany the evolution of surface creasing, a lattice-like periodic birefringence pattern is formed, which is frozen permanently by the strong polyion complex formation, even after the surface instability pattern of the gel disappears completely throughout the dynamic coalescence. In this work we rationally clarified that formation of such a long-range ordered non-equilibrium structure in the polyelectrolyte hydrogel by the rapid-heterogeneous swelling process requires the following three indispensable conditions: (i) swelling-induced surface creasing; (ii) polyion complex formation; and (iii) a semi-rigid or rigid dopant. This sort of non-equilibrium structure formation mechanism may help understand how biomacromolecules that are rigid polyelectrolytes, such as deoxyribonucleic acid, microtubules and actin filaments, form rich architectures during the growth of biological organs.

Hydrogel with Cubic-Packed Giant Concentric Domains of Semi-Rigid Polyion Complex

We report a novel giant oriented structure observed in plate hydrogels synthesized by photo-polymerization of cationic monomers with a cross-linker in the presence of a semi-rigid polyanion as the dopant. The giant structure, formed viaself-assembly of the semi-rigid polyion complex, consists of millimetre-scale cubic packed concentric cylindrical domains that are sandwiched by two homeotropically aligned outer layers. A universal relationship between the diameter of the cylinders D and the thickness of the swollen gel T is observed, as D = 0.5T, regardless the change in the concentrations of the polyanion and precursor cationic monomer. This result permits us to induce the giant concentric structure into hydrogels with tunable cylindrical sizes.

Self-assembled structures of a semi-rigid polyanion in aqueous solutions and hydrogels

Poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), a kind of liquid-crystalline (LC) molecule, has high molecular weight, negative charge and a semi-rigid structure. The aqueous solution of PBDT shows nematic liquid crystalline state above a critical PBDT concentration, CLC*, of 2 wt%-3wt%. Different from the flexible polyelectrolyte, PBDT shows a variety of self-assembling structures in aqueous solution with and without salt due to the semi-rigid nature and highly charged property. In addition, the hydrogels with ordered structure are developed by polymerizing a cationic monomer N-[3-(N,N-dimethylamino) propyl] acrylamide methyl chloride quarternary (DMAPAA-Q) in the presence of a small amount of PBDT below the CLC*. During the polymerization of cationic monomer, the polycations form a complex with semi-rigid PBDT through electrostatic interaction; these complexes self-assemble into ordered structures that are frozen in the hydrogel. Several different structures, including the anisotropic, dual network-like structure, and cylindrically symmetric structure, with various length scales from micrometer to millimeter, are observed. The hydrogels with ordered liquid crystalline assemblies and particular optical properties should promise applications in many fields, such as in bionics, tissue engineering, and mechano-optical sensors.