Ruobing Bai

• Doctor of Philosophy
• Professor (Assistant) at Northeastern University

Adhering hydrogels to various materials is fundamental to a large array of established and emerging applications. The last few years have seen transformative advances in achieving strong hydrogel adhesion, which is a supramolecular phenomenon. Two adherends connect through covalent bonds, noncovalent complexes, polymer chains, polymer networks, or...

Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration in 2001, it has received increasing interest with many recent studies of periodic and buckling behavior. Howeve...

There is current interest in developing photoactive materials that deform on illumination and can thus be used for photomechanical actuation. This is attractive since it can be affected at a distance, different frequencies can be used to actuate different modes and to sense, and corrosion-free lightweight fiber optic cables can deliver significant...

Soft adhesion has been rapidly studied and developed for various applications in recent years. Compared to existing toughening mechanisms based on the adherend or adhesive materials themselves, building architectures or patterns in soft adhesion offers an attractive way of enhancing adhesion without modifying the intrinsic material properties. Howe...

Elastocaloric polymers, whose performance typically relies on phase transformation between amorphous chains and crystalline domains, offer a promising alternative to traditional refrigeration technologies due to their solid-state characteristics and environmental benefits. While engineering polymer-network architecture has shown the potential to bo...

Liquid crystal elastomers (LCEs) are composed of rod-like liquid crystal (LC) molecules (mesogens) linked into elastomeric polymer networks. They present a nematic phase with directionally ordered mesogens at room temperature and an isotropic phase with no order at high temperatures, enabling large thermal-induced deformation. As a result, LCEs hav...

Pressure sensitive adhesives (PSAs) are viscoelastic polymers that can form fast and robust adhesion with various adherends under fingertip pressure. The rapidly expanding application domain of PSAs, such as healthcare, wearable electronics, and flexible displays, requires PSAs to sustain prolonged loads throughout their lifetime, calling for funda...

A hyperelasticity modelling approach is employed for capturing various and complex mechanical behaviours exhibited by macroscopically isotropic polydomain liquid crystal elastomers (LCEs). These include the highly non-linear behaviour of nematic-genesis polydomain LCEs, and the soft elasticity plateau in isotropic-genesis polydomain LCEs, under fin...

The increasing demand for wearable electronics calls for advanced energy storage solutions that integrate high electrochemical performances and mechanical robustness. Ionogel is a promising candidate due to its stretchability combined with high ionic conductivity. However, simultaneously optimizing both the electrochemical and mechanical performanc...

Anode-free all-solid-state lithium metal batteries (ASLMBs) promise high energy density and safety but suffer from a low initial Coulombic efficiency and rapid capacity decay, especially at high cathode loadings. Using operando techniques, we concluded these issues were related to interfacial contact loss during lithium stripping. To address this,...

Liquid crystal elastomers (LCEs) are made of liquid crystal molecules linked into rubber-like polymer networks. An LCE exhibits both the thermotropic property of liquid crystals and large deformation of elastomers. It can be monodomain or polydomain in the nematic phase and transforms to an isotropic phase at elevated temperature. These features ha...

Hydrogels composed of amorphous polymer networks are widely used as soft and stretchable components in diverse devices and machines. However, existing amorphous hydrogels are susceptible to fatigue fracture. Here, we propose that amorphous hydrogels with dynamic covalent bonds can resist fatigue fracture, so long as the dynamics of bond recovery is...

Photoactive liquid crystal elastomers are polymer networks of liquid crystal mesogens embedded with chromophores like azobenzene. They undergo large deformation when illuminated by light of a certain wavelength through photochemical reaction, inspiring exciting new applications. However, despite the recent progresses in both the experiment and theo...

Photoactive liquid crystal elastomers are polymer networks of liquid crystal mesogens embedded with chromophores like azobenzene. They undergo large deformation when illuminated by light of a certain wavelength through photochemical reaction, inspiring exciting new applications. However, despite the recent progresses in both the experiment and theo...

The emergent photoactive materials obtained through photochemistry make it possible to directly convert photon energy to mechanical work. There has been much recent work in developing appropriate materials, and a promising system is semicrystalline polymers of the photoactive molecule azobenzene. We develop a phase field model with two order parame...

Self-oscillating systems are powerful tools for transducing static energy inputs into repetitive motions without the aid of external control units. The challenge in sustaining the far-from-equilibrium motion of an oscillating material is to avoid the tendency of reaching thermodynamic equilibrium or get pinned at steady states in the dynamic proces...

The emergent photoactive materials through photochemistry make it possible to directly convert photon energy to mechanical work. There is much recent work in developing appropriate materials and a promising new system is semi-crystalline polymers of the photoactive molecule azobenzene. We develop a phase field model with two order parameters for th...

There is current interest in developing photoactive materials that deform on illumination. The strategy is to develop new photoactive molecules in solution, and then to incorporate these in the solid-state either by crystallization or by inserting them into polymers. This letter shows that the kinetics and the nature of the photo-induced phase tran...

When two stretchable materials (e.g., hydrogels, elastomers, and biological tissues) are adhered, the interface should be stretchable, without constraining the deformation and degrading adhesion. Here we develop methods to characterize stretchable adhesion. We do so by topological adhesion, using polyacrylamide hydrogels as adherends, chitosan as s...

Photoactive nematic elastomers are soft rubbery solids that undergo deformation when illuminated. They are made by incorporating photoactive molecules like azobenzene into nematic liquid crystal elastomers. Since its initial demonstration in 2001, it has received increasing interest with many recent studies of periodic and buckling behavior. Howeve...

Recent innovations highlight the integration of diverse materials with synthetic and biological hydrogels. Examples include brain-machine interfaces, tissue regeneration, and soft ionic devices. Existing methods of strong adhesion mostly focus on the chemistry of bonds and the mechanics of dissipation, but largely overlook the molecular topology of...

In article number 1804916, Ximin He and co‐workers present a universal (bio)chemical sensing platform based on hydrogel interferometry. The stimuli‐responsive gel thin film presents different colors, revealing rich environmental metrics. The unique local concentration effect and signal amplification effect lead to ultrahigh sensitivity up to the fe...

Developing ultrasensitive chemical sensors with small scale and fast response through simple design and low‐cost fabrication is highly desired but still challenging. Herein, a simple and universal sensing platform based on a hydrogel interferometer with femtomol‐level sensitivity in detecting (bio)chemical molecules is demonstrated. A unique local...

Most tough hydrogels suffer accumulated damages under cyclic loads. The damages may stem from breakage of covalent bonds, unzipping of ionic crosslinks, or desorption of polymer chains from nanoparticle surfaces. Recent experiments report that when a tough hydrogel is subject to cyclic loads, the stress-stretch curves of tough hydrogels change cycl...

Achieving strong adhesion between wet materials (i.e., tissues and hydrogels) is challenging. Existing adhesives are weak, toxic, incompatible with wet and soft surfaces, or restricted to specific functional groups from the wet materials. The approach reported here uses biocompatible polymer chains to achieve strong adhesion and retain softness, bu...

Polyacrylamide hydrogels are highly stretchable and nearly elastic. Their stress-stretch curves exhibit small hysteresis, and change negligibly after many loading cycles. Polyacrylamide is used extensively in applications, and is the primary network for many types of tough hydrogels. Recent experiments have shown that polyacrylamide hydrogels are s...

Living organisms ubiquitously display colors that adapt to environmental changes, relying on the soft layer of cells or proteins. Adoption of soft materials into an artificial adaptive color system has promoted the development of material systems for environmental and health monitoring, anti‐counterfeiting, and stealth technologies. Here, a hydroge...

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress–stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some...

A plastic liquid such as toothpaste and butter deforms like an elastic solid under a small stress, and like a plastic solid under a large stress. Recently, plastic liquids have been used as compliant electrodes for elastomeric transducers. Here we study the deformation of a plastic liquid adherent on an elastomer when the elastomer is stretched mon...

Tough hydrogels of many chemical compositions have been developed in recent years, but their fatigue fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study t...

This paper presents a comprehensive model coupling the effects of hydrostatic stress, surface/interface stress, phase transformation and the structure of electrodes. First, the governing equation of moving phase interface with hydrostatic stress is established. Under the effect of hydrostatic stress, phase transformation process is much faster, whi...

Some molecules change shape upon receiving photons of certain frequencies, but here we study light-induced deformation in ordinary dielectrics with no special optical effects. All dielectrics deform in response to light of all frequencies. We derive a dimensionless number to estimate when light can induce large deformation. For a structure made of...