Tunable Molecular Sieving by Hierarchically Assembled Porous Organic Cage Membranes with Solvent-Responsive Switchable Pores.

Molecular separations involving solvents and organic impurities represent great challenges for environmental and water-intensive industries. Novel materials with intrinsic nanoscale pores offer a great choice for improvement in terms of energy efficiency and capital costs. Particularly, in applications where gradient and ordered separation of organic contaminants remain elusive, smart materials with switchable pores can offer efficient solutions. Here, we report a hierarchically networked porous organic cage membrane with dynamic control over pores, elucidating stable solvent permeance and tunable dye rejection over different molecular weights. The engineered cage membrane can spontaneously modulate its geometry and pore size from water to methanol and DMF in a reversible manner. The cage membrane exhibits ≥585.59 g mol-1 molecular weight cutoff preferentially in water and is impeded by methanol (799.8 g mol-1) and DMF (≈1017 g mol-1), reflecting 36 and 73% change in rejection due to self-regulation and the flexible network, respectively. Grazing incidence X-ray diffraction illustrates a clear peak downshift, suggesting an intrinsic structural change when the cage membranes were immersed in methanol or DMF. We have observed reversible structural changes that can also be tuned by preparing a methanol/DMF mixture and adjusting their ratio, thereby enabling gradient molecular filtration. We anticipate that such cage membranes with dynamic selectivity could be promising particularly for industrial separations and wastewater treatment.

pH-responsive membranes: Mechanisms, fabrications, and applications.

Understanding the mechanisms of pH-responsiveness allows researchers to design and fabricate membranes with specific functionalities for various applications. The pH-responsive membranes (PRMs) are particular categories of membranes that have an amazing aptitude to change their properties such as permeability, selectivity and surface charge in response to changes in pH levels. This review provides a brief introduction to mechanisms of pH-responsiveness in polymers and categorizes the applied polymers and functional groups. After that, different techniques for fabricating pH-responsive membranes such as grafting, the blending of pH-responsive polymers/microgels/nanomaterials, novel polymers and graphene-layered PRMs are discussed. The application of PRMs in different processes such as filtration membranes, reverse osmosis, drug delivery, gas separation, pervaporation and self-cleaning/antifouling properties with perspective to the challenges and future progress are reviewed. Lastly, the development and limitations of PRM fabrications and applications are compared to provide inclusive information for the advancement of next-generation PRMs with improved separation and filtration performance.

Designable Micro-/Nano-Structured Smart Polymeric Materials.

Smart polymeric materials with dynamically tunable physico-chemical characteristics in response to changes of environmental stimuli, have received considerable attention in myriad fields. The diverse combination of their micro-/nano-structural and molecular designs creates promising and exciting opportunities for exploiting advanced smart polymeric materials. Engineering micro-/nano-structures into smart polymeric materials with elaborate molecular design enables intricate coordination between their structures and molecular-level response to cooperatively realize smart functions for practical applications. In this review, recent progresses of smart polymeric materials that combine micro-/nano-structures and molecular design to achieve designed advanced functions are highlighted. Smart hydrogels, gating membranes, gratings, milli-particles, micro-particles and microvalves are employed as typical examples to introduce their design and fabrication strategies. Meanwhile, the key roles of interplay between their micro-/nano-structures and responsive properties to realize the desired functions for their applications are emphasized. Finally, perspectives on the current challenges and opportunities of micro-/nano-structured smart polymeric materials for their future development are presented.

Artificial Perspiration Membrane by Programmed Deformation of Thermoresponsive Hydrogels.

Thermal management is essential for living organisms and electronic devices to survive and maintain their own functions. However, developing flexible cooling devices for flexible electronics or biological systems is challenging because conventional coolers are bulky and require rigid batteries. In nature, skins help to maintain a constant body temperature by dissipating heat through perspiration. Inspired by nature, an artificial perspiration membrane that automatically regulates evaporation depending on temperature using the programmed deformation of thermoresponsive hydrogels is presented. The thermoresponsive hydrogel is patterned into pinwheel shapes and supported by a polymeric rigid frame with stable adhesion using copolymerization. Both shape of the valve and mechanical constraint of the frame allow six times larger evaporation area in the open state compared to the closed state, and the transition occurs at a fast rate (≈1 s). A stretchable membrane is selectively coated to prevent unintended evaporation through the hydrogel while allowing swelling or shrinking of the hydrogel by securing path of water. Consequently, a 30% reduction in evaporation is observed at lower temperature, resulting in regulation of the skin temperature at the thermal model of human skins. This simple, small, and flexible cooler will be useful for maintaining temperature of flexible devices.

Stimuli-Responsive Capsule Membranes for Controlled Release in Pharmaceutical Applications.

BACKGROUND: In conventional drug delivery, the drug concentration in the blood raises once the drug taken, and then peaks and declines. Since each drug has a level above which it is toxic and another level below which it is ineffective, the drug concentration in a patient at a particular time depends on compliance with the prescribed routine. METHODS: To achieve more effective efficacy and fewer side effects of drugs, the drug carriers with desirable dosing and controllable release property of drugs are highly desired. Stimuli-responsive capsules with smart gating membranes or hydrogel-based membranes as capsule shells are ideal candidates. The smart capsule membranes enable efficient encapsulation of drugs within the large inner volume, and the responsive gating membranes or hydrogel-based membranes could control the release rate of encapsulated drugs in responding to environmental stimuli. The trigger stimuli could be either artificial or natural ones corresponding to specific diseases, such as temperature, pH, glucose concentration, specific ion, light, and magnetic field. RESULTS: This review highlights the recent development in stimuli-responsive capsule membranes for controlled release in pharmaceutical applications, including two types of stimuli-responsive capsule membranes with different architectures for on/off release and burst release, which can achieve potential uses of case-dependent on/off release and burst release. CONCLUSION: The preponderances of the smart capsule membranes are that the capsules are with controllable inner space for drug vehicles with desired dose and stimuli-responsive membrane as shell to release drugs at a desired site and/or moment. However, the actual difficulties for the stimuli-responsive capsule membrane systems to go before they can be applied widely in the biomedical fields are discussed. The future works should focus on the improvements of biocompatibility, biodegradability and stimuli-responsiveness of the capsule membranes, easy and scalable fabrication techniques with further decrease of the capsule size for more efficient in vivo applications, and the diversification of the multi-compartmental capsule architectures with multi-stimuli-responsive characteristics for controlled release.

Stimuli-Responsive Surfaces for Tunable and Reversible Control of Wettability.

Surfaces with controllable wettability can be fabricated by embedding superhydrophobic particles into stimuli-responsive hydrogels. When the hydrogel changes its size due to a specific stimulus, the wettability of the surface can be reversibly tuned from superhydrophobic to superhydrophilic. This general method is used to fabricate "smart" membranes for controlling the permeability of chemicals under the influence of multiple stimuli simultaneously.