Biomimetic Self-Deformation of Polymer Interpenetrating Network with Stretch-Induced Anisotropicity

The introduction of anisotropicity is the key step of developing stimuli-responsive polymer hydrogels into intelligent devices. Among these anisotropic materials, hydrogels with oriented structures have attracted tremendous attention. However, most of oriented hydrogels are fabricated through aligning nanofillers or microchannels with external fields. In this work, instead of adding inert materials, we report a facile method to fabricate temperature and pH dual-responsive hydrogels with orientated polymer chains. The hydrogels are based on the interpenetrating network (IPN) of poly(acrylic acid) (PAAc) and poly(acrylamide) (PAAm), which can form temperature- and pH-switchable hydrogen bonds (H-bonds). The two polymerized networks are synthesized sequentially, which allows the application of a stretching force on the primary PAAm network and to "memorize" the orientation through the photopolymerization of the secondary PAAc network. Thanks to the aligned polymer chains and the oriented H-bonds, the obtained gels (i.e., PINSIA) exhibit anisotropic volume phase transition in response to temperature and pH. Due to the bilayer structure generated by the photopolymerization, the directions of the internal stresses of these two layers, that is, the bottom and top layer of the PINSIA gels, are perpendicular, driving the gels to form helices when tailored into narrow strips. Furthermore, the helical structure can be programmed by changing the cutting angle and the width of the gel strip and regulated by switching pH and temperature. This work provides a facile method to construct anisotropic hydrogels that can deform into programmable shapes, which could inspire the design and fabrication of smart hydrogels with potential applications as actuators, information carriers, etc.