DNAzymes, screened through in vitro selection, have shown great promise as molecular tools in the design of biosensors and nanodevices. The catalytic activities of DNAzymes depend specifically on cofactors and show multiple enzymatic turnover properties, which make DNAzymes both versatile recognition elements and outstanding signal amplifiers. Combining nanomaterials with unique optical, magnetic and electronic properties, DNAzymes may yield novel fluorescent, colorimetric, surface-enhanced Raman scattering (SERS), electrochemical and chemiluminescent biosensors. Moreover, some DNAzymes have been utilized as functional components to perform arithmetic operations or as “walkers” to move along DNA tracks. DNAzymes can also function as promising therapeutics, when designed to complement target mRNAs or viral RNAs, and consequently lead to down-regulation of protein expression. This feature article focuses on the most significant achievements in using DNAzymes as recognition elements and signal amplifiers for biosensors, and highlights the applications of DNAzymes in logic gates, DNA walkers and nanotherapeutics.

Hydrogels, as classic soft materials, are important materials for tissue engineering and biosensing with unique properties, such as good biocompatibility, high stretchability, strong adhesion, excellent self-healing, and self-recovery. Conductive hydrogels possess the additional property of conductivity, which endows them with advanced applications in actuating devices, biomedicine, and sensing. In this review, we provide an overview of the recent development of conductive hydrogels in the field of strain sensors, with particular focus on the types of conductive fillers, including ionic conductors, conducting nanomaterials, and conductive polymers. The synthetic methods of such conductive hydrogel materials and their physical and chemical properties are highlighted. At last, challenges and future perspectives of conductive hydrogels applied in flexible strain sensors are discussed.

Recently, hydrogel flexible sensors have attracted wide attention in the field of wearable electronic devices. However, they need an external solid-state power supply to output stable signals. The huge modulus difference between solid-state power supplies and hydrogel flexible sensors is not conducive to the integration, portability, and application of hydrogel flexible sensors. Therefore, it is necessary to design power supplies with a similar modulus to those of flexible sensors and assemble them with flexible sensors to develop self-powered flexible sensing systems, which is important to facilitate the integration, portability, and application of flexible sensors. In this work, polyvinyl alcohol (PVA), sodium chloride (NaCl), and N-hydroxyethyl acrylamide (HEAA) were used to synthesize a fully physically crosslinked double network hydrogel (PNH DN hydrogel) with excellent mechanical performance (tensile stress/strain of 3.76 MPa/872.15%), high ionic conductivity (1.3 S m⁻¹), and rapid self-recovery (energy dissipation/peak stress recovery efficiency of 57.08%/99.06% after a rest time of 10 min). Based on PNH DN hydrogels, a hydrogel strain sensor and a flexible solid-state supercapacitor were prepared, respectively. The hydrogel strain sensor based on the PNH DN hydrogel showed good sensitivity (gauge factor = 1.74), a wide detection range (0–735%), and a negligible response delay time (0.06 s), which are helpful for timely and reliable detection of various mechanical deformations. Meanwhile, the supercapacitor based on the PNH DN hydrogel exhibited an areal capacitance of 2.9 mF cm⁻², a high cycle stability (areal capacitance retention rate of 85.71% after 2000 galvanostatic charge–discharge cycles at a current density of 0.5 mA cm⁻²), and mechanical stability (cyclic voltammetry curves were almost overlapped under 0°, 45°, 90°, 135°, and 180° bending). Finally, the hydrogel strain sensor and supercapacitor were integrated into a self-powered flexible sensing system, which could stably and effectively detect human motions, such as finger bending, neck bending, and frowning. Overall, this work provides a new idea to improve the integration, portability, and application of hydrogel flexible sensors and can broaden the application of hydrogels in the field of new energy.

Bilayer hydrogel actuators, consisting of an actuating layer and a functional layer, show broad applications in areas such as soft robotics, artificial muscles, drug delivery and tissue engineering due to their inherent flexibility and responses to stimuli. However, to achieve the compatibility of good stimulus responses and high mechanical properties of bilayer hydrogel actuators is still a challenge. Herein, based on the double-network strategy and using the synchronous ultraviolet (UV) polymerization method, an upper critical solution temperature (UCST)-type bilayer hydrogel actuator was prepared, which consisted of a poly(acrylamide-co-acrylic acid)[MC] actuating layer and an agar/poly(N-hydroxyethyl acrylamide-co-methacrylic acid)[AHA] functional layer. The results showed that the tensile stress/strain of the bilayer hydrogel actuator was 1161.21 KPa/222.07%. In addition, the UCST of bilayer hydrogels was ~35 °C, allowing the bilayer hydrogel actuator to be curled into an “◎” shape, which could be unfolded when the temperature was 65 °C, but not at a temperature of 5 °C. Furthermore, hydrogel actuators of three different shapes were designed, namely “butterfly”, “cross” and “circle”, all of which demonstrated good actuating performances, showing the programmable potential of bilayer hydrogels. Overall, the bilayer hydrogels prepared using double-network and synchronous UV polymerization strategies realized the combination of high mechanical properties with an efficient temperature actuation, which provides a new method for the development of bilayer hydrogel actuators.