Make flytraps great again! Scientists make the hydrogel “wave”

Biological systems use cross-scale dissipative systems to achieve signal coupling and feedback, providing dynamic functions such as homeostasis, circadian rhythm, environmental adaptation, and signal transduction. They inspired researchers to design new dynamic self-regulating materials that go beyond classical biomimetic stimulus-responsive materials.

In biosignal transduction, the perceived stimulus is converted into a transient intermediate signal that responds through a feedback loop. The thermodynamic non-equilibrium conditions promote the complex dynamic functions of the energy-dissipative system. In synthetic materials, despite recent advances in dissipative systems and feedback control systems, such as chemical-mechanical feedback and physical/chemical oscillators, the realization of homeostasis and signal transduction under non-equilibrium conditions remains undiscussed. If such a new system is realized, it will provide a new way for the next generation of bio-inspired smart materials.

Recently, the research group of Professor Olli Ikkala of Aalto University in Finland and the research group of Professor Arri Priimagi of the University of Tampere jointly reported on the homeostatic oscillation of feedback-controlled hydrogel systems and the study of material signal transduction inspired by Flytrap, entitled “Feedback-controlled hydrogels with homeostatic oscillations and dissipative signal.” transduction”, the results were published in the international journal Nature Nanotechnology.

Dr. Zhang Hang, a postdoctoral researcher at the Academy of Finland, is the first author of the paper, while Dr. Zeng Hao, a researcher at the Academy of Finland, Professor Arri Priimagi from the University of Tampere and Professor Olli Ikkala from Aalto University are co-corresponding authors.

How to make a hydrogel “wave”

Poly(N-isopropyl acrylamide) (PNIPAm) hydrogels with thermal phase change functions were selected for this work, and interconnected porous channels were formed during the preparation of the hydrogel by removing agarose as a sacrificial template. This design allows PNIPAm gels to remain transparent below the lowest critical solution temperature (LCST, 36°C) and have extremely strong light scattering properties above LCST (Figure 1). At room temperature, the laser beam passes through the transparent PNIPAm channel at the point of transmission and is reflected through a mirror to the point of incidence of the polyacrylamide (PAAm) gel. Since PAAm gels contain gold nanoparticles that absorb light efficiently, local photothermal heating at the incident laser spot can be achieved, while heat is delayed by heat conduction to PNIPAm gel at the transmission point. When the temperature at the transmission point is higher than LCST, the incident beam is blocked due to strong light scattering, which completely prevents the beam from reaching the PAAm side, causing the entire system to cool down. When the transmission point temperature is below LCST, the beam can pass through again, starting a new heating-cooling cycle. Thus, a photothermal self-oscillation system controlled by a negative feedback loop is realized. It provides stable temperature self-oscillation or damping oscillation, and the period and amplitude of self-oscillation can be adjusted by controlling the laser intensity or the distance between the projection and the point of incidence.

Figure 1: Design principle of negative feedback control hydrogel.

The hydrogel system has an internal homeostasis similar to biological systems, and its temperature at the transmission point can be stably maintained at 36 degrees Celsius, close to the body temperature of the human body. This temperature is determined by the phase transition temperature (LCST) of the PNIPAm gel. No matter what kind of stimulus is given externally, such as wind, touch, or changing the light intensity, the hydrogel can self-adjust the constant temperature of the transmission point through negative feedback control without external human intervention. Pour cold water on the system and it will go dormant. After the water has completely evaporated, its temperature oscillations will spontaneously recover to its original state, as shown in Figure 2.

Figure 2: Sleep resuscitation process for homeostasis.

In addition, stable self-oscillation can be used to drive the responding material for dissipative functionality. As shown in Figure 3, a dynamic color display can be achieved by combining photothermal self-oscillation with thermochromic dyes. Depending on the position and the choice of color-changing temperature, it is possible to achieve red and white flashing mode, a continuous pink display, or a solid black display. Converting temperature oscillations into dynamic colors demonstrates the potential of applications such as visual signals and sensing. In addition, heat-responsive liquid crystal elastomers (LCEs) can be arranged on gel tubes in a fin-like sequence, thereby achieving periodic bending deformation driven by constant light. Due to the different amplitudes and time delays between LCE fin actuators, frictional deviations can be created on the paper model cargo, resulting in directional horizontal displacement of the cargo. This example reveals the possibility of building autonomous active transportation systems under feedback control using non-modulated light sources.

Figure 3: Application demonstration based on stable oscillation.

Make flytraps great again

More importantly, the negative feedback system can mimic the mechanical signal-based signal transduction of biological systems. We all know the classic plant mechanistic response model system mimosa, which, when touched, causes the leaves to close quickly (Figure 4), while another example is flytrap, whose leaves can trigger rapid closure by feeling the vibrations felt by the hair to catch insects. Although the study of mechanical instability in flytraps is well established, and there have been many reports of artificial biomimetic systems inspired by it, no mimicry of its more advanced plant arithmetic response, i.e., plants can trigger responses by feeling the number and time intervals of mechanical touch. The authors propose a method to use a gel negative feedback system to achieve touch frequency perception, thereby surpassing the previous flytrap biomimetic system that only mimics mechanical instability or stimulus response deformation. In this system, the gel enters a feedback-controlled damping steady state after a certain period of steady oscillation. This state is highly sensitive to the type and amplitude of external stimuli, where temperature oscillations can be restored after external mechanical triggering. This mechanical responsiveness can be used to construct signal transduction based on a mechanical stimulus-temperature-mechanical response pathway under imbalance conditions. The authors first demonstrate a single-touch mechanical response based on LCE strips. When the oscillator is in a damping steady state, the strip remains stationary. When a finger touches the gel tube and changes the position of the light, resulting in an instantaneous increase in the intensity of the transmitted light at the transfer point and a subsequent thermal overshoot at the heating point, causing thermally induced bending of the LCE. When you stop touching the gel tube, the shape of the LCE automatically reverts. In another system, the authors attached a prestressed plastic cantilever to a gel tube with heat-sensitive glue. When the gel tube is touched multiple times in a row, higher frequencies result in a higher time average optical power input through the transmission point, causing melting of the glue at high temperatures and instantaneous rapid release of the cantilever (Figure 3J). Low-frequency mechanical stimulation does not result in a cantilever response, enabling frequency gating of mechanical touches. This work is of great significance for the realization of signaling of stimuli-responsive materials and the realization of “communication” between materials.

Figure 4: Signaling and bioinspired responses based on negative feedback systems.

brief summary

In this study, a light-driven negative feedback-controlled hydrogel system is proposed that can generate stable temperature self-oscillation or damping steady state under constant light energy input. The system has an endohomeostatic property that keeps the temperature of a certain area constant under different external stimuli. Under damping steady-state, the system is highly sensitive to external stimuli, and from this signal transduction of mechanical stimuli can be constructed to achieve bio-inspired functions such as frequency gating of biomimetic flytrap. (Source: Science Network)

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