Plants have very rich and complex vasculature that transports water and nutrients through their tissues to maintain normal metabolism. For example, in leaves, which represent the main organ of plants for photosynthesis, their veins can deliver the nutrients produced by photosynthesis to the rest of the body while also transporting water throughout the leaflet for transpiration. This vasculature has inspired us to develop artificial systems with embedded fluidic channels, such as biomimetic microfluidic devices . Moreover, plants have evolved the ability to respond to environmental changes to allow their vasculatures to function more healthily even in an ever-changing natural environment. This ability of individual systems to sense the external environment and adjust themselves accordingly is sometimes known as “environmental adaptability.” In particular, the nastic movement of plants is the fastest way to adapt their shapes to environmental changes in light, temperature, and humidity . For example, the genus Oxalis deploys the leaflets during the sunny day to promote photosynthesis and folds them at night to retard energy dissipation due to transpiration. However, this plant-like ability to interact with the environment is rarely mentioned in synthetic microfluidic systems . Endowing microfluidic systems with these stimuli-responsive shape-changing functions can pave the way for sophisticated, multifunctional, or intelligent fluidic systems with dynamic biomimetic design or environmental adaptability .
Now, microfluidic systems respond to the environment by relying on linked or onboard electronics and computer programming. This can result in a tethered, complex, and overall cumbersome system . In contrast, their natural counterparts contain veins as fluid-transporting microchannels and pulvinus as stimuli-responsive actuators in their thin, lightweight, and flexible leaflets. Besides, along with the nastic movement, the leaflets tend to fold or unfold into a regular geometry that has specific purposes. Two main aspects that restrict the development of stimuli-responsive morphing microfluidics are as follows: First, the mainstream materials used to fabricate microfluidic devices are inert materials without environmental responsiveness, such as polydimethylsiloxane (PDMS) and poly(methyl methacrylate) (PMMA); second, the design of microfluidic devices is usually based on considering the fluidic flow to design the embedded channel shape rather than the overall shape of the microfluidic device. Consequently, most common microfluidic devices do not show environmental responsiveness and targeted shape change, let alone the nastic movement, similar to those in leaves. To gain ability of nastic movement, the microfluidic device has to exhibit stimuli responsiveness at the device level, and the overall shape of the device has to be programmable. These microfluidic devices will enable previously unfeasible applications: For instance, the resultant environmentally adaptive photomicroreactors self-regulate the photosynthetic conversion rate according to weather changes .
Stimuli-responsive materials—such as shape memory polymers, liquid crystal elastomers, and responsive hydrogels—are capable of changing their shapes in response to external temperature, light, or humidity. They have functioned as both sensors and actuators synchronously in many morphable structures. Nonetheless, current microfluidic devices use responsive materials only as localized components, such as light-controlled valves or flow-switching channels, rather than in the overall morphing of a microfluidic device . To implement a preset overall three-dimensional (3D) morphing, the device’s dimensions, the positions of the responsive materials embedded in the device, and the target operation in response must be engineered specifically. Combining with the ancient art of origami, the desired 3D structures not only can be constructed from precursors via 2D processing techniques but also can mutually convert between a fully deployed 2D plane and a compact 3D form by folding and unfolding . Therefore, by fusing responsive materials into a microfluidic device designed with an origami geometry, it can transform between 2D and 3D states based on the preset structure via stimuli-triggered responses.
In this work, the nastic plants inspire our concept of a microfluidic device with environmental responsiveness, and the stimuli-responsive structures offer us the conditions to realize this concept. On the basis of these, we have developed a transformable microfluidic chip by integrating stimuli-responsive materials with a thin and foldable microfluidic chip . The entire device can respond to changes in temperature, humidity, and light irradiance by transforming along the preset origami folds. Thus, we name this transformable origami microfluidic approach TransfOrigami microfluidics (TOM). We demonstrate that TOM can be applied as an environmentally adaptive photomicroreactor. The transformation reconstructs the reaction channels, changes their light-harvesting capability, and eventually regulates the photosynthetic conversion. Positive feedback control is built into interactions between TOM and the environment. When the external conditions are favorable for the photoreaction, the feedback results in an enhanced photosynthetic conversion and vice versa. As the first of its kind, stimuli-responsive morphing microsystems, our TOM will inspire applications in energy, robotics, or biomedicine that require environmental adaptations, such as artificial vascular networks or flexible electronics with adaptive rhythmic movements.