Plant-like hooked miniature machines for on-leaf sensing and delivery

Sustainable strategies for preserving plant health are crucial for addressing environmental challenges. Bioinspired soft miniature machines offer potential for operation in forests and agricultural fields, adapting their morphology to plant organs like leaves. However, applications on leaf surfaces are limited due to leaf fragility, heterogeneity, and harsh outdoor conditions. This research addresses these limitations by drawing inspiration from the *Galium aparine* plant, which uses hook-like structures for efficient climbing. The strong shear-dependent leaf-attachment mechanism of *G. aparine* is exploited to create miniature systems with multifunctional microhooks enabling precision anchoring to leaf tissues. The goal is to develop devices capable of in situ monitoring and controlled delivery of molecules to plant tissues, thereby advancing sustainable strategies for plant preservation and ecosystem management. This contrasts with existing methods such as glue-based or clip-based sensors which can damage the delicate leaf structures. Microneedle-based patches are also being developed, but these often lack the reversible, shear-dependent attachment properties of the proposed microhook system.

The authors review existing technologies for on-plant sensing and delivery, noting limitations of current approaches (chemical glue, clips, microneedles). They highlight the increasing interest in bio-inspired soft robotics for environmental applications, specifically focusing on miniaturized machines able to interact with plants without causing damage. The literature review emphasizes the diverse anchoring strategies plants have evolved, with a focus on hook-like structures as a particularly effective mechanism for reversible attachment. The researchers build on their previous work on *G. aparine* microhooks, demonstrating their successful application in other contexts like textile attachment, before focusing on the adaptation to plant leaves.

The researchers employed a multi-stage methodology. First, they designed and fabricated two types of microhook-based devices: flexible IPS-made MH-based devices (IPS-MHDs) and self-dissolving isomalt@fluorescein MH-based devices (i@fluo-MHDs). Two-photon lithography was utilized, combined with micromoulding for the isomalt devices. The morphological and biomechanical properties of various leaf surfaces were characterized using SEM and optical profilometry. This included classification of leaves based on epidermal structure, roughness, and tissue hardness. Pull-off, shear locking, and friction force tests were performed to assess the attachment performance of the MHDs on different leaves. The influence of leaf surface microstructures, roughness, and hardness on the attachment behavior were quantified. Subsequently, the microhook-based systems were integrated into miniaturized, origami-assembled multiparameter leaf sensors for leaf microclimate monitoring (temperature, humidity, light). Finally, self-dissolving MHDs were utilized for localized molecular delivery to leaf vascular tissues, tracking the movement of fluorescein through the plant using fluorescence microscopy. A soft robotic proof-of-concept demonstrator (MH-based MiniBot) was built to illustrate ratchet-like motion on leaves, using light-activated fluidic actuators. Computational modeling was also conducted to investigate the effects of hook density and size on leaf attachment.

The study demonstrates that artificial microhooks can achieve strong, reversible, shear-dependent anchoring to various plant leaf surfaces. The maximum shear-locking forces achieved were up to 10.4 ± 2.0 N cm⁻², significantly higher than previously reported methods. The attachment strength was found to be highly dependent on leaf surface structure and hardness. Smoother, less stiff surfaces allowed for better hook penetration and stronger attachment. The origami-assembled multiparameter sensor successfully monitored leaf microclimate parameters (temperature, humidity, light) in both controlled and outdoor environments for extended periods. The self-dissolving isomalt@fluorescein MHDs demonstrated efficient localized molecular delivery to leaf vascular tissues. Fluorescein was observed to be transported within the plant vascular system after injection. The MH-based MiniBot successfully demonstrated ratchet-like locomotion on leaf surfaces. Computational modeling provided further insights into the relationship between hook density, size, and attachment strength, suggesting optimal hook densities for maximal attachment.

The findings address the research question by demonstrating a novel approach to on-leaf sensing and delivery that overcomes the limitations of existing methods. The strong, reversible, shear-dependent attachment of the microhooks enables robust and non-destructive interaction with plant leaves, opening up possibilities for continuous in situ monitoring and targeted molecular delivery. The successful development and testing of both the sensor and the delivery system showcase the versatility of the bioinspired design. The ratchet-like locomotion of the MiniBot further enhances the potential for creating more complex, mobile plant-interacting robots. These results have significant implications for precision agriculture and forestry, enabling more efficient and sustainable practices for monitoring plant health, optimizing resource utilization (e.g., water), and delivering targeted treatments. This approach aligns with the growing need for sustainable and environmentally friendly technologies in plant management.

This research successfully demonstrated plant-inspired miniature machines with multifunctional microhooks for on-leaf applications. The strong, reversible anchoring, combined with successful in situ sensing and molecular delivery capabilities, offers significant potential for advancements in precision agriculture and forestry. Future research directions include optimizing hook design for various leaf types, developing more sophisticated mobile robotic platforms for complex field operations, and exploring the long-term effects of isomalt-based delivery systems on plant health.

The study primarily focused on short-term effects of molecular delivery and long-term effects require further investigation. The robustness of the origami sensor in extremely challenging field conditions may need additional testing. The MiniBot is currently a proof-of-concept demonstrator, and challenges remain in developing fully autonomous, robust, and energy-efficient mobile robots for deployment in complex natural environments.