The Bio-Engineering Approach for Plant Investigations and Growing Robots

It has been 10 years since the publication of the first article looking at plants as a biomechatronic system and as model for robotics. Now, roboticists have started to look at plants differently and consider them as a model in the field of bioinspired robotics. Despite plants have been seen traditionally as passive entities, in reality they are able to grow, move, sense, and communicate. These features make plants an exceptional example of morphological computation – with probably the highest level of adaptability among all living beings. They are a unique model to design robots that can act in- and adapt to- unstructured, extreme, and dynamically changing environments exposed to sudden or long-term events. Although plant-inspired robotics is still a relatively new field, it has triggered the concept of growing robotics: an emerging area in which systems are designed to create their own body, adapt their morphology, and explore different environments. There is a reciprocal interest between biology and robotics: plants represent an excellent source of inspiration for achieving new robotic abilities, and engineering tools can be used to reveal new biological information. This way, a bidirectional biology-robotics strategy provides mutual benefits for both disciplines. This mini-review offers a brief overview of the fundamental aspects related to a bioengineering approach in plant-inspired robotics. It analyses the works in which both biological and engineering aspects have been investigated, and highlights the key elements of plants that have been milestones in the pioneering field of growing robots.

1. Introduction

How we see plants has changed significantly, as has the importance of protecting them for the benefit of the entire terrestrial ecosystem (Baluška and Mancuso, 2020). From an ecological role and evolutionary path, plants are producers in the food net of an ecosystem. They are photoautotroph organisms, so able to self-produce organic compounds by using mineral substances through photosynthesis. By exploiting substances directly from air and soil, they “do not need traditional locomotion,” but they evolved a number of singular strategies to interact with the environment, including complex movements, sensing, growing and propagation. New technologies, such as time-lapse recording, have demonstrated such abilities both above and below ground (Vincent et al., 2011Silverberg et al., 2012Vlad et al., 2014Poppinga et al., 2015Guerra et al., 2019Rambaud-Lavigne and Hay, 2020). Also robotics has contributed to this change in perspective by starting to mimic plants at both components and system level (Mazzolai et al., 20102011Sadeghi et al., 20142020Hawkes et al., 2017Nahar et al., 2017Wooten and Walker, 2018Must et al., 2019Bolt et al., 2020Geer et al., 2020).

This mini-review focuses on the vision of “science for robotics and robotics for science,” to highlight the results of a bioengineering approach in simultaneously driving innovative technological design and obtaining new biological insights.

2. From Plants to Robots

Plants are sessile organisms, and this means that they spend their entire lives at the site of seed germination. They have thus evolved a high level of plasticity enabling them to thrive, adapt and respond to changing conditions and survive under stress (Karban, 2008). Due to their exceptional adaptability, plants are the first living beings to colonize hostile environments, and have the unique capability to live contemporary in two different environments (e.g., soil and air, or water and air; Niklas and Spatz, 2012). These behaviors are linked to a complex and dynamic interaction between their morphology, distributed sensory-motor control, and the environment, which in turn represent the basic principles of what is called “morphological computation”(Laschi and Mazzolai, 2016): a modern perspective on intelligence in which the physical body has a primary role (Paul, 2006Pfeifer and Bongard, 2007) and the behavior depends strongly on the mechanical properties, the form/morphology, and the arrangement of the perceptual, motor and “processing units” (Zambrano et al., 2014).

Plants are thus the perfect candidates to be a model to deal with a key challenge in robotics: the capacity to function in unstructured environments. This skill requires heightened abilities of perception, efficient use of energy resources, and high adaptability to dynamic environments and changing situations. Plants offer several ideas for designing innovative technologies, such as: (1) indeterminate growing capabilities; (2) movements without muscles; (3) structural materials with morphological adaptability and variable stiffness; (4) distributed intelligence and sensory systems; (5) anchoring/attachment strategies; (6) intra-system and inter-system communication; and (7) energy-saving mechanisms. Belowground, plants represent the best example among living beings for efficient soil non-destructive and capillary exploration. They have a network of growing and branching roots, whose tips are highly sensorized and efficiently move the soil volume and search for nutrients. Aboveground, plants are a unique model for the design of low-mass low-volume robots capable of anchoring themselves, negotiating voids, and climb where current climbing robots based on wheels, legs, or rails would get stuck or fall.

3. Movements in Plants Without Muscles

From the growing of shoots and roots, to the opening and closing of stomata at the leaf surface, to the rapid snapping of carnivorous plants and the explosive launch of seed pods, plants have evolved a remarkable range of mechanisms to generate motions without the need for a muscular structure (Darwin, 1880Gilroy and Masson, 2008Forterre, 2013Jung et al., 2014Geitmann, 2016Echevin et al., 2019Morris and Blyth, 2019).

3.1. Water Transport

Plants turn out in a perfect hydraulic engine. At the macro level, plants exploit a sophisticated strategy, relying on water potential gradient, for moving water from the soil (i.e., at soil level ≈ −0.3 MPa) to the leaves (i.e., at the leaf level ≈ −7.0 MPa; McElrone et al., 2013). At the cell level, plants draw the water in and out of their cells using the osmotic gradient across semipermeable membranes. The turgor pressure thus changes creating a local change in the cellular volume and tissue stiffness, and by exploiting the thin and stiff cell-wall features, enables the large-scale tissue deformations