Steering the directional growth of plants with an autonomous system is a novel and original concept that was not reported before. The key here for us is to find out what's actually feasible. Our technique is the first automated method to grow plants into desired shapes and forms using robotic nodes to detect plants with their sensors and to switch the stimuli accordingly.
This method and the robots used here can be employed with dynamically growing structures where feedbacks determine the development of the biohybrid system. Maintaining the health of plants and detecting their presence at intersection points is challenging. Make sure that external light sources are not triggering phototropic responses in a failure state.
Visual demonstration is necessary to show the collaboration between plants and robots over extended time periods to grow user-defined growth patterns. Julian Ptezold, a grad student in our research group, will demonstrate the experiment procedure. Before beginning an experiment, select a plant species known to display a strong positive phototropism toward ultraviolet A and blue light in the growing tips.
We chose Phaseolus vulgaris or common beans because they grow fast and show a strong directional growth towards blue light. Compose each robot around a single board wireless local area network-enabled computer and interface the computer to sensors and actuators via a custom printed circuit board. Include one infrared proximity sensor per direction being tested for approaching plants including enough light-emitting diodes to deliver the blue light requirements per direction being tested for approaching plants.
Include hardware that enables local cues between robots and include a photoresistor for each direction of the neighboring robot to monitor their light emittance status. Include hardware to dissipate heat as required by the conditions of the selected blue diodes and the utilized robot enclosure using a combination of aluminum heat sinks, vents in each robot's case enclosure and fans. After confirming that the robot component directions are uniformly serviced, position the blue diodes to distribute an equivalent light intensity to each of the directions from which the plants may approach and orient each diode in the robot case such that the center axis of its lens angle is within 60 degrees of each axis of the mechanical support it services.
Position the infrared proximity sensors equivalently for their respective approaching growth directions within one centimeter of the attachment point between the robot and the mechanical support being serviced orienting each sensor such that its viewing angle is parallel to the support axis. Next, affix a 125 by 180 centimeter sheet of transparent acrylic to a 125 centimeter wide stand that is capable of holding the setup in an upright position. Integrate the robots into a set of modular mechanical supports that dually hold the robots in position and serve as climbing scaffolds for the plants to restrict the plants'likely average growth trajectories.
On each robot, include attachment points to anchor the specific mechanical supports including one attachment point for each direction by which a plant may approach or depart a robot. Position pots with the appropriate soil on the stand against the acrylic sheet and affix two robots to the acrylic sheet inserting the ends of the previously placed supports into the sockets in the robot cases. Then repeat the pattern to affix the remaining robots and support into the diagonally gritted pattern such that each row of robots is 35 centimeters above the previous row and that each robot is horizontally positioned directly above the robot or Y joint that is two rows beneath.
The mechanical supports should be arranged in a regularly gritted pattern that is uniformly diagonal with an angle of inclination at 45 degrees or steeper with uniform support lengths of a minimum of 30 centimeters per support. The preferred exposed length should be at least 40 centimeters to allow some buffer for statistically extreme cases of plant detachment. In the robot software, establish a stimulus state during which the robot emits blue light and a dormant state during which the robot either emits no light or emits red light.
Place the experimental setup under controlled environmental conditions and maintain plant photosynthesis using light-emitting diode growth lamps external to the robots and facing the experimental setup. After germinating, provide each plant its own pot at the base of the experimental setup and regulate the air temperature and humidity levels as appropriate for the selected species using heaters, air conditioners, humidifiers, and de-humidifiers as necessary and monitoring these levels using a temperature pressure humidity sensor. To test plant growth in the presence of multiple subsequent stimuli conditions, provide the robots with a global map of the pattern to be grown and continuously capture timelapsed videos of the experiments using cameras positioned at two or more vantage points with at least one camera view encompassing the full experimental setup.
Ensure that the captured images are of a high enough resolution to adequately capture the movements of the plant growing tips which are typically only a few millimeters in width. Then observe the plant attachment events and pattern of growth along the mechanical supports over an appropriate experimental growth period. Under conditions lacking blue light, positive phototropism is not triggered and the plants exhibit undbiased upward growth as they follow gravitropism.
The plants also displayed typically circumnutation. As expected under these conditions, the plants fail to find the mechanical support leading to the dormant robots and collapse when they can no longer support their own weight. The experiment should be stopped when at least two plants collapse.
In this representative single-decision experiment, the stimulus robots successfully steered the plants toward the correct support. In this representative multiple-decision experiment, the plants grew into a pre-defined zigzag pattern using dormant and stimulus robots to sense and stimulate the plant growth respectively until the pre-defined zigzag pattern was fully grown. Studying this kind of biohybrid system paves the way toward emerging techniques from architecture, biology and engineering to develop living adaptive structures and grow building components.
