Designing Plant-Driven Actuators for Robots to Grow, Age, and
Decay
Yuhan Hu

Jasmine Lu

Nathan Scinto-Madonich

Cornell University
Ithaca, USA
yh758@cornell.edu

University of Chicago
Chicago, USA
jasminelu@uchicago.edu

Cornell University
Ithaca, USA
njm76@cornell.edu

Miguel Alfonso Pineros

Pedro Lopes

Guy Hofman

Cornell University
Ithaca, USA
map25@cornell.edu

University of Chicago
Chicago, USA
pedrolopes@uchicago.edu

Cornell University
Ithaca, USA
hofman@cornell.edu

Figure 1: Designs of plant-driven actuators: (a) Multiple mung bean sprouts grow within a soft silicone enclosure, sprouting
to create unique and varied extrusions through growth and decay; (b) Plant sprouts inside soft silicone sleeves push against
embedded pockets, creating textures as they expand their size; (c) Plants embedded inside a robot body propagate height growth;(d) Plant growth powers mechanical gears, gradually propelling the locomotion of the vehicle.

ABSTRACT
Designing plant-driven actuators presents an opportunity to create
new types of devices that grow, age, and decay, such as robots that
embody these qualities in their physical structure. Plant-robot hybrids that grow and decay incorporate unpredictable and gradualtransformations inherent across living organisms and suggest analternative to the design principles of immediacy, responsiveness,
control, accuracy, and durability commonly found in robotic design. To explore this, we present a design space of primitives forplant-driven robotic actuators. Proof-of-concept prototypes illustrate how concepts like slow change, slow movement, decay, anddestruction can be incorporated into robotic forms. We describe
the design considerations required for building plant-driven actuators for robots, including experimental fndings regarding themechanical properties of plant forces. Finally, we speculate on the

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ACM ISBN 979-8-4007-0583-0/24/07
https://doi.org/10.1145/3643834.3661519

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potential benefts of plant-robot hybrids to interactive domains
such as robotics.

CCS CONCEPTS• Human-centered computing → Interaction design theory, concepts and paradigms; HCI theory, concepts and models; • Hardware→ Emerging interfaces.

KEYWORDS
living media interface; shape-changing display; plant-driven actuators; human-robot interaction
ACM Reference Format:
Yuhan Hu, Jasmine Lu, Nathan Scinto-Madonich, Miguel Alfonso Pineros,
Pedro Lopes, and Guy Hofman. 2024. Designing Plant-Driven Actuators for
Robots to Grow, Age, and Decay. In Designing Interactive Systems Conference(DIS ’24), July 01–05, 2024, IT University of Copenhagen, Denmark. ACM,
New York, NY, USA, 16 pages. https://doi.org/10.1145/3643834.3661519

1

INTRODUCTION

Currently, the typical robotic device is designed for permanence,
maintaining a constant appearance and functionality throughout its
lifespan. While this allows for highly reliable robots, it also emphasizes their lifeless, machine-like qualities. What if, instead, robots

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Hu and Lu, et al.

were designed to grow, age and decay, similar to living beings? In
this work, we explore an approach to disrupting permanence byusing plants for robotic actuation, harnessing the biomechanicalpower inherent to plant growth instead of traditional electromechanical forms of actuation. By doing so, a plant’s natural cycles ofgrowth, aging, and decay are refected in the robot. Additionally,using plant-driven actuators can challenge our implicit assumptions of robotic design. For instance, much of robotic design istraditionally oriented around immediacy, responsiveness, accuracy,and durability. Instead, designing around plant-driven actuators
invites imaginings of robots that are slow-changing, unpredictable,
and subject to decay. While this may seem counter to common
goals in Human-Robot Interaction (HRI) Research, such characteristics might open up new perspectives and insights for future robot
interaction designs.
To investigate this, more research into plant-driven actuators isneeded. Just as traditional motor-based actuation requires a powersource & circuitry, plant-driven actuators also require technicalconsiderations, namely an environment that facilitates growth (i.e.,
the presence of nutrients, water, and light). Similarly, while priorresearch has focused on leveraging plants as interactive displays or
sensors [50, 56], embedding plants inside mechanisms as a meansof actuation has been less explored. Such a strategy also introduces
new constraints and considerations as it involves introducing plants
to atypical growing environments (e.g., not in soil).
We contribute a design exploration of how plant-driven mechanisms can be incorporated into robotic forms, paving the way forconstructing plant-robot hybrids that grow and age at their own
pace. We detail the design opportunities for plant-driven actuators for robotic design, including materials and mechanisms thatallow designers to interface plant properties with robotic properties. A library of design primitives illustrate how plant-based
actuators can afect both the morphology and functionality of arobot design. We analyze these design opportunities and provide
conceptual designs illustrating the interactive potential for each
design primitive. Our design proposals are accompanied by built
prototypes of plant-based actuators, including the interactive dimensions of slow change, propagating functional movement, andconveying decay and destruction. Our designs are presented toinvite new imaginings of plant-driven actuators and to highlight
how plants can be leveraged as a material for actuation. They are
not functionally superior to existing traditional mechanisms andare not highly useful objects. Instead, we present them as artifact
examples to provoke more critical and speculative ideas of how
plant-driven actuators can be incorporated into mechanical designs
beyond their utility.
Notably, this work does not perform a deep exploration of how
plant-robot hybrids can infuence social dimensions of HRI. However, this work is deeply motivated by the potential of plant-robot
hybrids to transform long-term human-robot interactionsin social
robot research. Thus, we present plant-driven actuators that could
be incorporated into robotic systems and a mechanical study offorces in plant growth to contribute approaches to how plant-driven
robots could be realized.

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2

RELATED WORK

Designing plant-driven actuators for robots to grow, age, and decay builds on a legacy of theoretical and applied work in humancomputer interaction and human-robot interaction. Specifcally,
this approach builds on prior work that challenges conventions
around what robots look like and how they should function as wellas the design of diferent robotic mechanisms. Additionally, theapproach builds on prior work leveraging plants and other organisms in interactive devices that contribute translations between
electromechanical and biological phenomena.

2.1

Challenging Robot Conventions

For the most part, robots are assumed to be highly functional,
durable, and responsive which is refected in their typical materials, interactions, and forms. Several works have shown thatbreaking away from these conventions can allow for more creative
approaches to tackle many issues within human-robot interaction[2, 38]. Similarly, we see incorporating plant-driven actuators into
robotic designs as a means of introducing new perspectives forhuman-robot interaction, especially by introducing new materials
and as an approach to enhancing long-term human-robot interactions through temporal changes of robots.
2.1.1 Robotic Materials. Robots typically employ rigid materials
such as plastics and metals, but more recently, robotic research has
expanded to also incorporate other materials. For instance, robotics
research has incorporated soft and deformable materials, such aselastomers, to create adaptable behaviors and handle delicate objects [35, 54]. Interactive robots are sometimes covered with soft
materials like fur and foam to provide a soft and warm touch experience [7, 14]. Additionally, some robots utilize craft materials [57, 60],
such as fabrics and woods, allowing users to design and decorate
the robot, customizing their robots to increase uniqueness. Similar
to these works, this work explores how plant-driven actuators canintroduce new materiality to robotic design.
2.1.2 Social Uncanniness of Companion Robots. Social robots, as
opposed to industrial robots, are designed to enrich human lives
through various forms of interaction, including entertainment, mental therapy, companionship, and education. For instance, the Parorobot [23, 31], designed as a sociable seal, aims to provide mental
therapy for the elderly and Aibo [13], a robot dog, plays the rolesof both companion and source of entertainment.
However, a persistent challenge for such robots is maintaining long-term engagement [26]. The initial novelty efect tends
to diminish over time, and following initial interactions, people’sinterest in the robot tends to wane, leading to changes in theirattitudes and behaviors towards the robot [10, 36].
Hofman [17] suggested that a sense of “Social Uncanniness”
might explain some of the discomfort people feel with robotic companions. One of the possible reasons for this uncanniness might lie
in the discomfort of interacting with a never-aging agent. Hofman
suggests decaying and eroding natural materials to alleviate thisefect, which has also been proposed by other robotics researchers[2, 25, 38]. Inspired by [17], plant-driven actuators could introduce
alternative robotic forms that transform long-term interactions between humans and robots through growth and decay. This could

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add novelty and transience to their interactions over time, potentially enhancing long-term human-robot relationships.

2.2

Designing Robotic Mechanisms

This work builds on prior approaches to introducing shape-changing
mechanisms, textured skins, motion, and destruction to robotic design. While our work explores how these can be accomplished via
plant-driven actuators, we provide an overview of prior approaches
to these spaces.
2.2.1 Shape Changing Displays. Prior research has explored various mechanisms and materials capable of facilitating robotic shapetransformations. Shape-changing mechanisms generally make useof materials that can respond to stimuli, such as heat, power, pressure, or moisture, and change their morphology accordingly [22,63, 66]. For instance, the inFORM [12] explores the use of motoractuated pins for shape displays; NURBSforms [59] uses a shapememory alloy (SMA) driven interface for modeling curved surfaces;
bioLogic [64] uses natto cells for actuating shape changes in response to humidity, etc. In this work, we explore what possibilities
emerge when harnessing plants as shape-changing actuators inrobotics.
2.2.2 Sof Robotics and Robotic Skins. Several works on shapetransformation involve soft robots and soft materials [53, 67]. A
common design paradigm involves fuidic actuation, where theforce of a fuid (usually pressurized air) causes a cavity to deformin the direction of least resistance [6]. Soft robotics skins withpneumatically-actuated textures have been proposed, which candeform and alter the robotic skin features for expression. This
design modifes the mechanical structures of the elastomers toprogram shape-changing features once the inner cavity is pressurized. [19, 20]. Many of the designs proposed in this work follow asimilar logic, but rather than utilize fuidic forces, we harness theforces of plants sprouting and growing.
2.2.3 Movement and Destruction. Movement (whether via limbs orwhole-body) is often a key feature in robotic designs. Traditionally,
robotic movement is achieved by electromechanical actuators. Thiscan take the form of DC motors extended by mechanical gears[13, 31, 57] or pneumatic pumps extended by infatable cavities[6, 29, 53]. This work builds on such uses of electromechanicalactuators to produce movement but instead explores how they
can be actuated biomechanically - through the use of plant-driven
actuators. Rather than using DC motors or pneumatic pumps as thebase actuator, this work explores achieving movement by utilizing
plant growth forces as the base actuator and extending these forces
through mechanical gears or infatable cavities.
On the other hand, destruction is a less common feature inrobotic design, but one that has garnered increased interest [25, 38].
Previous work has explored how destruction and unmaking canbe achieved through various fabrication techniques and simultaneously, ofer novel interactive benefts [9, 55]. Inspired by theseapproaches, we explore how plant mechanics during both their
growth and decay can similarly be leveraged to bring destruction
to robotic design.

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2.3

Plant-Driven Interactions

Finally, this work builds on prior work in developing plant-driven
interactions. We discuss this in relation to the growing body ofwork in Living Media Interfaces and plant interfaces (displays androbots) specifcally. Last, we present a brief overview of the plantmechanics at the core of this work.
2.3.1 Living Media Interfaces. Using plants as a living materialintroduces
many challenges and opportunities that have been previously explored in Living Media Interfaces (LMI) research [27, 32, 39]. Other
researchers have explored designing interfaces around various
types of living organisms, including bacteria [8, 45, 65], mold [33],slime mold [1, 37], dinofagellates [46], algae [4], and plants [42,47, 50, 56]. While extremely diverse in motivation and application, these works discuss how living media interfaces emphasize
non-anthropocentric engagements and encourage new means ofnoticing and relating to non-human counterparts [39, 46] Additionally, they emphasize how living media can upend traditional
disciplinary boundaries - requiring collaboration between biologists, computer scientists, and designers for instance [39]. Manyof these works introduce novel strategies for connecting innate
biological phenomena to computational or mechanical language,
enabling highly expressive integration of living material in interactive systems [37, 50, 56]. Building on these explorations, our work issimilarly motivated by the benefts of introducing living organisms
to interfaces, but explores this specifcally in how we can translate
plant growth into actuator design.
2.3.2 Plant Displays and Plant-Robot Hybrids. Using plants fordisplays and in plant-robot hybrids has previously been exploredin several works [24, 49]. Botanicus Interacticus uses plants as atouch-sensitive device for a human-computer interface. [47] Babbage Cabbage [11] provides environmental indicators, through acolor change of cabbages. Infotropism [18] use live display plants
to encourage recycling behavior and highlight eco-consciousness
through the aggregated plant’s orientation over time. Flora Robotica [15] combines living plants and distributed robots into symbiotic robot-plant bio-hybrids and explores the potential of a plantrobot society. Phytowalkers [62] builds robots with plant materials
and drives them through traditional actuation methods. Cyborg
Botany [50] employs a novel biomachine, where the autonomous
system relies on plant signals to drive a robot, such as moving toward a light. While existing work utilizes plants as sensors, displays,
and materials within the robotic system, this work primarily employs plants as the actuation unit to drive the movement or changesof the robots.
2.3.3 Plant Mechanics. Crafting plant-driven actuators requires
an understanding of basic plant growth and response that canbe translated into interactive movement, growth, or expression.
With few exceptions, plants require a combination of sunlight,
water, carbon dioxide, and essential mineral nutrients to thrive andreproduce. In order to access these vital resources, plants sense
and respond to external stimuli through directed growth of shoot(stem and leaves) and root tissue. The directed growth in response toexternal stimuli, termed tropisms, are exemplifed by plant roots thatgrow away from light and with gravity, while the shoot tissue grows
towards the light and against gravity [30, 40]. Our understanding

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of these tropisms can be used to manipulate the growth of a plant.
For instance, after seed germination in the dark there follows arapid elongation of the stem, known as etiolation, until specifc
light cues are perceived by the plant [3]. This rapid elongation of
the stem is meant to propel the nascent plant to the soil surface,where gas exchange and sunlight interception can occur. Oncethese conditions are met, the seedling will slow stem growth andexpand leaves for light interception. Notably, the forces generated
by seedling growth are not insignifcant. For instance, the processof stem elongation in wheat and crimson clover has been reported
to generate a maximum force of 30 and 60 grams, and depending
on environmental conditions this force can be applied consistently
over many days [5, 43].
While these previous reports studied the fundamental forces
underlying plant emergence from soil, we are interested in how
these plant forces can be harnessed to actuate movement throughintegration in mechanisms. Doing so deviates from prior plant mechanics research as doing so introduces new dynamics like atypicalgrowing environments and how the cumulative forces of many
growing plants may be afected by competition in close quarters.
Efective translation of plant growth to engineered actuator requires
additional study of their behavior in these new contexts.

Figure 2: A plant-driven actuator is composed of the living
layer, the transmission layer, and the interactive layer.

3.1
3

DESIGNING PLANT-DRIVEN ACTUATORS

Motivated by how plants might ofer new capacities to robotic design, we strive to explore actuator design that leverages the agency,transient states, and non-determinate timescales of plant growth.
While there are various non-living materials that exhibit shapechanging reactions in response to temperature and humidity thatcan replicate those of plants [22, 63, 66], our approach focuseson enhancing and highlighting the innate characteristics of plant
growth rather than trying to direct or shape them towards traditional robotic mechanisms. Ultimately, three characteristics of plant
growth shaped the design of our plant-driven actuators.
• Slowness: The pace of the change in plant growth is slow,
typically spanning from days to years. Whereas electromechanical actuators are typically triggered instantaneously
and are highly controllable, plant-driven actuators need tooperate more gradually, accounting for the longer processof plant growth.
• Irreversibility: As living entities, the growth of plants is irreversible, meaning mechanisms cannot be ’reset’. As such,they emphasize the natural concept of transience by progressing through a lifecycle that includes states of decay anddeath. With inert materials, actuators traditionally do notconsider such states in their designs.
• Agency: Provided the environmental requirements of nutrients and light, plant seeds will tend to sprout. Similarly,
provided enough time, they will decay and die. However, if
a plant sprouts, how quickly it grows or when it begins todecay can only be infuenced unlike most electromechanicalsystems where interactions usually have an immediate efect.
With plants, however, users have less control over them.

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Components of Plant-Driven Actuators

When developing a plant-driven actuator, the designer has to consider three layers of design and fabrication: a living layer, essentiallythe growing plant, a translation layer, converting plant growthand decay into mechanical action, and an interactive layer that
integrates mechanical changes into an interactive feature. This
three-part abstraction is illustrated in Figure 2.
The living layer encapsulates the growing plant in all stages,
from seedling to full growth to decay, and provides essential resources including air, water, and light. As the living layer expands,
the transmission layer interfaces with the changing form of the living layer, using machine components such as pistons or deformablecavities to translate the living layer’s state into a more controlled
movement for the interactive layer. The interactive layer further
amplifes the mechanical changes to achieve interaction-related behaviors, such as changing morphologies or enabling and disabling
functionality. The boundaries between these conceptual layers are
not always physically distinguished. Layers can overlap, and multiple instances at each layer can be involved in the operation of aplant-driven actuator. This three layer abstraction is used throughout the paper to discuss integrating such an actuator into robotic
systems. These layers also ofer sites of play for designers to engineer actuators that enhance properties of a specifc layer. For
instance, the living layer may be optimized by incorporating soil tocultivate the growth of strong roots or the interactive layer may beprioritized to allow propagated changes to have large, interactive
benefts. Distinguishing plant-driven mechanisms through thesethree layers thus allows for more attention to the diferent needs
and priorities at each layer.

3.2

Constraints and Considerations for
Plant-Driven Actuators

Including plants in actuator design comes with several unique constraints and considerations. For instance, a plant is a living, growing,Designing Plant-Driven Actuators for Robots to Grow, Age, and Decay

DIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

Figure 3: Illustration of the design primitives, including morphological and functional transformation driven by the growthand decay of plants. The change in size and force produced by the life cycle of the living layer is translated into mechanicalmovement through three mechanisms in the transmission layer. These mechanisms aford six interaction primitives.
and evolving being. This allows for engaging interactive qualities
like noticing changes in plant growth on a day-to-day basis or
being surprised by the direction and rate of plant growth. As specifc dimensions of plant growth can only be nurtured rather thancontrolled, plants are incredibly dynamic and unique as material
to work with and observe. However, plants also requires propercare, nutrition, and protection that are not traditionally found inother mechanisms or devices. Designing actuators thus requires
accounting for how the plant will receive the needed nutrition tosurvive and grow. On a related note, without a proper environment,
plants can grow weak, die, and decay. In the case of plant-driven
actuators, this might require the replacement of dead or decaying
plants with new seeds to restart plant growth. As such, actuators
would need to account for how to make such replacements in thatevent. On the other hand, plants are incredibly resilient and cangrow to be very strong and lasting depending on the plant type.
Thus, there are many opportunities to lean into diferent temporaldimensions of plant life and growth. In our work, we focus primarilyon sprouts due to their speed of growth being amenable to prototyping. While we demonstrate plant-driven actuators through a seriesof design primitives, these could take drastically diferent forms

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and functions with diferent plant types and diferent interaction
priorities. Designing with plants afords many unique opportunities
to integrate their biological characteristics into mechanisms, butalso requires an approach to designing in collaboration with plants
rather than designing against their constraints.

4

DESIGN PRIMITIVES: MORPHOLOGY AND
FUNCTIONALITY

The proposed design space for a plant-driven actuators is basedon the above-mentioned division into three layers, the living layer,
the transmission layer, and the interactive layer (fgure 3). The
living layer goes through two main phases: growth and decay,
accompanied—respectively—by an expansion and contraction inscale and force. These changes can cause three types of mechanicalmovements in the transmission layer: linear motion through apiston, deformation through a soft cavity design, and breakdown
through a planned fabrication of weak points in a rigid structure.
We outline six design primitives in the interactive layer caused bythese transmission mechanisms, which we further group into twogroups: morphological changes on the one hand and functionalchanges on the other.

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Figure 4: (a) Plants can act as a linear piston to increase body size by being embedded inside an expandable structure. As
plants sprout, the body expands driven by the plants’ upward force, and as the plants decay, they lose their structural integrity
which leads the body to shrink again. Unique and variable growth occurs across instances. (b) A speculative sketch of how thisprimitive can be incorporated across all limbs and sections of a robotic body.

Figure 5: Cross sections of the increasing height prototype
in Figure 4.

We present the library of design primitives alongside exploratory
proof-of-concept prototypes exemplifying the how these actuators
could be integrated in robotic forms.

4.1

Morphology

Expressing growth and decay via morphological changes allows
robots to change their bodily appearance over time. Previous research has actuated such shape changes with pneumatics, shapechanging alloys, and other forms of electromechanical actuation [20,22, 59, 63, 66]. As presented in the following section, plant-driven
actuators can be used instead to induce morphological changes
across size, shape, textures, and rigidity.
4.1.1 Size. Just as growth and aging are expressed in living organisms by changes in body size and height, a robot’s growing andaging process can be refected in transformations of size.

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For instance, a sprout can extend its length seven-fold from its
initial seedling stage, a dramatic transformation that could also be
refected in robotic forms. With a proper mechanical transmission
structure to direct the plants’ growth within constraints, a robot
can be designed to realize controllable changes in size driven
by plants. This can be achieved by a piston-like mechanism thatharnesses plant growth in a specifc direction. The plants inside thestructure grow vertically, pushing the piston’s displacement to create a taller structure. Depending on the stability of the mechanicalstructure and the material, the decay of the living layer could either
cause the grown mechanical structure to collapse with the loss of
the rigidity in plants or maintain its grown size if the growth has
pushed the mechanism through multiple stable equilibrium states,
such as buckled beams or bi-stable plates.
The prototype in Figure 4 represents this concept in which a
robot progressively grows taller, fueled by the natural growth ofan embedded plant without any other power source. The design
features a ventilated head with openings on the top, ensuring adequate airfow and light access. The torso consists of two layers
for supporting the growth of the internal plants. The lower layer is
pre-flled with water and essential nutrients for plant development.
The top layer is separated with a 3D-printed base providing a solid
support structure for the seeds to rest upon while also elevatingthem above the hydroponic solution to prevent fooding of the seeds.
In some variations that have longer growth cycles, we attach plastic
tubing to the bottom layer to facilitate access to the hydroponicsolution for necessary refreshments (See Section 5.2).
Initially, the torso is concealed within the head’s protective shell.
As the plant grows vertically, it exerts pressure on the head’s inner surface, causing the head to elevate, the torso to emerge, andthe robot’s overall height to increase (as depicted in Figure 5). Notably, while the mechanism is designed to create increases in height,
the grown structures are highly variable across diferent growth
attempts. For example, the plants might sprout through the ventilation holes, failing to exert sufcient force to create displacement,

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Figure 6: (a) Soft fexible materials have an embedded cavity that can be flled with plant seeds. As the plants grow, they pushthe material to deform, thus transforming the overall object shape such as by rounding, bulging, and sagging. (b) A speculativesketch of how these shape transformations could be embedded in a robot’s body.

Figure 7: Cross sections of the transforming shape prototypes
in Figure 6.

or the robot’s head might tilt with uneven growth of plants, subsequently creating uneven displacement forces.
4.1.2 Shape. Just like metamorphosis in biology, shape transformation of a robot across life stages can infuence the changes ofrobot’s identity, capability, and interaction approach. For example,
shapes with rounded edges are generally perceived as softer andmore approachable [28]. In several works[41, 52, 58], robots were
designed to convey shape transformations. We present several examples of enabling transformations in shape driven by plant

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growth. These are depicted in Figure 6 where diferent body parts
can change during cycles of growth and decay.
Plant seeds are embedded inside a deformable cavity, and as theygerminate and expand, they exert pressure on the elastic walls of
the cavity. This increased internal pressure, caused by the growing
plant volume, leads to the deformation of the cavity walls.
The soft cavity is fabricated through a standard mold-castingprocess [21]. The design of the cavity structure is adjusted according
to the intended deformation and function as shown in Figure 5
To maintain adequate air circulation within the internal cavity,
1mm diameter holes are created in the walls. A 3D printed support
structure, featuring 4mm holes, is inserted before the cavity is
sealed. This layer is crucial for providing seeds with access to thehydroponic solution without overhead watering. Upon assemblyof the mechanisms, the cavity is flled with a hydroponic solution,
setting the stage for growth.
Through the appropriate design of features such as wall thickness, elasticity, and the strategic placement of resources like ventilation holes and nutrients, growth can be manipulated to a certain
extent. For instance, the most fragile and well-ventilated wall islikely to deform before the stifer and darker edges. As depicted in
Figure 6, the mechanism, which initially possesses a cubic shape,
gradually softens its edges and transitions into a rounded form
through growth.
The aging and decaying phase of shape transformations also
depend on the material properties, specifcally the plasticity of thedeformation. As plants decay, becoming smaller in volume andmore fragile, elastic materials tend to push the robot back toward
to original shape, thus reversing the growth-induced deformation.
4.1.3 Textures. Just like how humans grow hair and develop skinwrinkles, robots could also evolve textures that signify the progression of time and their various states during both growth and aging.
These primitives are inspired by past work in creating new skins
and textures for robots [19, 20]. Here, we explore their actuation
with plants.
To mimic the evolution of texture, seed pockets are incorporated
into the surface layer and capped with an elastic, permeable layer
with holes for ventilation. As the seeds germinate and grow over

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Figure 8: (a) The transformation in texture occurs as the plants sprout, forming raised bumps and dark spots or protruding on
the robot’s skin, resembling hairs that ultimately wither or detach from the skin through decay. (b) A speculative sketch of arobot head with plant-actuated skin and hair textures.

Figure 9: (a) Exemplifying, the shifting rigidity primitive, an initially soft mechanism increases its rigidity as the plant grows
through the internal channel and forms its “bone”. (b) A speculative sketch of how robotic limbs could incorporate theseplant-embedded rigid structures.
time, each pocket experiences deformation, transforming the initially fat surface into a raised area within the pocket. With multiple
embedded textures, the surface evolves into a rugged, textured landscape as each miniature pocket alters its shape. Additionally, thehue of the pockets transitions from the original seed color to thecolor of the sprouting plants and turns brown when the plants begin to decay, a process analogous to the formation of dark spots onhuman skin. The material selection and surface openings determine
whether plants will penetrate the elastic surface and become visible.
Some plants can develop intriguing textures, for instance, hair-likestructures as depicted in Figure 8 (middle). As these plants grow,they may alter the visual and tactile qualities of the robot.
The aging and decay of the plants may cause the textures andappendages to change color (such as transitioning from green tobrown), shape (shifting from upright to bent), and other material
properties (for instance, transitioning from soft and resilient to stifand brittle), and they may even detach from the skin. As the plants
decay, the robot may appear to be gradually changing and losing

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its “hairs”, similar to the human aging process where hair turns
gray and may eventually fall out.
4.1.4 Rigidity. Just like how creatures develop muscles and bones,
robots may change the rigidity of their structures to represent
diferent stages of growth and engage in diferent interactions.
The following plant-driven rigidity-changing design primitives
leverage the changing rigidity of a plant’s stem.
Plants’ rigidity changes at multiple points during the life cycle.
Typically, plant rigidity is low as the seedling emerges from theseed. As the plant grows and develops, the plant’s overall rigidity
increases helping it withstand environmental stresses such as windand rain. As the plants begin to decay, their rigidity decreases, andthey become susceptible to breakage. By integrating plants in askeletal structures, a robot’s overall rigidity could change as a plantdevelops and decays.
Figure 9 (right) illustrates a plant growing through a channel
inside a soft mechanism, forming its “bone” and afecting the rigidityof the initially empty, fexible structure. Depending on the type ofDesigning Plant-Driven Actuators for Robots to Grow, Age, and Decay

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Figure 10: (a) The growth of plants may trigger the activation of robotic joints by exerting pressure on a clutch, transitioning
from an initially uncoupled mechanism to an interlocked state. Plant growth from the bottom mechanism locks 4 joints into
place on the top mechanism. This action enables the initially passive component to connect and engage in coupled rotary
motion. (b) A speculative sketch of a robotic arm that has its joints properly connected via plant growth.

Figure 11: Cross sections of the transforming shape prototypes in Figure 10.
plant, the initially fexible structure may become rigid when theplant fully grows through it, making it difcult to bend. When theplant dies and loses its strength, the internal “bone” becomes more
fimsy thus transforming back to the initial fexible state.

4.2

Functionality

A robot’s design might include functional changes across diferent
life stages, such as activating or gaining functions in the earlystage through growth and development, or disabling functions or
unmaking their bodies in a later life stage. This has been exploredin [25, 38]. Plant-driven actuators can also play a role in activating
or disabling functionality.
4.2.1 Activating. Plants growing inside a robot have the ability
to bond and strengthen connections with intentional design.
For instance, a robotic joint can be activated through the plants’2489internal growth through the use of a clutch mechanism as seen in
Figure 10.
The joint is initially uncoupled, where the servo arm rotates
freely. With the plants embedded in the body growing into theclutch, bonding the two parts together, the torque generated by the
servo motor is passed by the materials to the passive limb, thus
activating the control of limb rotation. This process is illustrated in
Figure 11 and the supplementary video. With this feature, robots
gradually develop their abilities to perform activities as the users
encourage their growth. The newly activated functions may bringnew interactive experiences, but these interactive experiences will
also be lost if the plants enter a state of decay, weakening the bondand disconnecting the clutch.
Another prototype involves harnessing the force generated byplant growth to propagate robot movement via mechanicalDIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

Hu and Lu, et al.

Figure 12: (a) A prototype demonstrates how weak joints within a robot’s body can enable its destruction and unmaking. (b) A
speculative sketch of how this disabling and unmaking can extend to all parts of a robot body.

Figure 13: The prototype uses a piston with rack and pinion
mechanisms to harness the energy from the plant’s growthand drives gears and mechanisms to gradually transformover time.

will begin a process of deconstruction, malfunction, and eventual
shutdown. This will help the users to repurpose the robot’s components for other uses or recycle them for environmental reasons
when the robot is no longer in use. Figure 12 shows a robot being
disassembled through the expansion of plants within it.
In this prototype, the robot features a fragile layer between
its neck and shoulder joint connections. The head and torso are
3D printed separately and are then interconnected using loose
mechanical joints and consolidated with hot glue to create weak
and vulnerable layers for destruction. As a result, when the plants
inside the robot grow and compete for more room, they will exertpressure on the weakest layer and eventually break it, thereby
creating more space, air, and light for their growth (Figure 12).

5

PLANT SELECTION AND INTEGRATIONIn this section, we present the selection of plants and the processof integrating plants in the mechanisms for growth.
gears. In the prototype shown in Figure 13, the piston constrains
and transmits the forces of plants’ vertical growth, and a rack andpinion mechanism transmits the vertical expansion into rotary
movements. This mechanism enables the transformation of slow
plant growth into accumulative robotic movements by connecting
various end efectors to the gears of the mechanism, such as wheels.
Figure 13 and the supplementary video shows the sequence of
movements driven by the living layer housed within the piston.
As the plants grow and push the plunger or rack to move upward,
it propels the gear sets to rotate in the corresponding directions.
This mechanism can be used to activate diferent movements. For
example, by connecting the end gear with wheels, it is able to drive
the robotic mechanisms to move slowly, day by day.
4.2.2 Disabling and Destructing. The growth of plants can obstruct and disable a robot’s primary function. For instance,a plant may grow inside the robot’s motors, entangling its gears
and joints, block the robot’s sensors, which can impair its movement, reduce its perception and limit its ability to interact with theenvironment.
Another demonstration for the end of a robot’s lifecycle is todestruct the robot, utilizing the growth of plants to break down
the robot’s components. The robot is designed with an inherent
limitation to its lifespan. When its pre-programmed time arrives, it

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5.1

Plant Selection

A range of plants, notably mung bean, wheatgrass, maize (corn),and chia were explored as potential candidates for growth actuators(Figure 14). These were integrated in prototypes based on uniquecharacteristics like their seed size, growth rate, and stem-rigidity.
Diferent plants can be used for diferent actuation objectives.
For example, a rigid, vertically-growing plant like maize could facilitate force output in mechanisms like linear pistons. Soft plants,
such as wheatgrass, can contribute to pliability. Volume-increasing,free-space growing plants, like mung bean sprouts, can aid in modifying shapes of fexible enclosures. Seed size diversity can also
infuence the design of robotic systems. As the seed is the germinating plants’ source of nutrition, the reserve of nutrients in the seed
will infuence how long the seedling may grow before needing additional nutrient input. By integrating various seeds that have distinct
growth and decay rates, it is also possible to design and program
the sequence of activation and deactivation of robotic functions.
The diversity of plant types and their physical characteristics allow
for an abundance of unique plant-driven mechanisms and forms tobe produced for robotic applications.
In the mechanisms described in Section 4, mung bean sprouts
are primarily used for rapid prototyping due to their quick earlyDesigning Plant-Driven Actuators for Robots to Grow, Age, and Decay

DIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

Figure 14: Four plant types (chia, wheatgrass, mung bean, and maize) explored over the course of prototyping plant-driven
actuators. All have diferent physical characteristics, growing times, and seed size.

Figure 15: Plant integration process, including seed sterilization, seed imbibition, seed germination, seed sowing and growth.
growth and the ability of free-space expansion. However, plants
are extremely diverse and future designers could consider making
use of other characteristics of plants in robot design. For example,
we explored using stem and leaf tissue to power movement but
similar mechanisms could also use root growth and expansion in asimilar manner, particularly if the system is operated without light.
Additionally, future work could employ sturdier plant types, suchas tree branches and roots, that might take longer to grow but could
ofer greater resilience in terms of mechanical force and durability.

5.2

Plant Preprocessing and Integration

The preprocessing and integration of plants into the mechanicalsystems involves seed sowing, germination, and limited seedling
growth. The activation of the plants consists of seed sterilization,
seed imbibition and germination, and seed sowing. The processof seed surface sterilization includes a treatment of 70% ethanol(v/v) for 5 minutes, followed by 15% bleach (v/v) for 10 minutes,
and at least 5 washes with water [44] . The sterilization processincreases plant success by removing any possible pathogens that
may exist on the dry seed and could propagate with the plant as
the seeds are hydrated. We imbibe the seeds in 5 millimolar (mM)
CaCl2 solution for three days until successful germination, andvisually sort the seeds for uniformity. We then plant the selected
seeds either in the 3D printed base or the silicone cavity, ensuring
the root of the plant faces down into the hydroponic solution.
The hydroponic solution consists of de-ionized water supplemented with 5 mM CaCl2 . To prevent solution nutrient depletion,
the solution is refreshed every 2 days through the plastic tubing
connected to the base. The simple solution was used to aid seedgermination and, as opposed to tap water, allowed for standardizing
the nutrient intake across all plants in the robots. The objective is
to facilitate plant growth for a short time period (< 2 weeks), and it

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is expected that the combination of CaCl2 with nutrients from theseed will efectively sustain growth within this span. More complex and nutrient-rich solutions may not provide much additional
beneft at this stage, and their richness could potentially leave theplant growth set-up more prone to microbial contamination.

6

EXPERIMENTAL EVALUATION OF
PLANT-DRIVEN ACTUATORS

To comprehend the mechanical characteristics of plant-driven actuators, experiments were conducted to calibrate the deformation of
the plants in relation to time and under the infuence of mechanicalcompression loads. These investigations approximate the physicaland mechanical properties of the living layer using the piston mechanism. This section presents the experimental results, discussing
the growth curve, the post-experimental morphology of the plants,
and the implications for design including the considerations ofother environmental variables.
Setup. Experiments were carried out in a controlled chamber(Environmental Growth Chambers Model EGC TC2) maintained at
22 ◦ C with 24 hours of daylight. Light intensity was measured witha light meter (LI-COR Model LI-250) between 34 to 46 umol s �1
m �2 . Seeds of Mung Bean (Vigna radiata) were prepared using theprocess documented in Section 5.2. For time-lapse measurements,
images were acquired every 20 minutes with a Nikon D7100 camera.
For image analysis, photos were imported into the FIJI plant analysis
software [51], calibrated, and measured manually.

6.1

Sprout Growth Curve: Length, Time, andLoad

To test the efect of vertical load on displacement over time, mungbean seeds were planted under four diferent loads: (1) 7.8 g (control

DIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

Hu and Lu, et al.

Figure 16: Efect of load on vertical displacement driven by mung bean growth. (a) experiment setup; (b) piston displacements
over time, averaged across three repetitions; (c) the average width and (d) length of the post-experiment plants’ morphology.
condition); (2) 22.1 g; (3) 36.6 g; and, (4) 51.0 g. In all conditions, thesprout growing system was provided with 5 mM CaCl2 to foster
early plant growth, with the solution being refreshed every twodays. Three trials were carried out with the same environmental
parameters: one lasted 112 hours, while the other two each lasted
152 hours. The aforementioned time-lapse photography was used
and we extracted vertical displacement, in cm, at 8-hour intervals.
Figure 16 (b) depicts the mean piston displacement at each timestamp, color-coded by load, with the shaded areas highlighting
the variance across the three trails. The result suggests that these
vertical load treatments are sufcient to challenge the movementof the piston by the mung bean. Regardless of vertical load, allof the growth curves were approximately sigmoidal. The initial
sprouting process was slow-paced, as the seeds took considerable
time to access the energy present in the seed to germinate and growsufciently to push the piston. During this stage, the variations inmechanical load did not signifcantly impact the sprouting process.
At 50 - 60 hours post-sowing phase, the infuence of diverse mechanical loads began to emerge. The linear increase of vertical loads
did not result in a linear efect on the displacement of the piston

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or the length of the mung bean stem: the lightest load enabled theplants to grow at the highest speed; the next two loads, however,hindered growth without signifcant variation between the two.
The heaviest load substantially impeded the plants’ growth. Occasionally, the plants lacked sufcient strength to sustain growth,
illustrated by them pushing the piston level upward only to fallback down periodically. Eventually, the growth pace declined toa steady phase. The average maximum vertical displacement was
7.24 cm, 4.928 cm, 5.15 cm, and 4.03 cm for 7.8 g, 22.1 g, 36.6 g, and
51.0g, respectively.
In addition, the maximum vertical displacement occurred at 152
hours after the beginning of the experiment for 7.8 g, 22.1 g, and36.6 g, while it occurred at 128 hours for 51.0 g. This refects thatplants had halted while the other treatments were still capableof additional vertical growth. This slowdown occurred when theseedlings were unable to achieve greater vertical displacement due
to limitations in the material qualities of their stems or due to thedepletion of their energy reserves.

Designing Plant-Driven Actuators for Robots to Grow, Age, and Decay

6.2

DIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

Post-Experiment Plant Morphology

At the end of the experiments, the seedlings were removed fromthe pistons and imaged for fnal length and width measurements.
The results, as shown in fgure 16 (c–d), suggest that the seedlinglength was afected by the vertical load applied, with 9.01 cm, 6.97
cm, 7.28 cm, and 5.45 cm measured for 7.8 g, 22.1 g, 36.6 g, and 51.0
g, respectively. A one-way ANOVA shows statistically signifcant
infuence of vertical load on stem length (p = 2.75 x 10 �5 ). Seedlingwidth, as measured at the base of the plant stem, was not noticeably
afected by vertical load. The experiments reveal the capacity of
seedlings to achieve a growth rate between approximately 0.8 cm
to 1.4 cm per day, extending to 9 cm of total growth while also
withstanding constant mechanical loads.

6.3

Discussion and Implications ofExperimental Results

The introduction of mechanical load tended to decrease the overall
growth of the sprouts over the time period tested. The vertical displacement between diferent loads appeared relatively similar until
about 80 hours. This could be the result of plants under increased
vertical load expending more energy and depleting seed resources
quicker compared to their less-burdened counterparts. Anotherexplanation involves the material properties of the seedling stems,
and that after roughly 3 cm of extension they became unstable andbegan to buckle, though this wasn’t empirically tested. Dependingon the role of the plants in a robot system this may be an importantconsideration, especially if the function involves applying a force
or stabilizing objects.
Another signifcant observation pertains to the variability seenin the three repeated experiments. Concerted eforts were made tominimize variability by controlling for confounding factors—seedhomogeneity, temperature, light, and nutrients— yet there was still
variability between each repetition. This can likely be attributed toinherent variation within the seeds themselves. Each experimental
trial consisted of three seeds per treatment and the vigor of eachindividual seed had a large efect on displacement. One way ofcompensating for this variation would be to increase the numberof seeds incorporated in an actuator to minimize variability if that
quality is desired.
6.3.1 Lights and Nutrients. Light is integral to plant growth andthe light intensity and photo-period (period of light and dark in 24
hours) can greatly afect plant morphology and life cycle characteristics, such as fowering [48]. Designers should plan to manipulate
and monitor lighting conditions, including the placement, direction,and intensity of the lights to afect the temporal change of the robot.
Mineral nutrients also have a profound efect on plant growth
performance over time. To control this variable, we used de-ionizedwater with added CaCl2 that standardized our nutrient set-up andsupported plant growth. Those looking to incorporate plant growth
into mechanisms over longer periods of time should consider robust
fertilizer formulations incorporating essential mineral nutrients forplant growth [34]. In practice, researchers may need to conduct
literature searches and empirical testing to fnd a sufcient nutrient
solution to ft plant growth, and may design mechanisms to add

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nutrients or refresh the solution at a certain life stage for temporal
behaviors.
6.3.2 Competition and Spacing Factor. Spacing, air ventilation, andpathogens were also considered when designing the mechanicalhousing for seeds and plants. To prevent the build-up of microorganisms and pathogens during plant growth, good cultural practices
were incorporated into the plant actuator design, such as adequate
ventilation and spacing between seeds. Consequently, most of ourdesigns incorporated a mechanical layer to elevate seeds from thesolution, with a 5mm layer featuring holes for root growth. This
prevents the seeds and stems from becoming submerged in thesolution, which can lead to seed and stem rotting followed by the
proliferation of microorganisms [61].
In practice, it is essential to consider the competition between
plants. As plants are packed, they may compete for the limited
resources, with the growth of one potentially suppressing others or
the joint use of a limited nutrient pool leading to quicker growtharrest. This could be a problem in the design, or could alternatively
be a feature of the design. At any rate, the spatial design must be
carefully considered as it afects plant growth and infection.

7

DISCUSSION

In exploring using plants as actuators for designing robots thatgrow, age, and decay, this paper discusses our process and presents
a set of mechanisms to sketch potential embodiments of plant-robot
hybrids. This approach is motivated by the potential impact of plantrobot hybrids to the feld of human-robot interaction research. For
instance, plant-robot hybrids might introduce new approaches toalleviating the social uncanniness of companion robots, ofer alternative interaction norms, and challenge the established paradigms
prevalent in conventional robotics. Using plants as design materials
provides many unique opportunities for engaging users in interactions compared to traditional robotic mechanisms, incorporating
the inherent characteristics of plants’ life, including slowness, unpredictability, nurturing and inevitable decay. Our collaboration
is both interdisciplinary and brings together experts in HCI/HRI
design, mechanical engineering, and plant science. This eclectic
mix allows us to assess and enhance the design from diverse perspectives. Several insights emerge from the process of designing
plant-driven actuators and on the implications for future plantrobot hybrids in human-robot interactions.

7.1

Challenging Robotic Forms

Plant-driven robot design suggests an unmaking of traditional representations of interactive robots, which often take on anthropomorphic forms. While these anthropomorphizing strategies canserve robots well for facilitating empathy from human users, plants
themselves have often been found to elicit a diferent type of empathy. In fact, houseplants could be argued to serve a similar socialrole to what many social robots aim to do, but are much morecommon across households in the world than social robots and insome cases, have much more interaction longevity. How to bestbalance the traditional expectations of what a robot does and looks
like while allowing designers to be inspired by plants to unmakeand reshape those expectations is something we hope to provoke
in our work.

DIS ’24, July 01–05, 2024, IT University of Copenhagen, Denmark

7.2

Hu and Lu, et al.

Embracing Unpredictability

Plant-driven robots also introduce a higher level of agency compared to the traditionally inert materials used in robot design. A
robot powered by a plant inherits the biological medium’s variability: Its growth, change, and decay dynamics cannot be completely
predicted or controlled. As such, robot-plant hybrids are no longer
under complete user control but instead exist, change, and live on
their own accord. This approach resists traditional human-centric
relationships with robots where the robot is completely oriented
around service to the human user. Designers and researchers should
keep in mind that there is inherent unpredictability in plant behaviors and take this variability into account when designing plantdriven actuators. Variability and randomness are unavoidable when
dealing with living materials, and have to be integrated in the designfrom inception.

7.3

Promoting Empathy and Care

The requirement for biological care, along with the physical changes
in a robot’s body over time, may allow for more empathy and responsibility to develop in the interaction with a plant-based robot compared to an electromechanical device. Citing the possibility of living media to promote empathy and caring in users,
some used living information displays to encourage positive behavioral change [18], motivate users to engage in tedious therapeutictasks [16], and feel responsibility and a sense of reciprocity toward
an interactive device [37]. Plant-driven robots could provide similar
benefts.

7.4

Allowing Slowness

In the context of interactive robotics, the slowness of change afforded by a plant contrasts with the user’s expectations of instantbehaviors and responses. This deceleration provides opportunities
to overcome the novelty efect in which the user’s interest decreases
after the excitement about new interaction patterns disappears [36],and can promote social relationships in long-term interactions. Allowing slowness to be central to the design of robots encourages
us to rethink interactions that emphasize one-of user interactions
and instead design for the total life cycle of social agents.

7.5

lasting novelty. Even when using similar mechanical setups, using
diferent seed combinations could also produce varied actuation
patterns, thus creating personalized experiences and making eachinteraction cycle irreplaceable.
Through this practice, we hope to inspire new perspectives of robot design by incorporating plants’ temporal changes into robotic
systems, exploring how they may transform long-term humanrobot interactions. Some future use cases of plant-driven robots
may include serving as an educational tool for encouraging caring
behaviors in children; displaying information over time, such as
visualizing the progress of user actions to remind and encourage
positive habits forming and behavioral changes; eliciting empathy
and enhancing emotional bonds between users and robotic companions through their inevitable aging and decay. Future work will
include exploring robotic prototypes with plant-driven actuators,
understanding how users interact with the plant-robot hybrids overtime, and investigating how these robots might change long-term
human-robot relationships.

8

CONCLUSION

Many of the norms of conventional robot design can often limit
the types of interactions we have with robots. To explore an alternative, we present plant-driven actuators that can be integrated
into the robotic functions, allowing for robots that grow, age, anddecay. We discuss design opportunities for achieving a multitude ofinteractions that can be actuated via plant growth. We evaluate thecharacteristics of plant growth and decay under the stress of our
proposed systems through a series of experiments. Through thiswork, we aim to show the opportunities for incorporating plants in
actuator designs and pave the way for future plant-robot hybrids.
Future work might explore what new robot forms could emerge
when robot designers use these plant-driven primitives, how users
might respond to growing and aging with a plant-robot hybrid, andeven how growing plants for robotic systems might enable new
insights for plant biology. We hope that this work can serve as astarting point to inspire others to imagine and build alternative
forms of robotic actuators.

ACKNOWLEDGMENTS
This research was supported under NSF National Robotic Initiative Award NRI: 1830471, and NSF Division of Graduate Education
Award DGE: 1650441.

Limitations, Opportunities, and FutureWork

As described in earlier sections, actuators powered by plant growth
face challenges due to inconsistencies between intended and actual growth patterns. For instance, plants might expand throughventilation openings thus failing to exert enough forces needed totrigger movements; seeds could compete for resources, resulting inuneven growth and mechanical friction during vertical movements;
or seeds might not germinate due to infections or less-than-ideal
environmental conditions. These issues can lead to the failure ofintended mechanical functions and problems with reproducibility
under varying environmental conditions and seed variances. However, this unpredictable nature also presents unique opportunities
in interactions. Each plant-based actuator is inherently distinct,
and its behavior may be unrepeatable, ofering a shift in interaction perspectives. This can result in unforeseen behaviors, ofering

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