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Background: The green algae balls (Aegagropila linnaei), known as Marimo, are large spherical colonies of live photosynthetic filaments, formed by rolling water currents in freshwater lakes. Photosynthesis therein produces gas bubbles that can attach to the Marimo, consequently changing its buoyancy. This property allows them to float in the presence of light and sink in its absence. Results: We demonstrate that this ability can be harnessed to make actuators, biosensors and bioprocessors (oscillator, logic gates). Factors affecting Marimo movement have been studied to enable the design, construction and testing of working prototypes. Conclusions: A novel actuator design is reported, incorporating an enhanced bubble retention system and the design and optimisation of a bio-oscillator is demonstrated. A range of logic gates (or, and, nor, nand, xor) implementable with Marimo have been proposed.
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Phillips et al. Journal of Biological Engineering (2019) 13:72
https://doi.org/10.1186/s13036-019-0200-5
RESEARCH Open Access
Marimo machines: oscillators,
biosensors and actuators
Neil Phillips1* , Thomas C. Draper1, Richard Mayne2,1 and Andrew Adamatzky1
Abstract
Background: The green algae balls (Aegagropila linnaei), known as Marimo, are large spherical colonies of live
photosynthetic filaments, formed by rolling water currents in freshwater lakes. Photosynthesis therein produces gas
bubbles that can attach to the Marimo, consequently changing its buoyancy. This property allows them to float in the
presence of light and sink in its absence.
Results: We demonstrate that this ability can be harnessed to make actuators, biosensors and bioprocessors
(oscillator, logic gates). Factors affecting Marimo movement have been studied to enable the design, construction
and testing of working prototypes.
Conclusions: A novel actuator design is reported, incorporating an enhanced bubble retention system and the
design and optimisation of a bio-oscillator is demonstrated. A range of logic gates (OR,AND,NOR,NAND,XOR)
implementable with Marimo have been proposed.
Keywords: Marimo; Processor; Logic gate; Oscillator; Biosensor; Bioactuator; Biomotor; Algae; Unconventional
Computing; Biomimicry; TRIZ
Background
Increasingly scientists and engineers are realising that
nature, over many millions of years of evolution, has by
necessity developed energy efficient systems. Combin-
ing some of nature’s biochemical processes with human
engineering (biomimetic) has enabled novel devices to be
prototyped [1].
The variability in living systems, bred from evolutionary
pressures, can take the form of movement with minimal
energy consumption. In plants, responses that occur in
response to a directional stimulus are called tropisms [2].
One of the most commonly observed tropic responses
in plants is phototropism, in which plant stems grow
towards light. Heliotropism is the basis for rapid and
reversible movement in plants, which allows them to track
the sun. Plants can also exhibit non-directional responses
to stimuli (nastic movements), for example temperature
and humidity. The rate or frequency of these responses
increases as intensity of the stimulus increases [3].
*Correspondence: neil.phillips@uwe.ac.uk
1Unconventional Computing Laboratory, University of the West of England,
Coldharbour Lane, Bristol, BS16 1QY UK
Full list of author information is available at the end of the article
Nature has adapted to harness solar energy via pho-
tosynthesis, which provides the basic energy source for
nearly all organisms. The maximum photosynthetic effi-
ciency of algae is an active topic of debate [4]. The theoret-
ical potential (approximately 36% [5]) cannot be achieved
in practice due to limitations of the relevant bio-chemical
processes. At low light intensity algae can convert roughly
6% of Photosynthetically Available Radiation (PAR) into
biomass in a best case scenario [6]. This conversion rate
drops in full sunlight to avoid damaging the organism [7].
This investigation focuses on harnessing adaptive
responses of a live heliotropic, photosynthetic organism
for use in hybrid bio-artificial systems. Aegagropila linnaei
balls are large (exceeding tens of centimetres in diameter
in some cases) spherical objects [8,9] formed by the natu-
ral rolling and self-adhesion of filamentous alga over many
years in turbulent freshwater lake currents [10,11]. A. lin-
naei are known more commonly (and hereafter in this
paper) by the Japanese monicker “Marimo”, from the ubiq-
uity of the alga balls arising from Lake Akan, Hokkaid¯
o,
Japan [12,13]. Photographs of both an intact Marimo and
the cross-section of a Marimo can be seen in Fig. 1aand
Fig. 1b, respectively. In the cross-sectional photograph, it
can be seen that the filamentous nature of the Marimo is
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 2 of 10
Fig. 1 Photographs of an (a) intact and (b) cross-sectioned Marimo. Small grains of sand are visible in both images. The diameter of the Marimo is
62mm
continuous throughout. Additionally the outer edge is a
darker green than the core, which is believed to be due to
the photosynthetic pigment concentrating in the regions
that receives the most illumination, in agreement with
previously published works [10].
After considering a range of algae structures it was
concluded that Marimo was particularly promising for
utilisation in functional bio-artificial devices. Marimo can
grow in three forms: (1) epilithic, usually found on the
shaded side of rocks; (2) free-floating filaments, that can
form a carpet on the surface of the water; and (3) densely
packed algal filaments, that radiate from the centre form-
ing spherical shape [14]. For our purposes, the latter has
the advantages of being self-contained, mobile, and able
to photosynthesise using light from any direction [15].
Furthermore, Marimo appear to have an extraordinarily
long lifespan, with literature citing that natural balls are
formed over ‘many years’ [10] and commercial suppliers
advertising prised ornamental specimens over 10 cm in
diameter, which are reportedly grown over a period of
15 or more years. This suggests a long lifespan of any
proposed bio-artificial constructs.
Other researchers [16] have studied the natural char-
acteristics of Marimo; in particular, its ability to rise and
sink in water, which was found to result from generation
of oxygen via photosynthesis. Bubbles are formed on the
surface of, and at shallow depths within, the Marimo when
they are provided with illumination: it is assumed that
the filamentous nature of the alga both provides numer-
ous nucleation sites and creates a mesh through which
it is difficult for the oxygen bubbles to dissipate. The
observed phenomenon of a Marimo ball rising when pro-
vided with a means to initiate photosynthesis suggests that
the oxygen generation, and retention as bubbles adherent
to and within the moss balls, may exceed the rate at which
oxygen is lost through dissipation or percolation through
its filamentous structure.
Several research groups have reported on bioenergy,
usually through converting biomass into electricity or
secondary products [6]. Other groups have reported
biomimetic microsystems with buoyancy control using
features such as: Pt:Ag microbeads decomposing H2O2
[17], clay-coated catalase-containing microcapsules which
decompose H2O2[18], or metal-organic frameworks con-
taining catalase for the decomposition of H2O2[19].
However, using Marimo to directly power processors,
bio-sensors and actuators through exploitation of its pho-
tosynthetic ability has yet to be explored. The research
reported here represents a step towards the long term goal
of autonomous, light powered, biological systems which
can operate under real world conditions. To expand on the
benefits of using biological components for engineering
and computing applications, many characteristics of bio-
logical systems can be considered as desirable if exploited
for a useful task, such as self-growth, low energy con-
sumption, carbon capture (in photosynthetic organisms),
organisation and variation. This ethos is predicated on
minimising the use of conventional electronics, as bio-
artificial hybrid devices necessarily exhibit the drawbacks
of both types of material. Therefore, biological devices are
not considered as direct replacements for their artificial
counterparts (e.g. as ‘biological time’ is slower than elec-
trical communications, biological solutions are typically
not suited to time-critical applications), but as comple-
mentary systems.
Rather than using the biomass formed from the photo-
synthesis, we took the unconventional approach of util-
ising the gas generated during the photosynthetic pro-
cess instead. More specifically, the low density of the gas
(0.001g cm3)comparedtowater(1.0gcm
3)meansthe
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 3 of 10
gas rises in the form of bubbles to minimise overall Poten-
tial Energy (PE). The movement of the bubbles towards
the surface of the water can be harnessed to enable a
variety of systems.
We demonstrate that a variety of actuating and, poten-
tially, computing devices can be implemented by using
Marimo with a controlled patterns of illumination. We
propose experimental designs of Marimo motor and oscil-
lator and speculate about potential implementation of
Marimo logic gates.
Methods and materials
Marimo culture
Artificially-rolled Marimo were sourced from Amazon
UK and were kept in large, lidded aquariums in tap water
containing 0.1mL L1of commercially-available fertiliser
containing a source of phosphates and nitrates (TNC
Complete, The Nutrient Company). When not in use in
experiments, the organisms were exposed to a day/night
cycle but were kept out of direct sunlight. Aquarium water
and fertiliser was refreshed every 3 weeks. All Marimo
culture and consequent experimentation was conducted
at room temperature (18Cto22
C). For a detailed
measurement of illumination level, a Photosynthetically
Active Radiation (PAR) sensor was used (see Additional
file 1for bubble, light and calibration data).
Bubble retention
Cohorts of Marimo (60 mm diameter) were monitored
under the same conditions (light and nutrients). All were
observed to produce bubbles, but only some gained suffi-
cient buoyancy to raise off the floor of the aquarium and
float. The proportion of bubbles which stay attached to
algal filaments is noted to affect overall buoyancy. In other
words, bubble generation and retention is required to
achieve flotation. The distribution of bubbles on the sur-
face (of undisturbed) water above Marimo was monitored.
The density of bubbles towards the periphery doesn’t
increase in proportion to Marimo surface area which sug-
gests it is easier for the bubbles to escape from the algal
filaments when their lift (due to buoyancy) is upwardly
aligned with the filaments.
Buoyancy increase from bubbles
The positive buoyancy provided by bubbles was measured
by carefully adding weights to floating Marimo until they
sunk. To minimise unwanted rotation and associated loss
of bubbles the weights were formed from rings of cop-
per wire with a diameter smaller than the diameter of the
Marimo. Thetypical lift provided by bubbles on Marimo of
approximately 60 mm diameter is 1.5 g. The surface area
of a 60 mm diameter Marimo is 110 cm2. However, only
part of the bottom hemisphere usually holds larger bub-
bles, so the effective lift area is 50 cm2. Therefore, usable
lift is approximately 0.03g cm2. To investigate the influ-
ence of illumination on bubble generation Marimo were
located under (water filled) glass funnels with attached
syringes, such that any released bubbles would rise inside
the funnel and be captured. The change in volume of air
inside the syringes (50 mL) was recorded while maintain-
ing the same water level inside the funnel. One beaker
was intentionally left empty and acted as ‘control’ (no gas
was collected). Five funnel rigs were left running (simulta-
neously) for two weeks. To ascertain the influence of the
concentration of dissolved carbon dioxide on photosyn-
thesis, a Marimo was continuously illuminated (Martin
Rush Light Source, see Additional file 1)foraprolonged
period (10 days) in still water until bubble formation (pho-
tosynthesis) was observed to stop. Once ceased, frozen
carbon dioxide (dry ice, 8 g) was added to elevate the
concentration of carbon dioxide in the water.
Marimo motor
Ten Marimo (approximately 60 mm diameter) were
located inside transparent chambers (70 mm diameter,
85 mm length, with screw cap lids) equally spaced around
the periphery of two transparent, acrylic discs (340
mm diameter), see Fig. 2c. The discs were mounted on
bespoke ‘friction-free’ magnetic bearings (see Additional
file 1). The rotor assembly was dynamically balanced (see
Additional file 1) to within 50 g mm by adding correcting
weight to both discs as required. Alternatively, an auto-
matic balancing set-up could be exploited [20]. The per-
formance of the motor was separately investigated with
sunlight and illumination solely from an LED growth light
(120 W input) on the side of the aquarium, see Fig. 2a. The
aquarium water re-circulation pump was left switched off
to prevent recirculating water affecting measurements. A
camera (Nikon P900) was programmed to take a photo
every four minutes, in order to determine the speed and
direction of rotation. Testing revealed the location and
size of the bubble release slots has a significant impact on
speed and smoothness of rotation. Slots 6mm wide and
40mm in length were used, through trial and error, see
Fig. 2bandFig.2d.
Marimo oscillator
Two Marimo (approximately 60 mm diameter) were place
in transparent columns of water (110×110×300mm, see
Fig. 3), free to move up and down when illuminated.
The ‘right hand’ Marimo (nearest the lamp) blocks light
reaching the ‘left hand’ Marimo. The illumination (from
Martin Rush LED lamp, set to 18% red, 18% blue and 0%
green, see Additional file 1) was made more parallel by
passing through tube (80 mm diameter and 200 mm
length) with matt black surface. After a period of illumi-
nation the ‘right hand’ Marimo floats and light illuminates
the ‘left hand’ Marimo, which then floats. The light beam
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 4 of 10
Fig. 2 Marimo motor. aMotor in aquarium and LED lamp. bSide view of Marimo in plastic pots. cPlan view of rotor assembly. dBubble escape slot.
A video of the Marimo motor rotating can be seen in the Additional file 3
is then reflected by a pair of mirrors onto the floating
‘left hand’ Marimo. The ‘right hand’ Marimo then sinks,
blocking the light to the ‘left hand’ Marimo. This then
causes the ‘left hand’ Marimo to sink. This oscillating
cycle repeats continuously. In order to restrict Marimo
movement to the vertical axis, spikes were added to the
bottom of the water columns. The acrylic bases of the
spikes were coated in silicon to reduce nucleation sites for
bubble formation. The centre of mass of the Marimo was
lowered as discussed in the “Results” section. Addition-
ally, the level of the water in the columns was set such
that, whilst floating, illumination to bottom half of the
Fig. 3 Experimental rig of Marimo oscillator. (1) parallel light beam via matt black tube from LED lamp, (2, 3) mirrors, (4, 5) spikes and (6, 7) Marimo
with floats and weights
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Marimo was most prominent. The experimental set up
and diagram of the oscillations can be seen in Fig. 3.A
bespoke parallel (4) light beam (660 nm) was developed
to enhance performance (see Additional file 1).
Results
Buoyancy & movement enhancement
It was observed the typical rate of bubble generation (for
approximately 60 mm diameter Marimo) in moderate sun-
light is 1 mL d13mLd
1.Theratecanincreaseto
6mLd
1in strong sunlight. After a prolonged period
of illumination, the bubble production of the Marimo
was observed to decrease. After addition of dry ice, no
additional generation of bubble formation was observed,
suggesting that the carbon dioxide concentration is not
responsible for the decrease in photosynthesis.
The variations in movements of similar Marimo (n=
20) were recorded (via video time lapse) under different
conditions. Analysis of recorded images revealed irregular
shaped Marimo floated more often than spherical shaped.
It was hypothesised that their asymmetry reduces rotation
allowing them to retain more bubbles. Through experi-
mentation this characteristic was deliberately enhanced
by adding positive buoyancy above the Marimo and neg-
ative buoyancy below the Marimo, to lower the centre of
mass which in turn reduces rotation off the vertical axis.
For best performance, the combined change in buoyancy
of positive buoyancy (float), negative buoyancy (weight)
and connection (link) needs to be slightly positive. The
float and weight need to be free of nucleation sites to
prevent additional bubble formation (which can posi-
tively bias buoyancy). This can be achieved by selecting
objects with smooth surfaces, such as plastics. A Marimo
modified in such a way can be seen in Fig. 4.
To measure water absorption (and quantify loss of buoy-
ancy over time) a cohort of spheres made of different
types of plastics with and without holes were weighed
before and after submersion under water (see Additional
file 1for details). Testing confirmed polypropylene offers
greaterliftpervolumethanHDPE(consistencyofden-
sity and water absorption were both similar). Some other
types of plastics (such as polymethylpentene (TPX™) with
density of 0.82 g cm3) provide greater lift per volume,
but are not readily available as spheres of desired size.
The float or weight can be coloured or marked to pro-
vide visual differentiation for experimentation. As the
float and weight are solid (rather than hollow) they can
be drilled to provided an anchor point (2 mm diame-
ter). To secure the float and weight, whilst ensuring the
Marimo is still able to float freely, a narrow (1.5mm diam-
eter) skewer is inserted directly from the top to bottom
to ensure the float and weight are aligned with Marimo
centre of mass. The available reduction in density (com-
pared to water) of solid plastic (e.g. 0.9 g cm3for
Fig. 4 Marimo with artificially generated lower centre of mass, in
order to prevent rotation. The top sphere is polypropylene, the
bottom sphere is polyoxymethylene, and they are interconnected
through the Marimo with a nichrome wire
polypropylene (PP)) is less than the available increase
in density (e.g. 2.2 g cm3for polytetrafluoroethylene
(PTFE)) so the float requires a larger volume than the
weight to provide an equivalent change in buoyancy.
Example dimensions and material properties are shown in
Tabl e 1, however, alternative equivalent combinations are
possible. An additional consideration is water absorption
of the float over prolonged periods of immersion, this can
be minimised through material selection (e.g. plastic made
from non-polar organic species).
Marimo motor
When illuminated by daylight, speed of rotation is pro-
portional to intensity. For example, approximately 0.2 re
vh
1at 1 kW h m2and approximately 0.5 rev h1at
2.7 kW h m2. Daily illumination levels were obtained from
a publicly available dataset on the photovoltaic power gen-
eration at the nearby Hamilton House, Bristol, UK. The
energy of a single Marimo completing a single revolution
is 0.9 mJ rev1per Marimo, when rotating at 0.2 rev h1.
This energy will increase linearly as the speed of rotation
increases. The calculation can be seen in the Additional
file 1.
Illumination level needed to be within a suitable range:
too low and the rate of photosynthesis is too slow to gen-
erate the necessary oxygen to raise the Marimo, whereas
too high level of illumination caused damage to the
Marimo (as evidenced by their taking on a discoloured
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 6 of 10
Table 1 The density and related buoyancy of various materials and Marimo
Part Material Density Diameter (Length)Weight Buoyancy
gcm
3mm mN mN
Float (sphere) Polypropylene (PP) ~0.9 25.4 71 7.8
Marimo (sphere) Filamentous algae ~1.0 50 – 60 520-4.9
Bubbles Gas (O2,CO
2&N
2mix) 0.001 0 – 5 0 0 – 20
Weight (sphere) Polyoxymethylene (Delrin®) 1.42 11 9.0 -2.6
LinkNichrome wire 8.4 (75) 1.5 -1.3
Combined Overall 1to+19
Dimensions are given as diameters for spheres and as length for the ‘link’. The Marimo ball has a weight of 88mN once desiccated
appearance). A PAR light sensor (SQ-120, Campbell Sci-
entific Ltd) was located on top of the motor’s bearing
support arm. The sensor’s output voltage was measured
with a Fluke 8846A. For example, when illuminated by
Laputa ‘60LEDS grow light’ (set to 40% of full power)
the PAR sensor output was 22mV, which corresponds
to 110 μmol m2s1(approximately 1240 lumens), which
rotated the Marimo motor with an average speed of
0.25 rev h1.
Marimo oscillator
The Marimo balls in the experimental oscillator setup
executed phases movement (Fig. 5a) detected via video
recording and measurement of light passing through the
water columns (Fig. 5b). Data recorded (using protocol:
both Marimo low = 0 0, left low, right high = 0 1, both
Marimo high = 1 1, left high, right low = 1 0) and a video of
the oscillation sequence is available in the Additional files
2and 5. Probabilities, derived from experimental record-
ing, of transitions between states of the two-ball oscillator
are given in Fig. 6a.Theoscillationcanbeseenasaprob-
abilistic automaton with four states (00), (01), (10), (11),
corresponding to the situation of ‘left’ and ‘right’ balls
being down (0) or up (1). The probabilistic state transition
diagram of the automaton is shown in Fig. 6b. If only state
transitions with a maximum likelihood are allowed to take
place then the two-ball Marimo machine runs in the cycle
shown in Fig. 6c, thus illustrating the ideal oscillator.
Discussion
A wide range of logic gates (including: AND,NOR,OR,
NAND,XOR) have been envisaged using Marimo with light
as ‘input signal’, ‘output signal’ and ‘actuation power’. In
other words, depending on the desired functionality, light
is interchangeable between all three functions [input sig-
nal (data), output signal (data), actuation power (energy)].
This provides potentially zero electric grid consumption
when operating on sunlight. Further, processing based on
variable density units can be both modular and scalable.
A key benefit to the devices presented is, therefore, the
efficiency inherent in utilising photosynthetic (i.e. carbon-
capturing), self-growing, solar-powered devices whose
running costs are essentially null, regardless of whether
their application lies in creating unconventional comput-
ing devices (logic gates, oscillators) or engineering (motor,
e.g. applied to electricity generation).
In order to extend the systems’ capabilities into logic
gates, a refined set up has been designed. The layouts for
these gates can be seen in Fig. 7. The main alteration is that
the float has transparent and opaque sections, as required.
Inputs are represented by the presence or absence of
a light beam entering the system, and outputs by a
light beam exiting the system. Such a system could also
be implemented using other vertically-controlled units
[1719].
During the development of the Marimo logic gates it
was observed that Marimo can hold a stable position
between the ‘top’ and ‘bottom’ of the containment vessel
for prolonged periods (upwards of 30 min, video available
in the Additional file 4), with a suitable lighting configura-
tion. This enables the potential advancement of logic gates
to include multi-state operations. Further, by graduating
the light level, an analogue system can be conceived.
It was observed that Marimo often take a period (sev-
eral hours) before forming bubbles and movement in a
new/refreshed environment. This induction period may
be a combination of several factors including: rising con-
centration of dissolved gases in local water before bubble
formation, re-acclimatisation of the organisms to the rapid
change in environment, and the state of the Marimos’
photosynthetic systems prior to beginning the experiment
[21].
The 1D rotational movement (of Marimo motor) can
be extended to 2D movement. One can envisage such a
system whereby multiple Marimo are located inside indi-
vidual transparent chambers (spheres), which are them-
selves located near the inside surface of a larger partly
transparent sphere. Marimo exposed to illumination will
generate gas (via photosynthesis) creating positive buoy-
ancy. The net change in buoyancy will rotate the larger
sphere (rover) away from the light source. Each of the
smaller spheres would contain a gas vent (located at the
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 7 of 10
(a)
PPFD, µmol m-2 s-1
0
5
10
15
Time, min
0 500 1000 1500
(b)
Fig. 5 Dynamics of the Marimo oscillator. aScheme of a typcial sequence of actions. bLevel of illumination against time is shown by black circles
(the connecting black line between the circles is to guide eyes). ‘Up’ and ‘bottom’ position of left (red triangle) and right (green rectangle) balls are
superimposed on the plot
Fig. 6 Probabilities of state transitions of a two-ball Marimo oscillator. (a) Experimental transition probabilities going from one state at time ‘t’ to
another state at time t+1. (b) Probabilistic State Transition diagram, populated with values from (a). (c) Probabilistic State Transition diagram for an
ideal oscillator
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 8 of 10
Fig. 7 Logic gates based on Marimo: (a)NOR (b)AND (c)OR (d)NAND (e)XOR. The Marimo are represented with green balls. The floats are
represented by orange and/or black circles. The black section of a float is opaque, while the orange section is transparent. The input signals are
shown as A, B, C, or D. The output signal is observed by sensor F1 or F2. An ‘always-on’ light sources is portrayed by a black box
largest distance from centre of larger sphere) such that
the gas can escape when the smaller sphere moves to the
‘top’ of the larger sphere. It is worth noting that because
the surface area of a sphere increases to the square of
its radius, the potential power of a Marimo rover would
increase quadratically with its size. Potentially, such a
light-powered Marimo rover could autonomously travel
on land or in water (if outer sphere is suitably configured),
and navigate topology. The topology of the land could be
inferred by remotely tracking the movement of the rover
(e.g. add RF reflector and track via satellite). A prototype
rover is under construction and will be reported on in the
future.
Potential applications of actuators and individual logical
gates based on Marimo can be found in situations where
speed of operation is not essential, but device longevity
is. For example, a logical device such as the one pre-
sented here could be used as a light-sensitive controller
for a microbial fuel cell system, such as the one pre-
sented in Ref. [22]. The rationale behind this approach
is that such a controller — which could be effective for
the decades-long lifespan of the Marimo used and would
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Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 9 of 10
not require an electrical power source — would switch
between power usage or storage based on the users diur-
nal energy demands. Furthermore, any low speed of oper-
ation could be mitigated in the various applications of
Marimo motors, such as the aforementioned electricity
generation. If, for example, Marimo motors were used to
generate electricity via induction from only sunlight input,
device efficiency could be improved through scaling and
design optimisation. Whilst electricity generation could
not be expected to be enormous through these means,
we consider the concept worthy of further investigation
as a current opinion on alternative fuel sources is that a
variety of ‘green’ methods should be utilised in combina-
tion to achieve better climate outcomes. As such, certain
perceived ‘failings’ of Marimo-based devices may be mit-
igated by factors such as their ability to capture carbon
and not requiring heavily-refined or enriched materials in
theirproduction,asisthecaseforsolarpanels.
The potential speed of operation is an important con-
sideration in biomimetic designs, as it is comparable to
data processing and actuation time. While most plants
move relatively slowly (compared to animals), some have
evolved fast responses. For example, the Venus flytrap
(Dionaea muscipula) can capture a fly by closing its trap
(a modified leaf ) in less than one tenth of a second when
triggered (physical contact with plant’s hairs) [23]. There-
fore, the speed of operation cannot be considered to be
limited solely due to its biological nature. Whilst the cur-
rent investigation did not examine variation in Marimo
diameter, this would be an important aspect to focus on in
future studies as there could be a relationship between the
size of a Marimo and its movement.
Although the applications outlined above are proposed
specifically in applications where slow speed of operation
is not an impediment, the principles of operation out-
lined in this paper can be applied to a smaller scale which
could be used, in turn, to enhance the operation speed.
As photosynthesis is a characteristic of multiple species
of plants, algae, bacteria and protozoa, the limitation on
the size of a living oxygen-generating buoyancy system is
therefore bounded by the physical dimensions of a sin-
gle photosynthetic cell plus a surrounding medium for
capturing any oxygen generated, although smaller systems
may be considered as viable through the use of decellu-
larised chloroplasts. We propose that, if immobilised in
a suitable porous, water-stable, light-permeable substance
such as sodium alginate, small colonies of microalgae (e.g.
Micromonas pusilla, approximately 3 μminlength[24])
or cyanobacteria (approximately 1 μm in length) could
be utilised in much the same manner as the previously
described Marimo devices. The use of microfluidic tech-
nology has been demonstrated as a reliable method for
generating alginate beads with embedded substances in
the range of 10 μm[25],henceweproposethistobea
realistic estimate of the size of a single photosynthetic,
variable-density buoyancy system, for use in miniaturised
versions of the Marimo devices previously described,
which could be engineered to have much faster oper-
ation times through exploiting lower-mass components
and shorter travelling distances for the photosynthetic
elements.
Conclusion
We have demonstrated that a diverse range of devices can
be built using Marimo. Our systems act as (1) biosen-
sors, in that their output varies with external environ-
mental conditions; (2) actuators, in that we have con-
structed a Marimo-powered rotational motor; (3) and
logic gates, using the natural vertical motions of Marimo
we have established the first Marimo oscillator and pro-
posed schematics for a wide range of logic gates based on
the time-varied density of Marimo. The performance of
these example systems has been enhanced by optimisation
of the centre of mass and overall buoyancy of individ-
ual Marimo. The principle of using photosynthesis to
control operations has been demonstrated. The potential
to extend functionality to a autonomous, light-powered
rover has been proposed. Transferring the principles of
operation to the cellular scale would enable greater pro-
cessing density. Opportunities for further enhancements
to functionality have been identified and are under devel-
opment.
Additional files
Additional file 1:Additional supplementary information is available in an
online file. This includes construction information on the motor and
balancing rigs, details on the floats used, and details on the light sources.
(PDF 1806 kb)
Additional file 2:Video of the Marimo Oscillator in action. (MP4 13,077 kb)
Additional file 3:Video of the Marimo Motor in action. (MP4 12,292 kb)
Additional file 4:Video of the Marimo whilst vertically stable. (MP4 12,592
kb)
Additional file 5:Processed data of the Marimo oscillator video. (XLSX 12
kb)
Acknowledgements
This research was supported by the University of the West of England, Bristol,
UK.
Authors’ contributions
NP conceived and conducted the experiments. All authors discussed and
analysed the results. NP and TCD took the photographs and videos. All authors
drafted, reviewed, and approved the manuscript.
Availability of data and materials
The datasets made and analysed during the current study are available from
the corresponding author on reasonable request.
Ethics approval and consent to participate
Not applicable.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Phillips et al. Journal of Biological Engineering (2019) 13:72 Page 10 of 10
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1Unconventional Computing Laboratory, University of the West of England,
Coldharbour Lane, Bristol, BS16 1QY UK. 2Department of Applied Sciences,
University of the West of England, BS16 1QY Bristol, UK.
Received: 3 June 2019 Accepted: 8 August 2019
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... In our previous study [9] we extended prior research into photosynthesis-powered actuators and biosensors into more advanced systems. In this article we advance the concept of autonomous & self-powered movement, suitable for real applications, and describe a fully-functional prototype rover using a biological organism as its main component. ...
... For spherical geometry, two variants were tested: (1a) Marimo ball of ∼60 mm diameter were located inside transparent sphere of 60mm diameter. The size was selected to correspond with previously published measurements [9] (1b) Marimo ball of ∼30 mm diameter in transparent sphere of 60mm diameter. For hemispherical geometry, one variants was tested: (2a) Half (hemisphere) of Marimo ball of ∼60 mm diameter in transparent sphere of 60mm diameter. ...
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