Gareth Jones** - 麻省理工学院人工智能研究专业

Gareth Jones**
†
Philip H. 国王
Hywel Morgan**
Maurits R. 右. de Planque‡
Klaus-Peter Zauner*,**
University of Southampton

关键词
Fluid robots, molecular robotics,
proto-organism, lipid membranes,
self-assembly, Belousov-Zhabotinsky
reaction

Autonomous Droplet
Architectures

Abstract The quintessential living element of all organisms is the
cell—a fluid-filled compartment enclosed, but not isolated, by a layer
of amphiphilic molecules that self-assemble at its boundary. Cells
of different composition can aggregate and communicate through
the exchange of molecules across their boundaries. The astounding
success of this architecture is readily apparent throughout the
biological world. Inspired by the versatility of natureʼs architecture,
we investigate aggregates of membrane-enclosed droplets as a design
concept for robotics. This will require droplets capable of sensing,
information processing, and actuation. It will also require the
integration of functionally specialized droplets into an interconnected
functional unit. Based on results from the literature and from our
own laboratory, we argue the viability of this approach. Sensing and
information processing in droplets have been the subject of several
recent studies, on which we draw. Integrating droplets into coherently
acting units and the aspect of controlled actuation for locomotion
have received less attention. This article describes experiments that
address both of these challenges. Using lipid-coated droplets of
Belousov-Zhabotinsky reaction medium in oil, we show here that
such droplets can be integrated and that chemically driven mechanical
motion can be achieved.

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1 介绍

Organisms consist of one or more membrane-enclosed fluid drops. The contents of these cells as
well as the composition of their membranes vary widely across species and across cell types within
multicellular organisms. Common to all of them is an intricate organization and sophisticated
molecular machinery that is so far only partially understood. Faced with this complexity, the ques-
tion of what the simplest cell might look like arises. It has been contemplated in the context of the
origin of life and more recently in the context of synthetic biology. The latter aims at finding a recipe
for the fluid and the membrane that would endow a drop prepared according to the recipe with the
phenomena ascribed to living cells, 即, self-maintenance, 再生产, 和进化性 [25].

* Contact author.
** Electronics and Computer Science, University of Southampton, 南安普敦, SO17 1BJ, 英国. 电子邮件: gj07r@ecs.soton.ac.uk (G.J.);
hm@ecs.soton.ac.uk (H.M.); kpz@ecs.soton.ac.uk (K.-P.Z.)
† Computational Engineering and Design Group, Engineering Centre of Excellence, University of Southampton, Southampton S016 7QF,
英国. 电子邮件: p.h.king@soton.ac.uk
‡ Electronics and Computer Science & Institute for Life Sciences, University of Southampton, 南安普敦, SO17 1BJ, 英国.
电子邮件: mdp@ecs.soton.ac.uk

G. 琼斯等人.

Autonomous Droplet Architectures

While synthetic biology aims at building living organisms, the field of robotics has a long tradition of
building crude analogues of organisms. Here the goal is lifelike behavior, which is far less ambitious than
the aim of reproducing the characteristics of life; though some of the latter might be desirable, 他们是
in general not considered necessary in a robot. Given the universal success of membrane-enclosed fluid
drops as an architectural principle throughout the biological world, one can ask whether this concept
could be usefully employed for the more modest aims pursued in robotics. 因此, the focus of the
present article is on the extent to which current laboratory techniques allow for the implementation of
simple robots with drops of fluid and with aggregates of such drops.

For a drop to exist as a defined entity, a degree of immiscibility is required between at least two
阶段 (gas or liquid), so that the cohesion of molecules can give rise to surface tension. This in turn
brings about the shape of a droplet. 例如, we could have a drop of water in an air phase or,
to hinder evaporation, in an oil phase. The phase boundary constitutes an interface that is selective
with regard to the molecules that will pass across. In the water drop submerged in oil, ionic sub-
stances would preferentially stay within the droplet, while hydrophobic substances would leave the
droplet and dissolve in the oil phase. Some molecules possess spatially oriented hydrophobic and
ionic parts. These so-called aliphatic substances will enter the oil phase with one part and enter the
water drop with the other part. 最后, aliphatic molecules self-assemble at the phase bound-
ary and can coat the entire drop, resulting in a membrane.

A membrane around the drop has three important consequences. Firstly, it will alter the interface
between the drop and its environment. 第二, it can prevent adjacent drops from merging and
thus enable the construction of stable aggregates. And thirdly, the membrane can itself acts as a phase
boundary that allows, 例如, the compartmentalization of ionic drops in an ionic environment. 在
double emulsions or cells, more than two phases are present, and typically there is an intermediate
partially immiscible phase. In double emulsions there can be an aqueous-in-oil-in-aqueous configuration.
In cells there is the internal phase; the liquid lipid bilayer, which acts as a boundary; and an external outer
phase, which can be liquid or gaseous. Numerous topologies can be assembled according to this concept,
and hundreds of aliphatic substances (尤其, lipids and surfactants) are readily available. 这些
membrane-forming components are typically used in combinations to fine-tune the properties of the
interface. Even without considering the contents of the drop and the possibility of inserting more
specialized molecules (such as pores or selectively binding molecules) into the membrane, com-
binatorics of the aliphatic molecules spans a rich space of potential constructs. For simplicity we will
refer to any small quantity of fluid enclosed by a membrane as a droplet (见图 1).

Droplets and small networks of droplets can be created easily by hand with pipettes, Petri dishes,
and simple mixtures of oils and surfactants. Where large quantities of droplets are required, 微-
fluidics can be employed to generate several thousand droplets within minutes [2]. Hierarchical droplet
architectures where several droplets are encapsulated within a single larger droplet can also be fabri-
cated manually [39] and by microfluidic technology [30]. The relative ease of their production has led

数字 1. A droplet is a liquid-filled volume separated from, and interfaced with, its environment by one or more layers
of aliphatic molecules that form a membrane around the droplet. Often the outer phase is organic and the inner phase
is ionic, with a single layer of aliphatic molecules separating the phases. For biological cells both phases are ionic and
separated by a double layer of lipids.

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G. 琼斯等人.

Autonomous Droplet Architectures

数字 2. Neuron-type transduction in a droplet chain. Droplets filled with Belousov-Zhabotinsky (BZ) reaction medium
are immersed in oil. The BZ droplets are arranged in a 1-mm-wide trench and prevented from merging by synthetic
lipids, which have been added to the oil phase and have self-assembled at the droplet surface. The sequence of images
shows the propagation of an excitation from left to right through the droplet chain.

to droplets being utilized in many research activities, such as protein engineering, novel particle syn-
论文, and drug delivery systems [37]. In the last context, Štěpánek convincingly put forward the
idea that droplets equipped with sensory and actuation capabilities could be viewed as autonomous
chemical robots. His group considers remote-controlled and fully autonomous chemical robots and
their interactions [11].

More advanced—even lifelike—behaviors are possible with similar droplet systems. 例如,
Sumino and colleagues demonstrated that an oil droplet could interrogate a glass surface that had
been selectively treated with acid. The droplet was able to detect and avoid acid-treated areas [33].
Hanczyc showed that a randomly moving droplet could respond to the introduction of a pH
gradient, adjusting its movement towards an area of high pH [13]. Browne and coworkers were able
to demonstrate the self-division of oil droplets within an acidic aqueous solution [6]. 相同
group has since showed that a droplet can be used to solve a maze by following a pH gradient
set up between the entrance and exit of the maze [24]. The information processing of the autono-
mous droplets described above is performed at the interface by the rearrangement of surfactant
molecules. 尽管如此, these higher-level droplet systems indicate an ability to take environmental
signals, apply some information processing to the signals, and then perform some action based
on the result of the information processing. Consequently we refer to these systems as autonomous
droplets, following the terminology of [14].

The environment of such droplets can also be formed by other droplets, leading to multi-droplet
架构. An example of a multi-droplet structure in which droplets communicate by means of
chemical signals is shown in Figure 2. Lipid-coated droplets of the excitable Belousov-Zhabotinsky
(BZ) reaction medium under oil form lipid bilayer membranes at the interfaces between droplets. 在
the droplets at rest, the catalyst is dark red (reduced state). An excitation, visible by the light color
of the catalyst (oxidized state), travels across the interface from droplet to droplet, mediated by
chemical transmitters. After a droplet has been excited, a refractory period prevents immediate
further excitation and gives rise to directional travel of the excitation.

If we analogize individual autonomous droplets with single-cell organisms and aggregates of
multiple droplets with tissues, the question arises whether there is a droplet analogue to multicellular

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Autonomous Droplet Architectures

有机体. This would require an integrated form of the droplet aggregate that is interconnected
not only functionally but also physically. Several different techniques are available to achieve this.
Anchoring of single-stranded DNA to the lipid layer has been used to assemble droplets with droplet-
to-droplet specificity [12]. Here we focus on an even simpler technique to integrate heterogeneous
droplets into a single concerted system—droplets assembled as a multisome containing within it
a number of smaller functional droplets [39]—and consider its application to the BZ medium.

The BZ reaction is capable of forming complex patterns. Order can be imposed on the patterns,
temporal or spatial, to bring about functionality from the BZ medium, 例如, elementary
chemical logic gates [31] and logic circuits [1]. At the micro scale, the application of spatial order
has led to the discovery of lifelike behaviors reminiscent of quorum sensing [36]. Typically spatial
order is imposed on BZ at the micro scale through compartmentalization as droplets [3].

The compartmentalization of BZ mixtures as lipid-coated droplets surrounded by an oil phase
allows for the exploration of unconventional computing architectures such as artificial wet neuronal
网络. In the context of constructing functional networks of lipid-coated BZ droplets, 我们有
directed our efforts toward optimizing droplet stability and inter-drop communication. 然而, 这
chemical compositions of the reaction medium in a droplet, the lipid layer enclosing the droplet, 和
the oil phase surrounding the droplet can also be optimized with other objectives in mind. BZ
droplets exhibit a rich repertoire of behaviors that might be selected for such optimization [9].
数字 3 shows samples of the phenomena we have observed in our laboratory. In this report,
we indicate the viability of the application of BZ autonomous droplets through observations from
the literature and specific results from our own laboratory.

2 Design of a BZ Autonomous Droplet

The fundamental features of an autonomous droplet are the ability to sense environmental signals,
carry out some information processing on those signals, and subsequently perform an action. 在
our autonomous droplet design, we combine several BZ droplets to form a single overall larger
autonomous droplet (数字 4). Our current design sees the BZ droplets formed in an organic
phase as reverse micelles, with lipids providing compartmentalization. Other approaches to com-
partmentalizing the BZ droplets could be achieved through using surfactants, polysaccharides
(chitosan-alginate polymers), or inorganic membranes [3, 10, 7]. 现在, 然而, given the
potential of engineered protein pores to modulate the flow of information between droplets [15,
27], we have adopted a lipid-based approach.

数字 3. BZ droplets as a flexible model system. (A) A signal passes between droplets in a simple signal-processing
网络. (乙) Complex oscillatory behavior is observed upon fusion of droplets with different BZ mixtures. (C) 二
types of droplets, arranged on a microfabricated regular grid; the droplets communicate their chemical state to
neighbors through exchange of chemical signals. (d) Spontaneous pattern formation induced by modifying the oil phase.
The droplets in the images vary in diameter from 2 到 2.5 毫米.

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G. 琼斯等人.

Autonomous Droplet Architectures

数字 4. (A ) The concept of an autonomous droplet containing an internal network of functional units. (乙) 使用
a Braitenberg vehicle connection scheme (see text ), behaviors such as stimulus seeking and avoidance could be
programmed into the droplet.

As a starting point, our droplet will be constructed with a simple information-processing scheme
based on Braitenbergʼs vehicles [5], which requires only a pair of light sensors connected to a pair of
actuators. Following Figure 4b, if each light sensor is connected to an adjacent actuator and the
effect of increased illumination is to inhibit actuation, then in this configuration the droplet will
be capable of light-seeking behavior. 相似地, if each light sensor is connected to the opposite
actuator, increased illumination will cause the droplet to turn away from the light. To implement
such a crossing-over of the signal path, the 3D structure of the droplets on a surface is not sufficient.
此外, control over the signal flow, for example through membrane proteins [4, 17], is required.
More complex behavior arises when there are mixed connections between sensors and actuators.
A BZ medium prepared with a ruthenium catalyst becomes photosensitive and leads to inhibition
of the reaction when irradiated. At the macro scale this has been exploited to produce a photo-
chemical memory device [22], while at the micro scale it has allowed for programmed synchroniza-
tion of an array of oscillating BZ droplets [8]. In a similar manner, droplets containing ruthenium
catalyst may provide the light-sensing capability of our autonomous droplet.

Networks of BZ droplets contained within an immiscible medium offer much potential as an
information-processing scheme. Already studies have been conducted on the effects of altering
parameters such as pH, droplet size, and application of temperature gradients on oscillations as well
as overall pattern formation [38, 32]. Particularly interesting is the coupling between BZ droplets.
During the course of the BZ reaction a number of inhibitory and excitatory intermediates are
produced with different hydrophobic/hydrophilic properties, which could be modulated to created
artificial neuronal networks. There are oil-soluble intermediates (inhibitory bromine ions and ex-
citatory bromine dioxide radicals) as well as water-soluble intermediates (excitatory bromous acid
and inhibitory bromide ions). The coupling between droplets can therefore be optimized for
information processing through careful spatial arrangement of the droplets.

Most reported chemically derived motions from the BZ reaction have utilized polymer gels. 在
such systems the catalyst is incorporated within a polymer gel structure [28, 40, 41]. The gel-catalyst
construct is then immersed within catalyst-free BZ media that proceed to react, causing changes in
the oxidation state of the catalyst. The changes in oxidation state lead to abrupt alterations of the gel
体积, giving rise to cyclic swelling and deswelling. This behavior has been harnessed to produce
peristaltic-action object transporters, artificial cilia, and self-walking gels [29, 35, 26]. Although not
used in our autonomous droplet design (数字 5), it is certainly conceivable that BZ gels could be
incorporated as part of the design in the future.

Reports of motile BZ droplets are far more rare. Spontaneous motion driven by chemical oscil-
lation has been observed for a 1-mm-diameter BZ droplet, either floating or immersed in oleic acid

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G. 琼斯等人.

Autonomous Droplet Architectures

[18–20]. 相似地, by using a light-sensitive catalyst, light-directed motion was shown possible with
a BZ droplet floating in oleic acid [21]. The motion of these BZ droplets was far less prominent
than that exhibited by previous examples of motile droplet systems described in the introduction.
Typically the displacement achieved by these droplets in a singular movement was less than 200 是,
with the droplet remaining in close proximity to its origin throughout the course of the reaction.
The limited motion displayed by these systems makes their selection for use in creating autonomous
droplets, for now, unrealistic. 然而, as we shall see next, larger displacements can be achieved in
BZ droplets.

3 Belousov-Zhabotinsky Droplet Actuators

The shape of a drop of liquid resting on a solid surface is determined by the surface tension between
droplet, surface, and surrounding phase. The surface tension arises from the balance of cohesive
forces between the molecules of the liquid on the one side and the adhesive forces between liquid
and solid on the other. The surface tension between a solid and an aqueous liquid can be determined
from measurements of the angle between the solid-aqueous interface and, in the present context, 这
aqueous-oil interface. The so-called contact angle, when measured to exceed 90° (as in the case of
more spherical droplets), indicates greater cohesion of molecules within the droplet and decreased
wetting of the surface. We assume that the changes in droplet shape are driven predominantly by
changes in surface tension between the BZ droplet and the surface upon which it rests. To test our
assumption, profiles of a BZ droplet in contact with a surface can be measured optically during the
course of a reaction and the contact angles determined.

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数字 5. Laboratory setup for the autonomous droplet concept. (A) An early prototype autonomous droplet is created
by hand under a standard light microsope. (乙) Seven BZ droplets are encapsulated within an organic oil droplet
containing natural lipids. (C ) The organic oil droplet is submerged within an immiscible fluorinated oil. In this early
prototype some of the droplets exhibited limited oscillation.

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G. 琼斯等人.

Autonomous Droplet Architectures

数字 6. An oscillating BZ droplet on a surface. (A) A false color image created by combining images of the BZ droplet in
reduced and oxidized states. (乙) Contact angle measurements recorded from the oscillating BZ droplet. A 4-Al BZ
droplet was pipetted on to a flat UV glue sheet (NOA81 + 1% APTES) that had been immersed in a lipid-oil solution
(40 mg/ml lecithin in decane). Contact angle measurements were recorded every second using a DSA 30 drop shape
analysis system (Kruss, 汉堡) connected to a computer for automated data capture.

Shape changes induced by the BZ reaction have been investigated previously with the malonic
acid version of the reaction. Here we make use of cyclohexanedione (CHD) [23] as substrate instead
of malonic acid. We have measured oscillation-coupled shape changes of CHD BZ droplets in con-
tact with a surface (数字 6). 而且, 如图 7, we have found that directionally
specific shape changes of CHD BZ droplets can be induced when using a patterned substrate. 笔记
that by combining the CHD medium with a bathoferroin catalyst [34] even larger displacements can
be obtained from slugs of similar volume on the same substrate (Figure 7e–h).

Utilizing 3D-printing technology and fabrication methods described in [16], we are creating de-
vices to explore the combined effects of geometry, surface treatments, and oil formulations to sup-
port mobile BZ droplets. Our preliminary results illustrate that BZ droplets provide an experimental
model system that captures some of the functional flexibility prerequisite for evolvability. The results
also point towards the feasibility of chemically powered autonomous droplets.

数字 7. Oscillation-dependent shape changes in Belousov-Zhabotinsky slugs contained within an oil-filled trench device.
Both BZ slugs experience shape changes following the direction of the trench as they oscillate from the reduced to
the oxidized state. (A )–(d): The BZ slug containing a medium developed for information transmission shows some
shape change. (e)–(H): A much more pronounced actuation is possible if the medium is optimized with actuation as
an objective. The flexibility of the BZ droplets to adapt to different objectives hints at the possibility of using evolutionary
methods for functional specialization of droplets.

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4 结论

Autonomous Droplet Architectures

By the middle of the 19th century it became apparent that the smallest unit showing the properties
of life is the cell and that all organisms are composed of cells. Arguably the aspect of the cell that
can be reconstituted from synthetic components most easily is the lipid bilayer of its membrane;
given suitable conditions, it self-assembles. Even simpler to form are water droplets in oil coated
with a single layer of lipids or surfactants. Techniques to compartmentalize mixtures of water-soluble
substances by means of self-assembled membranes are now well developed, with manual (pipetting)
和自动化 (microfluidics) methods available for a variety of topologies and phase combinations.
The self-assembly of the interfaces in these architectures not only enables convenient mass production
at the nanometer scale, but also offers self-repair as long as a surplus of membrane constituents is
可用的. A droplet becomes functional on filling it with an active chemical medium, and the interface
(IE。, the membrane composition) of the droplet can be adapted to its function.

We have shown here how the concept of a single autonomous droplet, which may be analogized
to a single cell organism, could be extended to aggregates of functionally diverse droplets. 这
specialized droplets can be considered as modules from which droplet architectures can be com-
posed as long as the compatibility of the interfaces is maintained, as in the example of the Belousov-
Zhabotinsky droplet system considered here. Starting from a droplet design that was engineered for
signal transduction but exhibited some shape change on excitation, we have investigated how this
chemically driven actuation can be enhanced to arrive at droplets that convert chemical energy into
mechanical motion.

A fully functional Braitenberg droplet architecture is still a few steps away. 尽管如此, 这
progress so far indicates that droplets are versatile building blocks even if the synthetic droplets used
are far simpler than what is found in nature. Droplets are easy to produce, to manipulate, 和
to observe at the macro scale. 但, as nature amply demonstrates, they can be scaled down to
micrometer size and have a virtually unlimited upgrade path in functionality. We therefore view
autonomous droplets as amenable to evolutionary procedures and as a promising new direction
in robotics.

致谢
The research reported here is supported in part by Future and Emergent Technologies Grant
FP7-248992 “NEUNEU” from the European Union.

参考
1. Adamatzky, A。, Holley, J。, Dittrich, P。, Gorecki, J。, 科斯特洛, 乙. D. L。, Zauner, K.-P., & Bull, L. (2012).
On architectures of circuits implemented in simulated Belousov-Zhabotinsky droplets. 生物系统公司, 109,
72–77.

2. Agresti, J. J。, Antipov, E., Abate, A. R。, Ahn, K., Rowat, A. C。, Baret, J.-C., Marquez, M。, Klibanov, A. M。,
Griffiths, A. D ., & Weitz, D. A. (2010). Ultrahigh-throughput screening in drop-based microfluidics
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