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1. Introduction
This emerging technology combines modular robotics, systems nanotechnology and
computer science to create the dynamic, 3-Dimensional display of electronic information known
as Claytronics.
1.1 Nanotechnology
Nanotechnology is not a novel technology. It dates back to 1959, when it was first used.
Nanotechnology, shortened to "nanotech", is the study of the controlling of matter on an atomic
and molecular scale. Generally nanotechnology deals with structures sized between 1 to 100
nanometer in at least one dimension, and involve developing materials or devices within that
size.
Figure 1.1 Nano Tubes
Nanotechnology is very diverse, ranging from extensions of conventional device physics
to completely new approaches based upon molecular self-assembly, from developing new
materials with dimensions on the nanoscale to investigating whether we can directly control
matter on the atomic scale.
1.2 Computer Science:
Computer science or computing science (sometimes abbreviated CS) is the study of the
theoretical foundations of information and computation, and of practical techniques for their
implementation and application in computer systems. It is frequently described as the systematic
study of algorithmic processes that create, describe, and transform information. Computer
science has many sub-fields; some, such as computer graphics, emphasize the computation of
specific results, while others, such as computational complexity theory, study the properties of
computational problems. Still others focus on the challenges in implementing computations. For
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example, programming language theory studies approaches to describe computations, while
computer programming applies specific programming languages to solve specific computational
problems, and human-computer interaction focuses on the challenges in making computers and
computations useful, usable, and universally accessible to people.
The focus of computer science is more on understanding the properties of the programs
used to implement software such as games and web-browsers, and using that understanding to
create new programs or improve existing ones.
1.3 Modular Robots:
Modular robots are autonomous kinematic machines with variable morphology. Beyond
conventional actuation, sensing and control typically found in fixed-morphology robots, selfreconfiguring robots are also able to deliberately change their own shape by rearranging the
connectivity of their parts, in order to adapt to new circumstances, perform new tasks, or recover
from damage.
Figure 1.2 Modular robot
For example, a robot made of such components could assume a worm-like shape to move
through a narrow pipe, reassemble into something with spider-like legs to cross uneven terrain,
then form a third arbitrary object (like a ball or wheel that can spin itself) to move quickly over a
fairly flat terrain; it can also be used for making "fixed" objects, such as walls, shelters, or
buildings. In some cases this involves each module having 2 or more connectors for connecting
several together. They can contain electronics, sensors, computer processors, memory, and
power supplies; they can also contain actuators that are used for manipulating their location in
the environment and in relation with each other. A feature found in some cases is the ability of
the modules to automatically connect and disconnect themselves to and from each other, and to
form into many objects or perform many tasks moving or manipulating the environment.
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2. Universal Intelligent Matter
Intelligent matter is any material in which individual molecules or supra-molecular
clusters function as agents to accomplish some purpose. Intelligent matter may be solid, liquid,
or gaseous, although liquids and membranes are perhaps most typical. Universally
programmable intelligent matter (UPIM) is made from a small set of molecular building blocks
that are universal in the sense that they can be rearranged to accomplish any purpose that can be
described by a computer program. In effect a computer program controls the behavior of the
material at the molecular level. In some applications the molecules self-assemble a desired
nanostructure by “computing” the structure and then becoming in-active. In other applications
the material remains active so that it can respond, at the molecular level, to its environment or to
other external conditions. An extreme case is when programmable supra-molecular clusters act
as autonomous agents to achieve some end.
Although materials may be engineered for specific purposes, we will get much greater
technological leverage by designing a “universal material” which, like a general- purpose
computer, can be “programmed for a wide range of applications. To accomplish this, we must
identify a set of molecular primitives that can he combined for widely varying purposes. The
existence of such molecular operations might seem highly unlikely, but there is suggestive
evidence that it may be possible to discover or synthesize them.
2.1 Applications
The possible applications of UPIM are, static or dynamic (or interactive). By a static
application we mean one in which the intelligent matter computes into an equilibrium state, and
is inactive thereafter. Therefore static applications are most often directed toward generating
some specialized material with a computationally defined nanostructure. On the other hand,
dynamic or interactive applications never terminate, but always remain ready to respond to their
environment in some specified way; they are the truly “smart” materials.
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2.1.1 Static Applications
Programs are ideally suited to creating complex data structures which can be converted to
complex physical structures by means of UPIM. Networks, chains, tubes, spheres, fibers, and
quasi crystalline structures are all straightforward to compute. The network resulting from such a
computation will be composed of computational groups (e.g., S, K,A) as well as inert groups,
which are manipulated by the computation but do not affect it. Typically, in these applications
the computational phase will be followed by a chemical phase in which the computational groups
are replaced by substances appropriate to the application (a sort of “petrification”). In addition to
the examples already mentioned, such an approach could be used to synthesize membranes with
pores or channels of a specified size and arrangement (determined either deterministically by the
program or stochastically by molecular processes).
A number of applications are suggested by the requirements of implementing small,
autonomous robots. Some of these will be controlled by very dense analog neural net- works, but
to achieve densities comparable to mammalian cortex (15 million neurons per square cm., with
up to several hundreds of thousands of connections each), we will need to be able to grow
intricately branching dendritic trees at the nanoscale. Generation of such structures is
straightforward with UPIM (e.g., using L-systems). The sensor and effector organs of micro
robots will also require very fine structures, which UPIM can be programmed to generate.
Of course, we should not neglect the potential of UPIM to do conventional computation,
such as solving NP-complete problems by massively parallel computation. For example, we
might replicate many copies of a program to test a potential solution, then mix them in a reaction
vessel with structures representing possible solutions, and wait for equilibrium to determine
actual solutions. The advantage of our approach to this kind of search problem over others, such
as DNA computation, is that our nanoscale test molecules are programmable.
2.1.2 Dynamic Applications
Dynamic intelligent matter is interactive in the sense that it is continually monitoring its
environment and capable of responding according to its program. That is, it is in a state of
temporary equilibrium, which can be disrupted by changes in the environment, resulting in
further computation and behavior as the material seeks a new equilibrium.
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For example, a membrane with channels, such as mentioned above, could be made active
by having the channels open or close in response to environmental conditions, including control
commands transmitted optically or chemically. The program located in each channel is simple: in
response to its sensor state it executes one or the other of two effectors, blocking the channel or
not. The sensor and the medium would determine whether the channel is sensitive to global
conditions (e.g.. overall chemical environment or ambient illumination) or to its local
environment (e.g., molecules or light in its immediate vicinity).
Similarly, unanchored or free-floating molecular clusters (e.g., in colloidal suspension)
may react to their environment and change their configuration, thus affecting physical properties
of the substance, such as viscosity or transparency. Or they might polymerize or depolymerize on
command.
Unanchored supramolecular networks might also operate as semiautonomous agents to
recognize molecules or molecular configurations, and act upon them in some intended way (e.g.
binding toxins or pollutants). However, such applications will require the agents to operate in a
medium that can supply the reactants needed for computation.
These sorts of active intelligent matter will find many applications in autonomous micro
robots. For example, active membranes can serve as sensory transducers, responding to
conditions in the environment and generating electrical, chemical, or other signals. They can also
be programmed to self-organize into structures capable of preprocessing the input (e.g., artificial
retinas or cochleas). Further, it is a simple modification of a membrane with channels to make a
membrane with cilia that flex on command. By means of local communication, the cilia may be
made to flex in coordinated patterns. Similarly we may fabricate artificial muscles, which
contract or relax by the coordinated action of microscopic fibers. UPIM may also provide a
systematic approach to self-repair of autonomous robots and other systems, since if a robot’s
“tissues” were created by computational processes, then they can remain potentially active, ready
to restore an equilibrium disrupted by damage. Less ambitiously, materials can be programmed
to signal damage or other abnormal conditions.
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3. Programmable Matter
Programmable matter is the dynamic application of Universal Programmable Matter. In
the past 50 years, computers have shrunk from room-size mainframes to lightweight handhelds.
This fantastic miniaturization is primarily the result of high-volume nanoscale manufacturing.
While this technology has predominantly been applied to logic and memory, it’s now being used
to create advanced micro electromechanical systems using both top-down and bottom-up
processes.
One possible outcome of continued progress in high-volume nanoscale assembly is the
ability to inexpensively produce millimeter-scale units that integrate computing, sensing,
actuation, and locomotion mechanisms. A collection of such units can be viewed as a form of
programmable matter.
Programmable matter refers to a technology that will allow one to control and
manipulate three-dimensional physical artifacts (similar to how we already control and
manipulate two-dimensional images with computer graphics). In other words, programmable
matter will allow us to take a (big) step beyond virtual reality, to synthetic reality, an
environment in which all the objects in a user’s environment (including the ones inserted by the
computer) are physically realized. Note that the idea is not to transport objects nor is it to recreate
an objects chemical composition, but rather to create a physical artifact that will mimic the
shape, movement, visual appearance.
3.1 Synthetic reality
One application of an ensemble, comprised of millions of cooperating robot modules, is
programming it to self-assemble into arbitrary 3D shapes. Our long-term goal is to use such
ensembles to achieve synthetic reality, an environment that, unlike virtual reality and augmented
reality, allows for the physical realization of all computer-generated objects. Hence, users will be
able to experience synthetic reality without any sensory augmentation, such as head-mounted
displays. They can also physically interact with any object in the system
in a natural way.
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4. Claytronics
The enabling hardware technology behind synthetic reality is Claytronics, a form of
programmable matter that can organize itself into the shape of an object and render its outer
surface to match the visual appearance of that object. Claytronics is made up of individual
components, called catoms—for Claytronic atoms—that can move in three dimensions (in
relation to other catoms), adhere to other catoms to maintain a 3D shape, and compute state
information (with possible assistance from other catoms in the ensemble). In our preliminary
designs, each catom is a self-contained unit with a CPU, an energy store, a network device, a
video output device, one or more sensors, a means of locomotion, and a mechanism for adhering
to other catoms.
Creating a physical replica of an arbitrary moving 3D object that can be updated in real
time involves many challenges. The research involved in addressing these scientific challenges is
likely to have broad impact beyond synthetic reality. Particularly relevant to the ASPLOS
community, for example, is how to build and program a robust distributed system containing
millions of computers that must cooperate extensively in a harsh environment where their
configuration and goals are constantly changing? It is already possible to reproduce static 3D
objects. To create a dynamic 3D object, however, we will build upon ideas from such diverse
areas as modular and reconfigurable robotics and amorphous computing.
A Claytronics system forms a shape through the interaction of the individual catoms. For
example, suppose we wish to synthesize a physical “copy” of a person. The catoms would first
determine their relative location and orientation. Using that information they would then form a
network in a distributed fashion and organize themselves into a hierarchical structure, both to
improve locality and to facilitate the planning and coordination tasks. The goal(mimicking a
human form) would then be specified abstractly, perhaps as a series of “snapshots” or as a
collection of virtual deforming “forces”, and then broadcast to the catoms. Compilation of the
specification would then provide each catom with a local plan for achieving the desired global
shape. At this point, the catoms would start to move around each other using forces generated
on-board, either magnetically or electro statically, and adhere to each other using, for example, a
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nano fiber-adhesive mechanism. Finally, the catoms on the surface would display an image;
rendering the color and texture characteristics of the source object. If the source object begins to
move, a concise description of the movements would be broadcast allowing the catoms to update
their positions by moving around each other. The end result is that the system appears to be a
single coordinated system.
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5. Catoms
The driving force behind programmable matter is the actual hardware that is
manipulating itself into whatever form is desired. Claytronics consists of a collection of
individual components called claytronic atoms, or catoms. In order to be viable, catoms need to
fit a set of criteria. First, catoms need to be able to move in three dimensions relative to each
other and be able to adhere to each other to form a three dimensional shape. Second, the catoms
need to be able to communicate with each other in an ensemble and be able to compute state
information, possibly with assistance from each other. Fundamentally, catoms consist of a CPU,
a network device for communication, a single pixel display, several sensors, an onboard battery,
and a means to adhere to one another.
Programmable matter consists of a collection of individual components, which we call
claytronic atoms or catoms. Catoms can:
•move in three dimensions in relation to other catoms
•adhere to other catoms to maintain microrobot; we believe that implementing a completely
autonomous microrobot is unnecessarily complex. Instead, we take a cue from cellular
reconfigurable robotics research to simplify the individual robot modules so that they are easier
to manufacture using high-volume methods.
At the macro-scale, catoms have a diameter > 1cm and weigh many tens of grams. In
light of the design principles stated above, the only viable force that can be used to move and
adhere catoms is magnetic; which sets a lower limit on the size and weight of a catom as the
magnets have considerable weight and volume. Furthermore, the circuitry needed for the high
currents necessary to switch the magnets increases the weight. At this scale, it may not be
possible to adhere to the “no static power” design principle. Our current prototype, as shown in
Figure 5.1, is a system composed of catoms that only operate in two dimensions. In this case
gravity holds the individual catoms to a surface and we do not have to deal with the adhesion
problem.
Much of the weight and size in a macro-scale catom comes from packaging, e.g., chip
packages, pins, wires, PCBs, etc.. In fact, in our planer prototype currently under construction we
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estimate that more than 20% of the weight is packaging and more than 77% goes to the magnets
and their support circuits. This is partly because we are hand-assembling the catoms. Our next
version will use machine assembly which will allow us to shrink the packaging, resulting in an
estimated savings of twofold in weight and fivefold in volume.
Figure 5.1: The side and top views of a partially assembled planar prototype Catom.
Micro-scale catoms have diameters between 1mm and 1cm and weigh less than 1 gram.
Almost all packaging is eliminated and the catom is constructed by bonding VLSI dies directly
to MEMS-based sensor and actuation dies. The forces necessary to move a catom are now
sufficiently small that electrostatic forces become an option. Macro-scale catoms would require
electric field strengths in excess of the dielectric breakdown of air. Programmable nanofiber
adhesives (PNA), can be combined with electrostatic actuation to attach catoms without using
any static power. PNA is the next step in bio-mimetic adhesives which mainly use vander Waals
effects similar to that of the Gecko. The active components for micro-scale catoms are within the
realm of current engineering practices using micro- and nanoscale fabrication techniques. The
container could be manufactured by casting a spherical transparent polymer film and sputtering
indium-tin-oxide (ITO) electrode patches on the film. The patches of ITO would be used both to
connect catoms for distributing power as well as used to create capacitive coupling between
catoms in order to control their movement.
The next discontinuity occurs when catoms are manufactured using nanotechnology, e.g.,
as in chemically assembled electronic nanotechnology. In this regime, the catoms are small
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enough, e.g., < 10 microns in diameter, that they are aerosol. While this is currently beyond the
state-of-the-art in manufacturing, further scaling of lithographic features in VLSI, and advances
in MEMS capabilities combined with advances in nanotechnology will enable the integrated
construction of such catoms.
In addition to capabilities, the different regimes (macro, micro, and nano) have
significantly different economics.
Macro-scale catoms require the assembly of multiple parts into a single unit. We expect
that this will make the realization of life-size synthetic reality prohibitive due to the cost per
catom. The micro-scale catoms may also require assembly, but with many fewer parts, e.g., no
boards and the elimination of magnets in favor of electrostatics. At this scale the entire catom
will be constructed using a parallel process, e.g., photolithography. Just as VLSI-based
computers are commonplace (as opposed to vacuum tube based computers), in this regime,
catoms are inexpensive enough that synthetic reality, though expensive, becomes viable. For
example, assume a 2mm catom controlled by 3mm 2 of silicon. If half the cost is in the silicon,
then, to construct a human sized form with an average depth of 10 catoms would require
approximately 1.25x10 7 catoms or less than one million dollars. In the nano regime they will
become sufficiently inexpensive that synthetic reality could be used not only for applications
such as telepario, but could be as ubiquitous as embedded processors are today.
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6. Ensemble Principle
Realizing this vision requires new ways of thinking about massive numbers of cooperating
millimeter-scale units. Most importantly, it demands simplifying and redesigning the software
and hardware used in each catom to reduce complexity and manufacturing cost and increase
robustness and reliability. For example, each catom must work cooperatively with others in the
ensemble to move, communicate, and obtain power. Consequently, our designs strictly adhere to
the ensemble principle: A robot module should include only enough functionality to contribute to
the ensemble’s desired functionality. Three early results of our research each highlight a key
aspect of the ensemble principle:

easy manufacturability

powering million-robot ensembles

surface contour control without global motion planning
6.1 High Volume manufacturability
Some catom designs will be easier to produce in mass quantity than others. Our present
exploration into the design space investigates modules without moving parts, which we see as an
intermediate stage to designing catoms suitable for high-volume manufacturing.
Figure 6.1 Claytronic atom prototypes. Each 44-mm-diameter catom is equipped with 24
Electromagnets arranged in a pair of stacked rings.
In our present macroscale (44-mm diameter), cylindrical prototypes, shown in Figure 6.1,
each catom is equipped with 24 electromagnets arranged in a pair of stacked rings. To move, a
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pair of catoms must first be in contact with another pair. Then, they must appropriately energize
the next set of magnets along each of their circumferences.
These prototypes demonstrate high speed reconfiguration—approximately 100 ms for a
step-reconfiguration involving uncoupling of two units, movement from one pair of contact
points to another, and recoupling at the next pair of contact points. At present, they don’t yet
incorporate any significant sensing or autonomy.
The current prototypes can only overcome the frictional forces opposing their own
horizontal movement, but downscaling will improve the force budget substantially. The resulting
force from two similarly energized magnet coils varies roughly with the inverse cube of distance,
whereas the flux due to a given coil varies with the square of the scale factor. Hence, the
potential force generated between two catoms varies linearly with scale. Meanwhile, mass varies
with the cube of scale.
6.2 Powering Microrobot Ensembles
Some energy requirements, such as effort to move versus gravity, scale with size. Others,
such as communication and computation, don’t. As micro robots (catoms) are scaled down, the
onboard battery’s weight and volume exceed those of the robots themselves.
To provide sufficient energy to each catom without incurring such a weight and volume
penalty, we’re developing methods for routing energy from an external source to all catoms in an
ensemble. For example, an ensemble could tap an environmental power source, such as a special
table with positive and negative electrodes, and route that power internally using catom-to catom
connections.
To simplify manufacturing and accelerate movement, we believe it’s necessary to avoid
using inter catom connectors that can carry both supply and ground via separate conductors
within the connector assembly. Such complex connectors can significantly increase
reconfiguration time.
Several members of the Claytronic team have recently developed power distribution
algorithms that satisfy these criteria. These algorithms require no knowledge of the ensemble
configuration lattice spacing, ensemble size, or shape or power-supply location. Further, they
require no on-catom power storage.
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6.3 Shape Control Without Global Motion Planning
Classical approaches to creating an arbitrary shape from a group of modular robots
involve motion planning through high-dimensional search or gradient descent methods.
However, in the case of a million-robot ensemble, global search is unlikely to be tractable. Even
if a method could globally plan for the entire ensemble, the communications overhead required
to transmit individualized directions to each module would be very high. In addition, a global
plan would break down in the face of individual unit failure.
To address these concerns, we’re developing algorithms that can control shape without
requiring extensive planning or communication. While this work is just beginning, Claytronics
researchers have had early success using an approach inspired by semiconductor device physics.
This approach focuses on the motion of holes rather than that of robots per se. Given a uniform
hexagonal- packed plane of catoms, a hole is a circular void due to the absence of seven catoms.
Such a seven-catom hole can migrate through the ensemble by appropriate local motion of the
adjacent catoms.
Holes migrate through the ensemble as if moving on a frictionless plane, and bounce
back at the ensemble’s edges. Just as bouncing gas molecules exert pressure at the edges of a
balloon, bouncing holes interact frequently with each edge of the ensemble without the need for
global control. As Figure 6.2 illustrates, edges can contract by consuming a hole or expand by
creating a hole, purely under local control.
Figure 6.2 Hole motion. Edges can (a) contract by consuming a hole or (b) expand by
creating a hole, purely under local control.
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We initiate shape formation by “filling” the ensemble with holes. Each hole receives an
independent, random velocity and begins to move around. A shape goal specifies the amount
each edge region must either contract or expand to match a desired target shape. A hole that hits
a contracting edge is consumed. In effect, the empty space that constitutes the hole moves to the
outside of the ensemble, pulling in the surface at that location. Similarly, expanding edges create
holes and inject them into the ensemble, pushing its contour out in the corresponding local
region.
Importantly, all edge contouring and hole motion can be accomplished using local rules,
and the overall shape of an ensemble can be programmed purely by communicating with the
catoms at the edges. Hence, we use probabilistic methods to achieve a deterministic result. Our
initial analyses of the corresponding 3D case suggest surface contour control will be possible via
a similar algorithm.
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7. Software
The essence of claytronics a massively distributed system composed of numerous
resource-limited catoms raises significant software issues: specifying functionality, managing
concurrency, handling failure robustly, dealing with uncertain information, and controlling
resource usage. The software used to control claytronics must also scale to millions of catoms.
Thus, current software engineering practices, even as applied to distributed systems, may not be
suitable. We are just beginning to explore the software design principles needed.
We have broken down the software issues into three main categories: specification,
compilation, and runtime support. Our goal is to specify the global behavior of the system in a
direct and descriptive manner. The simplest model we are investigating with respect to
specification is what we call the Wood Sculpting model. In this model, a static goal shape is
specified. We are investigating two alternative compilation methodologies, both of which fit into
the general category of single-program-multiple data (SPMD) programming models. In the first,
we are compiling the specification into a planning problem. In this approach we are inspired by
work done in communicating soccer robots and in the context of reconfigurable robots, by the
constraint-based control framework in which a high-level description such as a particular gait
which is translated to a distributed, constraint-based controller. Our second approach is based on
emergent behavior. Prior work by seems particularly appropriate in this latter approach with
respect to claytronics. At the highest level of abstraction a shape is specified in terms of Origami
folding directives. Through a process of planning, these folding directives are translated into
low-level programs for autonomous agents; achieving the shape by local communication and
deformation only.
Underlying the user-level software is a distributed runtime system. This system needs to
shield the user from the details of using and managing the massive number of catoms. Our initial
steps in this direction use emergent behavior to determine a catoms location and orientation with
respect to all catoms as well as to build a hierarchical network for communication between
catoms. Efficient localization is achieved by having the catoms determine their relative location
and orientation in a distributed fashion. Then as regions of localized catoms join up they unify
their coordinate systems. Our algorithm takes O(1) time if the network is capable of broadcast.
With a network limited to point-to-point connections the algorithm takes O(√3 n) time in 3D.
Once catoms are localized we form a hierarchical communication network, again using simple
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local programs on each catom. A tree is formed in parallel by having nodes join with their
neighbors until all the nodes are in a single tree. This simple algorithm produces a surprisingly
efficient tree from which can then be further optimized.
7.1 Meld: A Declarative Approach to Programming Ensembles
Meld is a programming language for modular robots, i.e., for independently executing
robots where inter-robot communication is limited to immediate neighbors. Meld is a declarative
language, based on P2, a logic programming language originally designed for programming
overlay networks. By using logic programming, the code for an ensemble of robots can be
written from a global perspective, as opposed to a large collection of independent robot views.
This greatly simplifies the thought process needed for programming large ensembles. Initial
experience shows that this also leads to a considerable reduction in code size and complexity.
Modular robotics is based on the concept of a (typically homogenous) collection of
robots working together to carry out a task. Most of the designs proposed to-date involve the
robots attaching together to form larger robotic structures. These connected ensembles gain
capabilities beyond those of the individual robots, including: greater strength from concerted,
parallel efforts; flexible, reconfigurable form to better deal with the environment and obstacles;
and greater capacity to redundantly sense the surroundings. However, planning and control for
the ensemble increases in complexity due to the large number of robots and exponentially large
configuration space. Compounding the problem is the fact that computation, sensing, and
actuation are distributed among the relatively limited robots—in general, no one robot is strong
enough to move all of the other robots, nor is it possible, typically, for any one robot to
determine the positions of every other robot or where they should be. These limitations in
strength and knowledge multiply the number of possible failure modes. For example, a robot
might move to the wrong location or physically impede the desired movement of other robots.
The ensemble as a whole might move into an unstable structure and risk collapse. To succeed,
the individual robots in a modular robotics system must work cooperatively.
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8. Scaling and Design Principles
A fundamental requirement of Claytronics is that the system must scale to very large
numbers of interacting catoms. In addition to previously stated principles for the design of
modular robots we have the following four design principles:
Macro
Micro
Nano
Dimensions
> 1 cm
> 1 mm
< 10 microns
Weight
10’s of grams
100’s of mg
< 1 mg
Power
< 2 Watts
10’s of mW
10’s of nW
Locomotive
Mechanism
Programmable
Magnets
Electrostatics
Aerosol
Adhesive Mechanism
Nanofiber Adhesives
Programmable
nanofiber adhesives
Molecular surface
adhesion
and covalent bonds
Manufacturing
methods
Conventional
manufacturing
and assembly
Micro/Nanofabrication
and micro-assembly
Chemically directed
self-assembly and
fabrication
Resolution
Cost
Low
$$$/catom
High
$/catom
High
Millicents/catom
Table 8.1: A summary of the characteristics of the different catom design regimes.
1. Each catom should be self-contained, in the sense of possessing everything necessary for
performing its own computation, communication, sensing, actuation, locomotion, and adhesion.
2. To support efficient routing of power and avoid excessive heat dissipation, no static power
should be required for adhesion after attachment.
3. The coordination of the catoms should be performed via local control. In particular, no
computation external to the ensemble should be necessary for individual catom execution.
4. For economic viability, manufacturability, and reliability, catoms should contain no moving
parts.
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Catoms share many of the same features as individual robots in modular (self
)reconfigurable robots .In both claytronics and MRR the behavior of interest is group behavior.
The design goals of the individual robots and catoms are also similar in that the individuals are
self-contained. 1 The differences are however many. MRR are designed to reconfigure their
shapes in order to achieve functionality such as physical movement. While scaling to many
robots is a stated goal of MRR, it is not primary since typical goals (e.g., such as moving across a
terrain) require relatively few cooperating MRRs. In contrast, the original motivating goal of
claytronics is to create high-fidelity macro-scale 3D objects from micro-scale components, as
reflected in our design principles.
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9. Process in Claytronics
As the size of the modules in a self-reconfiguring modular robotic system shrinks and the
number of modules increases, the flexibility of the system as a whole increases. In this paper, we
describe the manufacturing methods and mechanisms for a 1 millimeter diameter module which
can be mass produced. The module, which we call a catom, is the basic unit for claytronics, a
modular robotic system designed to scale to millions of units.
Figure 9.1 The Catom: Tube + Chip.
Figure 9.2 Fabricated Catom.
Successfully realizing claytronics rests firmly on the ability to mass-produce submillimeter scale catoms. Each catom must have a small robust physical structure integrated with
the ability to transfer energy to neighboring units, store energy, move, communicate with its
neighbors and selectively adhere to neighboring units; all under the local control. To invest each
unit with all of this functionality requires a novel approach towards the design and manufacture
of the catom.
Our design philosophy seeks to simplify the catom design by following the ensemble
principle, which states that: A unit should include only enough functionality to contribute to the
desired functionality of the ensemble. Adherence to the ensemble principle forces us to use local
computation and nearest-neighbor communication to simplify the physical mechanisms in each
catom as much as possible. In this instance, we use one physical mechanism (conductive plates
on, or near, the surface of the unit) for locomotion, adhesion, communication, and power
transfer. The local control of the catom has the ability to move charge between the plates. This
charge generates electrostatic forces between the robots to accomplish all the aforementioned
functions.
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9.1 Introduction Path
Related Path
The design and implementation of a sub-millimeter autonomous robot, even one which
only needs to function in an ensemble, is challenging and therefore will be done over several
design generations. The first generation, detailed here, aims to develop the fabrication process
and implement the core functionality of the catoms: manufacturing, energy transfer, energy
storage, and actuation. Once this is achieved, functionality can be added to the catom through
relatively minor process improvements and adding additional circuitry.
Our approach, motivated by the need to mass produce the robots, is to integrate as many of the
individual components as possible. Our first design decision was to make each robot a sphere.
This simplifies manufacturing, while providing significant flexibility in the lattice arrangement
of the individual modules. A sphere can be created through stress-driven assembly of a photo
lithographically defined die. Locomotion is achieved by having the robots orbit each other using
electrostatic forces as described in these electrodes can be readily fabricated in two dimensions
before the device folds up. Having a solid approach towards realizing the structure, the next step
is to integrate active circuitry into the catoms. In order to simplify manufacturing and assembly
challenges, we decided to focus on integration and temporarily work on cylindrical tubes rather
than spheres. Using tubes relaxes the spatial constraints on interior components and allows us to
focus on improving the folding process to enable regions with changing radii, design of the
actuators, and integration of power transfer and active circuitry. Note that all of the techniques
developed are directly applicable to the final spherical shape so that simply decreasing the driver
circuit size and changing the initial two dimensional patterns will enable us to move from the
tube back to a spherical shape. The unit we present here represents all of the core capability
required in a catom so that moving forward is a matter of implementing new circuits and adding
functionality to the die.
Following the ensemble principle, we use conductive plates near the surface of the catom
to provide actuation. A first generation circuit capable of rectifying an incoming AC signal and
actuating the catom has been designed. Testing of the circuit has shown promise, but several
critical components require a redesign. These circuits are implemented using a commercial
CMOS foundry.
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9.2 Manufacturing The Tube Micro robot
9.2.1 Fabrication of the Tube
The process for fabricating the catoms is critical to the goal of creating ensembles of
thousands to millions of them. Densely packed functionality must be provided through highly
repeatable low-cost mass production. Therefore, the process we are developing is based around
commercial CMOS processes. These processes are ideal for producing large numbers of
repeatable, compact, highly integrated circuits.
In the long run, our fabrication process will have three distinct steps: CMOS foundry
fabrication, pick and place bonding, and three dimensional micromachining. The first two steps
in this process are both widely available from a number of sources, and therefore, our work to
date has focused on step three; a three dimensional micromachining process compatible with
prefabricated CMOS circuits and pick and place components. In developing this process, we
have chosen an approach based on stress driven self assembly of three dimensional structures.
Figure 9.3 The tubes are embedded in the wax
(black) after the backing Si is etched.
Figure 9.4 The process steps for catom fabrication.
The fabrication process used for the tubes is illustrated schematically in Fig. 9.4. It begins
(1) by thermally growing a 2μm thick layer of SiO2 on a silicon wafer. (2) The SiO2 is then
patterned using buffered oxide etch (BOE) to form rectangular islands that will later become the
tube microrobots. (3)Aluminum (Al0.99CuSi) of various thicknesses (0.1 μm to 0.5 μm) is
sputtered onto the SiO2 and patterned using a lift off process. At this point, the materials
required to form the tube are all in place and the next step is to free them from the original
silicon wafer. In order to do this, it is necessary to protect the aluminum, any CMOS circuits, or
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mounted die from the silicon etch process. (4) Therefore, the wafer is mounted, device side
down, onto a sapphire handle wafer using Apiezon Wax W. This wax is very stable and is not
significantly attacked during the remaining process steps. Once the devices are mounted on the
handle wafer, (5) the back side of the silicon device wafer is etched. Since it is necessary to etch
over 0.5 mm of silicon, an initial etch is done using HNA (HF-Nitric-Acetic Acid). When the
device wafer thickness is below 100 μm, the rest of the etch is done using XeF2, an isotropic gas
phase silicon etchant that does not significantly etch most other CMOS circuit materials
including SiO2, Al, or photo resist. The final XeF2 etch stops cleanly on the SiO2 and wax
surfaces as shown in Fig. 4. (6) The devices are then released from the wax by immersion in
trichloroethylene (TCE).
Upon release of the tubes from the wax (7), the rectangular shaped tubes bend into
cylinders due to residual stresses in the SiO2 and Al. During the SiO2 thermal growth process,
oxygen atoms are introduced into a pre-defined silicon lattice SiO2. Not surprisingly, the
resulting oxide layer is highly stressed (≈ 350 MPa, compressive). In contrast, the sputtered Al
films can take on a wide range of stress values depending upon the deposition parameters. Under
most conditions, these films will have tensile stress with a lower magnitude (typically < 60 MPa,
tensile).
Upon release, the SiO2 expands more than the Al, causing the rectangular structure to
bend into a cylinder, with the Al on the inside. The physical shape of the oxide patch and the
pattern of the Al electrodes are used to control the direction of bending. In general, a rectangular
oxide patch will bend so that the long direction rolls up leaving the shorter axis as the length of
the tube.
The radius of the tube depends heavily on the thickness of the layers. In general it is
possible to have different thicknesses or patterns of aluminum on the same tube, to create parts
with different radii. For example, in Fig. 9.3, the bottom six tubes have a solid aluminum patch
on the left side, which, when released, bends with a smaller radius of curvature. By controlling
the bending process in this way, it will be possible to mount a CMOS die on one end of the prereleased structure. During folding the region the die is bonded to will stay flat, then a second
region starting at the edge of the chip will have a smaller radius of curvature, followed by an
outer region with a larger radius of curvature .Properly controlling the radius of curvature in
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these two sections will enable us to keep the CMOS die close to the center of the tube. This is
crucial since a non-centered chip would make actuation more difficult.
Figure 9.5 A) After bonding the SOI CMOS chip to the tubes. B) After back-etching the chip. C) A released tube
which requires re-rolling.
To further improve actuation performance and reduce manufacturing difficulty we etch
the back side silicon of the Silicon-on-insulator (SOI) CMOS die down to the SiO2 insulator
layer after it is bonded to the tube (See Fig 9.5). This substantially reduces the weight of the die
and makes the entire structure smoother, enhancing the release process.
9.2.2 Analysis of the Curvature
The radius of curvature for the released tubes can be predicted using the methods. In our
case, we have a two layer structure composed of SiO2 and Al. Each layer has a thickness ti,
Young’s modulus of Ei, a residual stress of σi. Fig. 6 shows calculated diameters for a range of
layer thicknesses assuming EAl = 70GPa, ESiO2 = 60GPa, σal = 60MPa, σSiO2 = −550MPa.
We measured thicknesses of tubes with both solid (Fig. 6, circles) and patterned Aluminum (Fig.
6, crosses). The patterned tubes have 100μm wide stripes of Aluminum electrodes with a pitch of
100μm, so the total aluminum coverage of the tube surface is 50%.
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Figure 9.6 Tube diameter for different thicknesses of SiO2 and Al.
We see that for tSiO2 = 2μm and tAl = 0.5μm the expected diameter of a solid Al tube is
0.97mm. We choose to use tAl = 0.49μm and were able to achieve a diameter of 0.87mm. The
same thicknesses resulted in a 1.2mm tube when the aluminum is patterned. This increase in tube
diameter is expected, as with less aluminum coverage, there is less stress and thus less bending,
resulting in a larger diameter.
As shown in Fig. 9.6, the diameters of the tubes we manufactured are in agreement with
our analysis. Although thinner SiO2 gives a broader range of realizable diameters, the resulting
tubes are too fragile. Using a thickness of 2μm yields robust tubes with useful diameters.
9.3 Actuation
An important design goal in realizing claytronics is to use a single mechanism to perform
as many of the required functions as possible. The actuation mechanism for catoms is the most
challenging of the functions; it is essential that catoms are able to move and reconfigure in an
ensemble. In this section, we describe an actuation mechanism for catoms based on
electrostatics.
Catoms move by rolling along the surface of neighboring catoms. They use electrostatic
forces to do this. Each catom has conducting plates (“electrodes”) covering its outer shell; they
run along the length of the tube, e.g., in and out of the page in Fig. 7. The electrodes of
neighboring catoms align and form capacitors (see Fig. 9.7). When a voltage is applied between
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the plates of a capacitor, attractive electrostatic forces are created due to the accumulated charge
on the electrodes. These forces then cause the catom to move.
Figure 9.7 The force created between the electrodes of adjacent catoms
moves the catom.
Since catoms are not electrically connected, they cannot share a common ground. In
order to generate a voltage across the electrodes, adjacent catoms are coupled using at least two
pairs of electrodes. This way, a series circuit loop is formed. The total electrostatic force created
between the electrodes causes the catoms to roll around each other until equilibrium is reached.
By switching the voltage on the electrodes, and successively charging/discharging the next pair,
a continuous rolling motion is possible.
9.3.1 Force Created Based On Voltage
Before we analyze how much force can be created, it is important to understand how
much force is needed to actuate a catom. For a catom to roll around a fixed catom, in the worst
case, the torque created by the electrostatic forces must be enough to overcome the moment
created by the weight of the catom, with respect to the point of contact. The moment created by
the weight of a catom is:
τrequired = MgR = 6.8 × 10−10Nm
where R is the radius of the catom, M is the total mass of the catom, including the chip.
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Figure 9.8 The electrodes on neighboring catoms are paired to form
capacitors.
As shown in Fig. 9.8, the electrodes are placed radially on a catom, each spanning an
angle of θw. Adjacent electrodes on the same catom are separated by a gap, so that they do not
experience dielectric breakdown. In Fig. 9.8, when the catom on top is to roll clockwise around
the bottom catom, a voltage difference is applied between all the electrodes that are located in
the bottom right quadrant of the top catom and the electrodes that are located at the upper right
quadrant of the lower catom.
9.3.2 Required Voltage based on Eccentricity
Since no fabrication technique is perfect, is it reasonable to expect the catom to have a
non uniform shape after the fabrication steps explained. In this section we investigate the effect
of imperfect catom shape, and calculate the torque that is required in this case for a catom
moving horizontally.
We model our catom as a tube with an elliptical cross section, with the major axis a =
1mm, and a variable minor axis, b. We define the eccentricity of the catom as
:
.
Eccentricity is simply a measure of how distorted the catom is. With our
current fabrication technique as described, we are able to fabricate tubes that have minor-tomajor axis ratios ranging from 0.9 to 1, corresponding to an eccentricity of e = 0.43 to 0.
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Figure 9.9 Ellipse with the maximum moment arm. For a : b = 4 : 3, the corresponding eccentricity is 0.66.
Using an analysis, a torque requirement is computed based on the eccentricity of catom.
We find the biggest moment arm of the center of mass of the catom, with respect to the point of
contact. Fig.9.9 shows this moment arm, k, which can be calculated analytically as
For the worst case measured eccentricity value of e = 0.43, the required voltage to
overcome the torque due to the imperfect shape is V = 18volts. This is well below the operation
voltage of the tube, V = 100V . Even if the chip size were to grow twenty fold, the required
voltage would still be manageable.
We conclude that even in the worst case, it is possible to create a torque more than two
times needed to actuate the tubes made using our fabrication technique which yields tubes with
an eccentricity less than 0.43.
9.4 Realizing The Ensemble Principle
In the previous section we described the structures and methods for actuating catoms with
electrostatic forces. Here we briefly describe how these same structures can be used for adhesion,
communication, and power. The capacitively coupled electrodes of adjacent catoms create
electrostatic forces which are used for actuation and can clearly be used for adhesion, that is,
when the catoms come to equilibrium, the capacitively coupled electrodes hold them in place.
The use of capacitive coupling for inter-module latching was investigated in. Our analysis
showed that the bulk yield strength of an ensemble of catoms can be as high as that of plastics.
Power transfer is also possible using capacitively coupled electrodes. When one catom
puts an AC signal on its half of the plates of the capacitors, it generates an AC signal on the
adjacent plates of the neighboring catom. This signal can be rectified and converted to DC to
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power the neighboring catom. In the worse case we obtain power efficiencies of 50%. In the best
case, when the catoms are trickle charging each other we can achieve efficiencies of nearly 80%.
Figure 9.10 Different radius tubes.
Finally, communication using the same electrode pairs is even simpler. Data modulated
with an AC signal can be easily transferred across the capacitors, and demodulated at the target.
9.5 Realizing The Catom
There are many steps involved in realizing a working catom. To this point we have made
progress in both manufacturing and actuation. The CMOS compatible fabrication process for
forming three dimensional tubes from a two dimensional process has been developed to the point
where tubes with two (or more) radii can consistently be fabricated at the same time, and even on
the same die. The actuation process has been modeled and simulated allowing the actuator
design to be optimized. Further, the simulations give us good confidence that the catoms can
provide sufficient force to operate once they are implemented, even accounting for nonuniformities such as eccentricity. Proof of principle experiments have been done to demonstrate
rolling tubes using external actuation. Therefore, the next step is to integrate active control
circuits that collect and store energy and then use this energy to drive the catom.
Implementing the chip begins by selecting the appropriate CMOS foundry process. From the
actuation design work, it is clear that standard CMOS will not work as we may need voltages of
up to 100V to drive the electrodes. Further, we would like a process that uses Silicon-oninsulator so that it can later be used as part of the shell forming process. Based on these
constraints, we selected XFAB’s XI10 High Voltage
BCDMOS process since it can support voltages in excess of 100V.
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Figure 9.11 The chip.
Fig. 9.11 shows the schematic diagram of the circuits required to implement the first
catom. These circuits have all been to fit on a single die. The power required to operate the chip
is supplied through capacitive coupling. The tubes have electrodes running along the
circumference on both ends, called the “coupling electrodes” (See Fig. 16). An AC signal
generated at the source, i.e., a powered floor of another catom, is transferred to the chip through
the coupling electrodes. These AC signals are rectified to generate a DC output. The DC power
is then stored in a capacitor and used to power the integrated circuit and drive the electrodes at
voltages of up to 100v.
When completed, this first prototype catom will demonstrate the viability of a massproduced mm-scale robot with internal control and the use of a single mechanism—external
plates for capacitive coupling—which will be used for adhesion, actuation, and power transfer.
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10. Applications
10.1 A 3D Fax Machine:
This is a novel application of modular robotic technology. Many researchers expect
manufacturing technology will allow robot modules to be built at smaller and smaller scales, but
movement and actuation are increasingly difficult as dimensions shrink. We describe an
application—a 3D fax machine — which exploits inter-module communication and computation
without requiring self-reconfiguration. As a result, this application may be feasible sooner than
applications which depend upon modules being able to move themselves. In our new approach to
3D faxing, a large number of sub millimeter robot modules form intelligent “clay” which can be
reshaped via the external application of mechanical forces. This clay can act as a novel input
device, using intermodule localization techniques to acquire the shape of a 3D object by casting.
We describe software for such digital clay. We also describe how, when equipped with simple
inter-module latches, such clay can be used as a 3D output device. Finally, we evaluate results
from simulations which test how well our approach can replicate particular objects.
Consider a hypothetical scenario in the near future, where a paleontologist is searching in
a remote location for interesting fossil remains. Having found an interesting bone fragment, she
records the site information, using GPS and local surveying instruments, and takes numerous
digital photographs to record the context of the find. She then takes an impression of the
fragment in clay or plaster to facilitate its later reproduction for wider study as well as display in
museums. Given present technologies, the latency between discovery and access to fossil and
object replicas will generally be measured in months. However, imagine in this future world, that
the clay-like material used to take the impression is electronic in nature, comprised of a myriad
of tiny modules which are capable of inter-module communication and computation. This
intelligent clay measures its own shape and, by reflection, the shape of the embedded fossil
fragment, and generates a digital representation of the fossil’s three-dimensional structure.
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11. Conclusions
11.1 Future Design

3Dmovement of catoms.

Catoms will be able to emit variable color and intensity of light, allowing for dynamic
physical rendering.

Onboard battery sufficient to power catoms will exceed the size of the catom itself, so an
alternate energy solution is desired

Research is being done into powering all of the catoms in an ensemble, utilizing the
catom-to- catom contact as a means of energy transport

Develop a unary connector for the catoms in order
Our initial research results suggest it may be possible to construct, power, and control large
microrobot ensembles to model 3D scenes. While many difficult problems remain, successful
implementation of a dynamic physical rendering system could open the door to a new era of
human computer interface design and new applications. Economic feasibility also poses a high
bar to the manufacture and deployment of multimillion-robot ensembles. However, the
innovative application of high- volume manufacturing techniques bridged a similarly large gulf
in cost and physical scaling in computer hardware. Achieving the Claytronics vision won’t be
straightforward or quick, but by taking on some of the problems associated with operating and
building these ensembles, we hope to advance the state of the art in modular reconfigurable
micro robotics and encourage others to undertake related research.
The advancements in nanotechnology and computing necessary for claytonics to become a
reality are feasible, but the challenges to overcome are daunting and will require great
innovation.

Many challenges need to be resolved before bringing this technology to real world.

The hardware aspect of this technology deals with making the crude and tiny robots to
move and configure.

The software aspect is still highly staggering.

It concerns with controlling the movements of minute catoms and programming these
millions to work together.

Still, claytronics is a promising field of engineering
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As the capabilities of computing continue to develop and robotic modules shrink, claytronics
will become useful in many applications. The featured application of claytronics is a new mode
of communication. Claytronics will offer a more realistic sense to communication over long
distance called pario. Similar to how audio and video provide aural and visual stimulation, pario
provides an aural, visual and physical sensation. A user will be able to hear, see and touch the
one communicating with them in a realistic manner. Pario could be used effectively in many
professional disciplines from engineering design, education and healthcare to entertainment and
leisure activities such as video games.
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