How To Make Your Own Soft Robot

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A description of how to build a life sized Baymax is here.

Humans are "ugly giant bags of mostly water." (video)

For the purposes of this page, a soft robot is a movable structure with deformable parts. We need to create soft structure, and then actuate it to move. Focusing on structure, we can mold or sculpt some form of deformable material, such as silicone. We can make a much lighter structure by inflating some sort of bag. Our soft robot could have a rigid skeleton, or internal ligaments. Actuation can be provided by the same kinds of actuators used in rigid robots attached to tendons, or by moving pressurized air or liquid into volumes that can change shape (pistons, McKibbon actuators). Currently available sensors as well as sensors made possible by soft materials can be used. We can mix and match all of these approaches to create a wide variety of robots. Let's first focus on how to build an inflatable structure and then consider how to build robots out of deformable material. We will outline how to build both types of structure under computer control (3D printing and other rapid prototyping techniques). We will then talk about actuation and sensing techniques made possible by soft materials.

It is helpful to think more generally about what is an inflatable robot. One definition is a robot that uses membranes to provide tensile resistance to stretching, and a pressure difference to provide resistance to compression. Examples include balloons, inner tubes, balls for sports, inflatable pool toys, blimps, boats such as Zodiacs, buildings (more) and space suits. The compression material in these examples is air, but it could also be liquid (cells in biology, many plants, worms), or some sort of solid but deformable material (fiber in plush toys, particles in sand bags and bean bags: recently there has been a lot of interest in jamming grippers with particles surrounded by a membrane). The membrane does not have to fully enclose the compression material (sails, kites, parachutes, hot air balloons, underwater air lift bags, air dancers). Some buildings, large inflatable lawn decorations, and inflatable costumes require continual inflation to remain pressurized. The structure can be dynamic (automobile air bags). Inflatables can be quite tough, and are used for Mars landing systems and automobile jacks. There could be just one compartment, or many compartments (bubble wrap, foam, biological tissues).

We and others have built relatively simple inflatable robots, with outer membranes enclosing a small number of volumes filled with air, with explicit flexing joints. [PICTURES] Future research will explore robots made out of bubble wrap or foam (flying foam robots?). The key idea is to make robots like we make clothes: cut sheets of membrane material according to a pattern, and then assemble the three dimensional structure by sewing, glueing (part 2) or welding seams, or some combination of these techniques. This is much less expensive than machining metal. We borrow techniques for making patterns from instructions on making clothing as well as making shaped balloons. There are many academic papers on generating a pattern for a giving three dimensional shape ( example, can't find link to work) One way to experiment with glues and materials is to buy inflatable repair kits. Another way to get materials (and complicated parts like nozzles) is to scavenge them from broken inflatables (or just buy cheap inflatables such as buying beach toys in the fall). For our inflatable robots, we use polyurethane film (McMaster-Carr). To form seams, we use an impulse sealer to weld straight seams. A thermal welding machine such as a hot air welder can be used to make curved seams. A soldering iron or hot air gun can be used to weld seams by hand. Research areas including using materials with different stiffnesses, and the effect of internal fibers similar to carbon fiber reinforced polymers.

Another way to make soft robots is to carve, sculpt, machine, dip, spray, or mold soft material, such as silicone. Carving, sculpting, or machining is used with solid material, while dipping, spraying, and molding work with a liquid material that solidifies. Examples of recent molded robots include Glaucus (4 legged robot: more, more), Trefoil (tentacle robot: more, more, more), crawler (more), jumper, fish, grippers, fingers, heads, skin, actuators (more), and tires. There is an attempt to mold a complete full size humanoid (Inmoov blog, Inmoov hand), as well as an Iron Man suit. There is a lot of information on the web about molding silicone. Molds can be directly carved from clay, wood, silicone, or machined using metal. Sometimes it is easier to use an object directly to create a mold (such as a human face for a mask or a hand for a Halloween decoration) or carve or sculpt wax or clay or machine metal to create the desired shape and use that to create a mold. In this case liquid mold material is poured around the desired shape and allowed to harden. Molds can be designed to be reusable or non-reusable Non-reusable molds often remove the original master shape by turning it into a liquid by solubalizing or melting it (typically a wax master), or the object is removed physically if it is sufficiently deformable. This kind of mold is typically used once and then destroyed to free the molded object. The advantage of a non-reusable mold is that it avoids seams. Reusable molds can be hard or soft. Hard mold materials (plaster) often require the mold to be made in parts in order to allow it to be removed, leading to seams, while soft mold materials (silicon, alginate [REF], ...) can be deformed to remove them from the original object and subsequent molded objects. More complex molding sequences are used involving sequences of positive and negative versions of the object, often to change materials used in the final molding process. Challenges in molding soft robots include internal bladders, seams, air lines, wire conduits, and space for actuators, tendons, sensors, computers. batteries, and/or fuel. Modern versions of lost wax casting solubalize internal structure that creates these voids. If it is acceptable to leave these materials inside the final object, plastic straws or other forms of flexible tubing and balloons can be used to create voids in the molded object and simply left inside it. Another process that can be used to mold robots by creating an object that represents the void is dipping and spraying. It is sometimes hard to tell the difference between an inflatable robot and a molded robot (molding balloons, for example) or a pneumatic actuator in a molded robot. Compare these two inflatable structures: molded and made from sheets.

How can computers be used to control the manufacturing of soft robots? In the case of inflatables, computers can be used to plan the pattern, and robots can be used to cut the material and make some or all of the seams. Laser cutting is useful here. Challenges in this case are similar to the challenges in automating clothing manufacture: handling flexible materials, sensing material location, and correctly cutting and making seams. In the case of machined robots, CNC machining and laser cutting can be used to sculpt the desired object. It is often difficult to machine or cut accurately soft materials that do not maintain their shape under gravity or when machine tools are applied. Can soft robots be 3D printed directly? One needs a 3D printer that can print liquids that harden, such as silicone ( more, more, more, more, more ). 3D printing soft materials is similar to 3D printing food and often the same tools can be used. Challenges in this case include liquids that do not accurately maintain their shape as they harden, especially when additional material is depositied nearby due to surface tension effects or on top due to gravity. If the deposition of material is slow enough to allow a layer of deposited material to harden, printing an entire object may be very slow. In addition, soft objects do not maintain their shape even when the material is "dry", leading to further inaccuracies. Festo directly prints deformable plastic parts for a wide variety of air powered robots using selective laser sintering (SLS) of polyamide powder. In the case of molded robots, 3D printing, laser cutting, and CNC machining can be used to create the desired shapes and/or molds out of metal or plastic. Wax or soluble material is used where internal molds will be solubalized and removed. Many of the soft molded robots mentioned above were created using 3D printed molds, including Glaucus (more), Trefoil, jumper, fish, grippers, fingers, heads, humanoid, and an Iron Man suit.

There is spectrum of designs that mix these approaches to soft structures. Support and actuation is often provided by both inflation and soft solid material. In addition, there is the possiblity of adding flexible or rigid internal or external structure to the soft robot as a skeleton. An example of a flexible skeleton is the Gumby toy.

skeleton: blimp vs. zeppelin
skeleton does not need to be connnected:
battens on a sail,
compression structure: skeleton (endo, exo)?
internal tensile structure
Compression structure could be inflatable (kite for kite sailing has
inflatable forward edge)

Elastic vs. plastic soft robots.

Silicone vs. bean bags.

Joints can be defined by where a rigid skeleton can bend, or they can be defined by concentrations of deformation in soft structure (soft joints) How can we make soft joints? In inflatables, this is easy to do by creating seams that close off a volume and allow a segment to easily bend. Joints can also be created by internal membranes and ligaments, or by creases in the outer membrane. External Tension elements can be used to create a crease along their path.

rolling surfaces
electrostatic or electromagnetic actuation along rolling surfaces.
continuum bending or just bend at joints

How to actuate?

standard actuators pulling on tendons (Sanan inflatable arm, continuum arm).
rigid actuators: motors, pistons.
ultimate deformable actuator: muscle.
deformable actuators: McKibbun actuator, SMA
actuate: tendons connected to force sources
move filler material (piston, balloon)

Soft Sensing.

In addition to sensors currently used in rigid robots, soft robots provide
additional opportunities for sensing. The potentially large
deformations of a membrane or
three dimensional material can be measured with 
strain gages [REF,Park,Majidi] as well as 
(optical tactile sensing).

Garth Zeglin's suggestions:

Your pre-proposal text calls for some crazy new technologies, but I think
the ball could get rolling with conventional materials: cast silicone, sheet
rubber, fabric, pneumatics, fsr fabrics, 3d-printed molds, laser-cut molds,
rubber flexures, tubing, etc.  All of that should be easily available.

1. Laser-cut laminated molds cut from sheets of acrylic for casting
silicone.  Obviously, features in Z are pretty coarse, and internal
voids are tricky, since they involve building up internal mold parts.  I
did this with Smooth-on silicone and it worked fine; it would have
worked even better with a vacuum degassing step.

This could make flexure bearings and shaped solid structures.

It would not be hard to add inclusions such as electronics.

It shouldn't be too hard to add reinforcing fibers by stretching them
between parts of the acrylic structure prior to casting.  I think this
has a lot of potential for making structures which bend in interesting
ways by including tension elements.  I have not tried this.

It would be very difficult to make an internal air bladder, but a
separate balloon could be attached after the fact.

2. 'Lost-wax' casting.  At first I thought you were joking, since
lost-wax usually involves hot metal melting out the wax during the pour.
 But many waxes melt at lower temperatures than silicone, so yes, you
could cast rubber around a wax positive and melt it out.  This could
lead to a whole discussion of positive versus negative casting, and
multi-step casting processes.

So how about laser-cut/laminate or 3d-print a negative mold and cast a
single-use wax positive in it.  Wax will shrink a little after cooling,
so perhaps material could be cast into the space between the negative
mold and the wax, then the wax melted out?  This is speculative.

3. Dip-casting, e.g. condom manufacture.  3d-print a positive mold, then
iteratively dip it into liquid latex and allow thin layers to cure.
There are apparently sprayable rubbers which might allow more bulk
material to be applied outside a mold.  This could possibly be inverted
to coat the inside of a hollow mold by rolling it around during curing.

4. Blow-molding is the way soda bottles are formed, by heating and
inflating a small blank inside a mold.  It's a bit of stretch, but
people have tried this at home to make rigid thin-shelled objects:

5. Are you going to include a discussion of actuation?  E.g.
High-pressure versus low-pressure air, pneumatic valves, pumps, winches
pulling tension elements.

6. Are you going to discuss sensing?  In the Physical Computing course,
we've been introducing ideas for simple resistive fabric sensors, those
are very maker-friendly.  This is well-trodden ground:

7. How do you define the scope for 'soft robots'?  Is it just
inflatables, or does it it includes soft structures and clothing?

structures with rubber flexure bearings, e.g

fabric gadgets including actuators
(search "lilypad actuators", although most projects are LED lights)

(search "inflatable dress", e.g.

8.  General materials ideas.

silicone rubber
latex rubber
fibers (Spectra, nylon, kevlar)
"dipping rubber" (often latex)
Sprayable rubbers
plaster for molding

9. Suppliers and specific products

10. Iterative processes.  I think that 3D-printing tends to encourage a
design-and-print tunnel-vision, but all these casting and forming
processes tend to be multi-step.  Those intermediate steps *can* include
hand-work, i.e., heating and bending wax positives by hand, carving,
etc.  They can also combine both additive and subtractive steps. (e.g.
Aaron Dollar's hand process).

And a practical soft robot will likely include multiple elements:
premanufactured air bladders, molded flexure joints, embedded tension
elements for creating soft linkages, embedded sensors and electronics,
3d-printed rigid housings for rigid parts (batteries, motors, pumps).
It might be useful just to name out all the systems and see which ones
can or can't be made 'soft'.

So did I tell you anything you didn't already know? :)  In the long run,
this would work better as a small brainstorming session with a few
design constraints (e.g. practical problems).

  - Garth

Carmel Majidi's notes:

One technique is to use VHB tape.  That's probably the easiest way for
hobbyists to make a soft robot.  For my 24-673 class, my students have found
it much easier to use shape memory alloy instead of pneumatic inflation to
power the robot limbs.

VHB tape is a soft and stretchable double-stick adhesive that is easy to
laser pattern and has a high thermal resistance.  Therefore, we can bond it
to SMA without melting through during activation.

The design with VHB tape and SMA is very simple -- something you can do with
scissors and a ruler.  


Yong-Lae Park's notes:

1. Shape Deposition Manufacturing (SDM):

SDM was originally developed by Lee Weiss et al. at CMU more than 15 years
ago.  It was introduced as a new manufacturing concept to build
multi-material structures.
Web page

However, due to its capability of using heterogeneous materials in one
structures and easiness of embedding components, it is widely used for
building robot parts and robots, such as legs and bodies
of running and climbing robots, end-effectors, grippers etc. For this
application, usually a wax block is machined to a mold using a CNC milling

There are multiple reasons of using wax as a mold material: i) wax is easily
machinable and does not melt down during machining in spite of its
relatively low hardness and low melting point, ii) wax can be easily melted
away if it is captured in a complicated structure, and iii) wax does not
stick to various polymer materials (e.g. polyurethane, silicone etc.) after
casting and easy to removable. 

One good thing about SDM is that the entire process can be done at room
temperature. Baking process at an elevated temperature is not necessary
although it may help expedited curing of polymer materials.  However, it may
involve a degassing process in a vacuum chamber to remove bubbles in the
polymer material.

SDM is not a specific manufacturing process but more like a concept, so it
can be modified to build unconventional and complicated robotic structures
depending on the user's ideas.  The following paper shows a good example of
modification of SDM to build fully three-dimensional composite robotic

2. Layered Molding and Casting (LMC) 

LMC is simplified version of SDM that does not involves a direct machining

Instead of directly machining a wax block and an actual structure, a 3D
printer is used to prepare molds.  In this process, the target structure is
divided into multiple 2D layers in design to form the target 3D structure
later.  Separate 2D layers are first made of 3D printed plastic molds and
later stacked up.  Since the molds are prepared by a 3D printer, the mold
preparation is easier and faster than SDM.  However, the resolution of the
target structure is dependent on the resolution of the 3D printer, which is
not as good as that of a CNC machine.  Also, the mold material is sometimes
limited to the material that can be used in the 3D printer.  To bond
multiple layers, spin-coating technique is commonly used.  All the process
can be done at room temperature like SDM.  This method may also involve a
degassing process. 

The following paper describes a detailed LMC process to build multi-layered
highly stretchable and flexible sensors:

This process is more effective with soft materials that cannot be easily
machinable can be easily used for development of soft robots.  Soft
materials can be easily removable from a hard plastic mold, and the mold can
be reusable multiple times.  For structures with hard materials, SDM can be
also combined with this process.  The mold can be made out of wax that was
machined and used for building multiple layers.  More simply, a wax 3D
printer can be used to directly make a wax mold. 

Siddharth Sanan's notes:

Stuff that I think might not be included already:

Other notes (move to resources?)

check out these links

Coros, Sanan, Whitney

Whitesides group, Harvard


Wyss institute

How to make foam? aerogel?

exhaust heat can be handled easily by exhausting hot air

Often the question of robustness comes up. Here are some applications
of inflatables:
space suits
air bags
mars landing bags
inflatable boats (Zodiak), kayak, paddle board
inflatable water trampolines
kite surfing kites
inflatable buildings (tennis court covers are popular)
pool toys, water slides,
inflatable amusement park:
pool, hot tub
holiday lawn decorations
bounce house/bouncy castle: moon bounce, jumpers, obstacle courses
pillows for jumpers in fire rescue and stunt men to jump onto

water wings
blood pressure cuffs

A discussion of two materials:
PVC and Hypalon
google "hypalon" for more.

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