IEEE Robotics & Automation Magazine - March 2017 - 61

Acetate Insert
i)

Top-Layer Mold

Bottom-Layer Mold

Thread
ii)

ii)

Mold Insert

Glue Layer
iii)

iii)

iv)

Elastomer Plugs
v)

vi)
(a)

(b)

(c)

Figure 3. The comparison of three methods for manufacturing of soft fluidic actuators. (a) The two layers are cast separately and then
glued together using a layer of silicon rubber. (b) A mold with an insert is used for casting. The insert is later removed and the bladder
is plugged at both ends to create an airtight chamber. (c) A sacrificial acetate layer is held in the middle of the mold with thread, and
it acts as a zero-thickness air chamber.

have reported difficulties in using the toolkit information due
to complex manufacturing methods and a requirement for
specialized machine tools and expensive consumables. Thus,
we identified a need for new instructional materials based on
manufacturing methods that are less complex, more reliable,
and require only low-cost and easily accessible materials.
More Accessible Manufacturing Methods
and Instructional Materials
Our focus in developing new manufacturing methods has been
on actuators rather than sensors or control hardware and, in
particular, on fluidic soft actuators. Such actuators can achieve
complex motions due to mechanical programming using only
fluidic pressure as a simple input. Soft fluidic actuators consist
of airtight chambers surrounded by materials of varying stiffness. Upon pressurization, the channels in the soft actuator
expand in the direction of lower stiffness. A variety of motions,
including extension, contraction, bending, and twisting, can
be programmed into the actuator through the morphology
and materials used in the construction of the fluidic chambers.
Most of the actuators documented in the SRT consist of
silicone rubbers cast in complex, multipart molds produced
using 3-D printers or other rapid manufacturing tools. The
silicone rubbers themselves, which are commonly used in
model-making and special effects, are affordable and easy to
obtain. However, the requirement for rapid prototyping
equipment remains to be a barrier for many. To address this
issue, we have designed molds that can be built from accessible materials such as paper and cardboard. One advantage
of this approach is that mold designs can be shared via twodimensional templates that can be printed on paper,

adhered to a sheet of cardboard, and cut out to create components that can then be assembled into 3-D molds. This
approach has the potential to drastically lower the entry
barriers to soft robotics and to thereby support the wide dissemination of robotic-hardware designs.
The challenge when designing molds for fluidic actuators is
creating airtight chambers. Common approaches include
casting layers separately and subsequently gluing them together [Figure 3(a)] or using a mold insert during casting that can
be removed and plugged afterward [Figure 3(b)]. Both methods require multiple casting steps and are time-consuming.
The molding technique presented here requires fewer steps
and is more accessible for novice users [Figure 3(c)]. This technique produces simple airtight bladders that serve as building
blocks for more complex actuators. Our approach uses a sacrificial insert made of acetate sheet that creates a zero-thickness
air chamber. The acetate layer is suspended between cardboard walls using thin thread (Figure 4). A two-part silicone
rubber is cast in the mold. Once the silicon rubber is cured, the
thread is removed. The sacrificial acetate layer remains in the
bladder but does not stick to the walls of the bladders, thereby
creating the fluidic chamber.
The bladders produced using this technique can be assembled in different combinations to create a variety of actuators.
For example, external reinforcement layers can be used to
mechanically program the motion of an actuator in response
to internal fluid pressure [17]. Kevlar thread wrapped around
the circumference of the actuator restricts its radial expansion. Layers of fabric prevent axial expansion in parts of the
actuator. Figure 5(a)-(c) shows three examples of actuators
that can be achieved from identical bladders simply by
March 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2017