# Fabrication

Elastomer foams can be made by any variety of foam-forming process, though to date, the lost salt process and castable polyurethane foams have shown the most promise.

When using the lost salt process, this tool is useful to calculate amounts of elastomer and salt to combine. By inputting the desired elastomer amount and volume fraction of salt (which is approximately equal to the subsequent porosity of the foam) as well as the elastomer and salt density, this tool outputs the mass and volume amounts of each component.

Below, two methods to fabricate foams using lost salt processes are described. The first uses ammonium bicarbonate, the second uses table salt. Whereas the ammonium bicarbonate foam-forming process is faster, the table salt process requires no hazardous chemicals.

#### Ammonium Bicarbonate in Silicone

This method of producing foams uses ammonium bicarbonate, (NH4)HCO3 (also known as ammonium hydrogen carbonate). Ammonium bicarbonate is useful as a porogen because it i) thermally decomposes readily and entirely above 50 °C, ii) yields easily managed products (water, CO2, and NH3) leaving the PDMS foam residue free, and iii) is inexpensive and readily available.

Of the decomposition byproducts, water is the highest boiling. Considering this, if ammonium bicarbonate is decomposed above 100 C, all of the byproducts will be removed as gases. We used a vacuum oven to remove the salt quickly. Depending on the part size, the ammonium bicarbonate  was removed within 2-8 hours.

We selected Mold Max 10 PDMS (MM10; Smooth-On, Inc.) as the elastomer due to its low tensile modulus ( E ≈ 500 kPa) and large ultimate strain, which enables large deformations at low inﬂation pressures (typically Δ P < 70 kPa). Additionally, MM10 is beneﬁcial as it exhibits a short room temperature cure (≈3 h) and is compatible with ammonium bicarbonate porogen since it cures via an organotin-catalyzed condensation reaction.

One important consideration when using ammonium bicarbonate is that it will signiﬁcantly inhibit the platinum catalyst in addition-cured PDMS resins. This means that it should primarily be used with tin-cured PDMS resins.

#### Table salt soft foams manufacturing process

Materials:

• Ecoflex 00-10 Part A + Part B
• Table salt
• THI-VEX thickener (optional)

Manufacturing steps:

• Mix Ecoflex 00-10 Part A and part B 1:1 ratio, by weight;
• Add the table salt to the EcoFlex mixture according to the desired porosity;

Open cells porosity can be described as follows:

$$ϕ_p=V_S/(V_E+V_S )=(M_S/d_S )/(M_E/d_S +M_S/d_S )$$

Where:

VS is the table salt volume, [cm3]

VE is the total Ecoflex volume (Part A + Part B), [cm3]

MS is the table salt weight [g]

ME is the total Ecoflex weight (Part A + Part B), [g]

d= 2.17 is the table salt density, [g/cm3]

d= 1.04 is the Ecoflex density, [g/cm3]

• Manually stir the mixture to obtain an homogenous compound;
• Wait for 15-20 minutes;
• Sculpt or cast the soft foam;

To sculpt the desired shape, an additional of THI-VEX (Smooth-On) thickener (4% weight of part A) is needed, in order to increase the mixture viscosity. Directly add it once the salt crystals are homogeneously distributed in the mixture. Otherwise, it is also possible to partially cure the mixture for 20-25 minutes before sculpting. To cast the soft foam, just pour the mixture in the desired mold and wait until the mixture is cured (about 15-20 minutes). To speed up the curing, it is also possible to put the sculpted/casted mixture in the oven (15 minutes at 80°C).

Table salt soft foam casting:

Table salt soft foam sculpting:

• Remove the salt particles after curing.

Once the mixture cured fully, it is necessary to dissolve the salt in warm water (~50°C) overnight. To speed up dissolution, an ultrasonic bath could be useful. In addition, it is also possible to periodically stretch the foams under water to increase salt-to-water contact. After drying the foam, we used compressed air (at flow rates accessible by computer keyboard air dusters) to remove any residual salt, which ensured an interconnected fluid pathway by breaking thin walls between closed cells (see also the following video)

Strain Limiting Layer insertion

The strain limiting layer allows the actuator to bend in a precise direction when the actuator is pressurized. We add a paper sheet (with the desired shape) as inextensible layer. We used paper because it is easily accessible, low cost, and can be cut to any shape using a laser cutter or scissors. As paper is porous, this layer merges completely with the sealeant elastomer while curing. Thanks to this manufacturing technique, the inextensible layer can be directly added while sealing the soft foam (see next step).

Foam Sealing

We used two different methods to form the external seal: painting and molding. Painting can be faster (especially for complex shapes) by eliminating the need to design a mold; however, it can result in an uneven seal. Molding enables a precisely designed, reproducible seal and thus was our preferred technique for blocking force test samples. This one-step seal cure avoids bonding issues between asynchronously cured seal layers, as opposed to the multi-step cure often used in traditional approaches. Though the foam and seal cure at different times, the bond is sufficiently strong for inflation due to the large bonding area available on the foam's porous surface.

Some examples of manufactured actuators with different sealings are shown in the following picture:

#### Castable Polyurethane Foams

It is also possible to form foam actuators using castable polyurethane foams. Unlike foams made from the lost salt process, these foams do not require additives to form pores. These polyurethanes are formulated to generate and retain gas during their cure which results in a porous elastomer foam. The advantages of using these materials is that they can be cast as a very low viscosity liquid (prior to foam formation) and that they generally attain high porosity foams (leading to rapid actuation). The Flexfoam-It series of foams (available from Smooth-On) are a great basis for these actuators. As with all of the foams described on this page, these foams can be combined with a sealing elastomer and a strain-limiting layer to form a foam-based actuator