© 2017 HCII Institute, Carnegie Mellon University

Sequential Folding of Biocompatible and Edible Hydrogels


By composing a set of biocompatible and edible hydrogel materials with different swelling rate during water absorption, we can create 2D films made of common food materials (protein, cellulose or starch), which can be transformed into 3D shapes upon hydration. Temperature and material composition can be manipulated to program the sequential transformation of such hydrogels.

We found most of the food gels belong to hygromorphic materials, which can hydrate and change volume when absorbing water through hydrophilic interaction within their molecular or inter- molecular structure. Starch is an example of a hydrophilic polymer, since it has -OH groups present on its surface. One way to quantify a material’s ability to hydrate is to use the swelling index.

We measured the swelling index of a few major edible components, which showed that protein (gelatin), carbohydrate (starch) and soluble fiber (agar) can all absorb about five times their own weight within 10 min (in film form), while insoluble fiber (ethyl cellulose film) cannot absorb any water. This finding is the basis for designing shape transformation through volume change by introducing heterogeneous swelling behavior within food composites.

In addition, we studied the swelling behavior of these edible materials at different temperatures in water. As expected, at high temperature, edible film can absorb water at a faster rate than when in cold water.This can be explained by Fick’s law.

Desining Substract Film
We fabricate substrate film that has heterogeneous density distribution, which can cause differntial expansion upon hydration. We found that, when preparing film in a petri dish and evaporating water only from the top of the film, the desired film heterogeneity was achieved. The solid-air boundary contains a higher con- centration of material due to migration and aggregation of the solids upon drying. After forming a dried top layer, water evaporation in the lower portion of the film is slowed. This results in the formation of a dense top layer, as well as a loose, porous bottom layer, of gelatin films.


Adding Shape Constraints
In order to achieve controllable bending behavior, an ethyl cellulose strip is introduced as both a shape constraint and a water barrier on top of the film. This semi-rigid strip structure could help regulate the binding direction and create dynamic shape changing by modulating the top surface’s water adsorption rate (majorly due to the decreased water adsorption area).

This cross-section of the material composite shows the anisotropic swelling rate of the composite in water. The middle region is made of ethyl cellulose at the top and gelatin at the bottom; both sides are made of gelatin. The microscopic images show that the regions on the side expand at a greater swelling rate compared to the region in the middle.
We hypothesize that the cellulose strip has a dual role in controlling shape changes in the whole structure. First, the cellulose layer can be seen as a barrier for water diffusion, which modifies swelling behavior of the gelatin film beneath it. Second, the cellulose layer is also a mechanical constraint and can be used to tune the bending deformation of the structure.We performed finite element simulations with the ABAQUS. Both gelatin and cellulose are modeled as neo-Hookean material.
Beyond using straight lines as shape constraints to achieve 1D folding, we have also developed 2D folding by using either 2D constraints or distributing 1D constraints on a 2D surface. By using curved lines or surface constraints, complex structures, such as saddle shapes, cone shapes and flower shapes, were obtained. This folding principle provides us with a basic grammar to design more complex shape transformations by simply manipulating geometry and constraints.
The composite material undergoes a sequential transformation, while the sequential states to some extent are programmable. We have illustrated the three transformation sequences for a simple strip sample. Such sequences are repeatable on complex patterns as well. For instance, the flower folds sequentially. It is a unique sequential transformation, since it happens with a single, universal stimulus: the aquatic environment.