Hi Ibrahim, you have discovered the exciting world of swelling hydrogels!

A couple of the key areas of use for swelling hydrogels are thought to be soft muscle like actuation and sensors. There have been many review articles and book chapters dedicated to this topic but a nice place to start could be this paper:

Article Hydrogel-based actuators: Possibilities and limitations

Or a more detailed overview here:

https://www.mdpi.com/2313-7673/3/3/15

However if you use search terms such as hydrogel actuation, hydrogel sensors polymer swelling you will find plenty of information.

There is some excellent science behind the swelling of hydrogels, the basics are covered nicely here purely in terms of crosslinked gel + liquid = swelling:

https://www.stevenabbott.co.uk/practical-solubility/polymer-swelling.php

Aside from just absorbing a liquid to cause swelling, hydrogels can respond to a broad range of stimuli including pH, temperature, electric field, ionic strength to name a few.

A term you will come across a lot is polymer 'cross-link' sometimes it can be nice to start with the Wikipedia page and get familiar with a topic:

https://en.wikipedia.org/wiki/Cross-link

I hope you enjoy learning more about the interesting world of swelling polymers!

The swelling behaviour of gelatin can be attributed to crosslinked network structure which provide for ample empty spaces or voids which accommodate lot much of liquid when in contact thereby expand as you say.

Hi Adrian,

Thank you for your informative reply, very interesting articles! any similar experience with gelatin or agar??

Dear Dr. Kulkarni,

Thank you very much for your reply.

Hi Ibrahim, I have never worked with gelatin or agar specifically. I think their gelling mechanisms are similar though, they are both carbohydrates which tangle up upon cooling which creates physical crosslinks and allows the normally water soluble carbohydrates to hold together and become a gel instead of just a dissolved polymer. The tighter the crosslinking the less the gel will swell but sometimes the polymers urge to swell (driven by the flory-huggins paramter) can be so strong that the gel rips itself apart.

I'm not sure what factors affect the swelling levels of gelatin and agarose. This article mentions that pH (a common gel stimulant) affects the strength of an agarose gel but no mention of the swelling properties.

http://www.agargel.com.br/agar-tec-en.html

In a quick google search I found people are further creating chemical crosslinks in gelatin to increase its stability as the physical crosslinks are likely not as permanent or stable as chemical ones.

Did you have a specific issue with your gelatin/agar you are trying to solve? Do you want them to swell less? Swell more? Be more stable?

Hi Adrian, I am really thankful for your help in this regard. Originally, I wanted to shrink the gelatin, which I thought would normally happen during gelation. Surprisingly, almost no noticeable shrinking happened. Now, I changed my plan to swell the gelatin (as much as possible). After reading several articles about structural characterization of gelatin, I could not really reach a clear methodology to make it swell (either spontaneously or with induction). I tried to use the osmolarity of the solvent hoping it will swell when it contacts water, but nothing at all happened. In the previous example (in the question), it showed considerable swelling on contact with water, but this only happened after histological processing (including paraformaldehyde, and sequential dehydration).

This publication has a nice description of swelling specifically gelatin but they use the term 'actuation' rather than swelling. I think gelatin has a lot of complexities so I'm hesitant to give any definitive descriptions of its behavior. However, it looks like it is a pH sensitive gel so I would start with acidic and basic solutions. If it is an anionic gel, it will swell in basic solution and shrink in acidic solution.

Article Polyelectrolyte Soft Actuators: 3D Printed Chitosan and Cast Gelatin