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Revision as of 23:54, 6 January 2009 by MichaelPetralia (Talk | contribs) (Final Project)

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About me

I am working on soft mechanical systems at the Harvard Microrobotics Laboratory. My research focuses on creating changes in stiffness and damping properties, macroscopic motion, and force generation using 'soft' materials. The buzz word is artificial muscles, but the scope is much broader. I'm thinking about how we can create compliant devices that out perform the typical 'hard' systems engineers usually design.

Fun facts on soft matter

Typically, we think of changing the degree/type of cross-linking in order to change the mechanical properties of a polymer. Lengthening of a polymer is accomplished by aligning the polymer chains. It seems that there are polymers where the monomer-monomer interactions are not covalently bonded and thus the lengths of the polymer chains themselves can change.

“. . . in other long-chain objects the subunits are joined not by covalent bonds, but by physical ones. 
Examples of this are the giant worm-like micelles formed in some amphiphile solutions, and the long 
chains of compact protein molecules which constitute, for example, actin filaments. Such objects are 
sometimes called ‘living polymers’; their characteristic is that they can change their length in response 
to changes in the environment. This contrasts with the more usual covalently linked polymers, in which 
the length of the molecules, or the distribution of lengths, is fixed during the polymerization process.”
page 73, Jones Soft Condensed Matter

Additionally, we do not need to be so rigid in our thoughts on cross-linking.

“Linear polymers may be connected by physical, rather than chemical, bonds, giving a thermoreversible 
gel such as a gelatin. ” 
page 95, Jones Soft Condensed Matter

Final Project

One 'soft' actuation technology I'm looking into are polymer gels. Gels are materials that fit somewhere between a solid and a liquid, consisting of a polymer network swollen with an interstitial fluid. The properties of the gel are defined by the polymer network, the interstitial fluid, and the interaction between them.

Jones tells us

“A gel is a material composed of subunits that are able to bond with each other in such a way that one
obtains a network of macroscopic dimensions, in which all the subunits are connected by bonds. If one 
starts out with isolated subunits and successively adds bonds, one goes from a liquid (a sol) to a material 
with a non-zero shear modulus (a gel). A gel has the mechanical properties characteristic of a solid, even
though it is structurally disordered and indeed may contain a high volume fraction of liquid solvent.” 
page 95,  Jones Soft Condensed Matter, 3

All gels process the unique ability to undergo abrupt changes in volume, often as a result of small changes in external conditions such as temperature, pH, electric fields, and solvent and ionic composition [1]. This phase change is a result of a shift in which forces dominate (entropic, attractive, repulsive).

There is some criticism in the soft robotics community about the usefulness of polymer gels for artificial muscle type technology. A recent review article by Madden [2] intentionally omitted their consideration. Madden claimed that the response time is typically slow (anywhere from seconds to minutes--I've seen it as short as fraction of a second and as long as weeks) and they are relatively weak (~100 kPa--I'm assuming he means this is the tensile stress). Despite these short comings, I believe they still have merit because of the breadth of stimuli that can be used to activate them (light, heat, pH, electric and magnetic fields, ionic strength) and the control we have over their swelling properties. What I lack is a good understanding of the physics and chemistry at work in polymer gels necessary to judge whether this technology is worth investigating.

So I picked up de Gennes classic Scaling Concepts in Polymer Physics to see if I could learn a few things.

Ideas without a home

The change in physical shape of polymer gels is dominated by diffusion, and over long time scales will be isotropic. In order to create useful motions, it is likely that the gels will be placed in systems which constrain part of their volume expansion, or geometries which will create anisotropic swelling over short time scales.


[1] Brock, MIT A.I. Memo No. 1330 (1994)

[2] Madden et al., IEEE Journal of Oceanic Engineering 29, 706 (2004)

[3] Jones, Soft Condensed Matter (2002)

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