Progress through Mechanics: Soft Active Materials
Copyright ©2007, Robert M. and Mary Haythornthwaite Foundation
Xuanhe Zhao
Graduate Student at Harvard University
Cambridge, MA
xzhao@seas.harvard.edu
Nature chooses polymers as the material for life: the most important components of
a living cell (proteins, carbohydrates and nucleic acids) are all polymers [1]. Polymers and
polymeric gels are representative of “soft materials” as opposed to “hard materials” (e.g.,
metals and ceramics). Soft materials can be made active in that they can undergo large
deformation in response to diverse stimuli, including mechanical stresses, electric fields, and
trace amount of enzymes.
Soft active materials (SAMs) are being intensely studied in recent years.
Examples include dielectric elastomers, shape-memory polymers, and stimuli-responsive gels.
Once a voltage is applied to a layer of VHB acrylic elastomer, one kind of dielectric
elastomers, a strain over 200% is easily achieved [2, 3], two orders of magnitude larger than
the maximum strain from piezoelectric ceramics [4]. Polymers have been developed to
change among three shapes with temperature variation between 30ć to 60ć[5]. Few
opponent shape-memory alloy or ceramics exists. In response to a small change of pH, a
hydrogel may undergo an abrupt volume change and hence hold or release a large amount of
solvent [6]. The hydrogel may degrade eventually in the human body [7]. These examples
illustrate the merits of SAMs: large deformation, high sensitivity, and biocompatibility. As a
result, SAMs have been (or will be) playing essential roles in many applications. Some
scenarios that may be realized in the near future are briefly described below.
Soft actuators: Conventional actuators have limitations in one or more
performance parameters [8]. On the other hand, dielectric-elastomer actuators, exhibiting
high actuation strain (>200%) and stress (0.1 to 2 MPa), fast response times (<1 ms), and
potentially high efficiencies (80 to 90%), represent a general-purpose actuator technology
with excellent overall performance [2]. Future space exploration requires robots that are
robust, lightweight, efficient, and easy to control. It is expected that the use of
dielectric-elastomer actuators will make a fundamental improvement on future space robots
[9]. Cancer surgeries guided by a magnetic resonance imaging (MRI) and a remote-controlled
robot have been long dreamed of, because the robotic needle could be aligned directly with
the tumor as seen by the MRI. But until now, there has been no commercial device capable of
operating inside MRI without distorting the images, which depend on a strong magnetic field.
As dielectric-elastomer actuators are nonmetallic, they work well inside MRI scanners
without affecting the images [10]. Consequently, robots based on dielectric-elastomer
actuators pose one most promising solution for MRI-compatible surgery. In both examples,
an accurate electromechanical model of dielectric elastomers is of essential importance to the
design of actuators. In addition, stimuli-responsive hydrogels can act as actuators to control
microfluidics [11] and adaptive microlenses [12].
Controlled drug delivery: Delivery systems that can target drugs to specific body
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sites or precisely control drug release rates have had an impact on nearly every branch of
medicine [13]. The use of SAMs in drug-delivery technology begins an era of “polymer
therapeutics” [14]. For example, biocompatible hydrogels sensitive to pH in blood have
provided a potential autonomous treatment of insulin-dependent diabetes [15]. The process
works as a feed-back loop. Following the enzyme reaction that converts glucose to gluconic
acid, the pH in blood is temporarily lowered, causing the hydrogel to swell and enhancing the
release profile of insulin. Once insulin is released, the glucose level drops, resulting in a pH
increase that stops further release of insulin. To precisely control this process, a quantitative
understanding of the deformation of hydrogel and diffusion of insulin is necessary. As
another example, SAMs for targeted drug delivery have been achieved by immobilizing drug
molecules linked to a polymeric backbone via enzyme-cleavable linkers [14]. Consequently,
a cancer-specific enzyme secreted by tumor cells can be used to break the linkers and trigger
the release of the therapeutic agent.
Functional self-assembly: SAMs can also be used for directed assembly of
functional structures. One example is the spontaneous folding of a 2D elastomer sheet,
patterned with magnetic dipoles, into a simple 3D electric circuit [16]. The path of the
self-assembly is determined by a competition between mechanical and magnetic interactions.
This simple example demonstrates the potential for fabrication of complicated 3D
micro-electro-mechanical systems (MEMS) and microelectronic devices via self-assembly of
SAMs.
In summary, SAMs represent a variety of technologies for promising applications.
The progress in these technologies will depend on how well we understand the basic
properties of SAMs. Inside a SAM, multiple physical fields tend to coexist and interact,
including electric, magnetic, temperature and chemical-potential fields. These fields are in
general coupled with large deformation of the materials. Consequently, the rise of SAMs
has created great opportunities for mechanicians of our time. For example, mechanicians
are particularly well equipped to answer some of the crucial questions in studying SAMs:
What is the effect of mechanical forces on the electric-breakdown voltage of a
dielectric-elastomer actuator [17, 18]? How do the various fields inside an elastomer or a
hydrogel interact with each other and induce large deformation [19-24]? How does
viscoelasticity and diffusion affect the durability and performance of devices? How do we
model the effect of enzymes on swelling of a hydrogel? The list continues and the questions
are intriguing. They call for fundamental advances in mechanics.
While nature has made its choice of soft materials for life, human beings are
inventing ways to complement our life with soft active materials. Through mechanics, we
mechanicians will make unique contributions to the progress of the exciting new
technologies.
Xuanhe Zhao
September 17, 2007
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