Stimuli-responsive polymer gels

Stimuli-responsive materials have sparked enormous interest in recent years due to their potential applications in micro-machines, soft robots, biomedical systems, etc. [1-6]. A variety of intelligent polymeric materials such as shape memory polymers [7, 8], polymer gels [9, 10], conducting polymers [11, 12], and dielectric elastomers [13,14] have been developed for these applications. Compared to other stimulus-driven methods including pressure [15], heat [16, 17], electric field... 

Smart polymers Smart polymers include those polymers that act in response to very little change in the surrounding environment or to external stimulus, and thus they are also called environmentally sensitive or stimuli-responsive polymers or intelligent polymers. The unique character which makes these polymers intelligent and smart is their capability to respond even to minor changes in the nearby environment. These responses on one hand are fast and microscopic and on the other hand are reversible, which further enhance the imique character of these polymers. The response can be visualized or measured in terms of change in shape, solubility, sol-gel transition, surface characteristics and formation of complicated assembly of molecules, etc. [12,13]. This class of polymers will be discussed in detail later in the chapter. 

Sutani, K., Kaetsu, L, Uchida, K. and Matsubara, Y. (2002). Stimulus responsive drug release from polymer gel Controlled release of ionic drug from polyampholyte gel. Radiation Physics and Chemistry, 64,331-336. 

Gels having dangling chains Attachment of linear polymer chains on the gel particles is another approach to increase the response rate of hydrogels (Yoshida et al. 1995). DangUng chains in a gel easily collapse or expand upon an external stimulus because one side of the dangling chain is free. 

As a model to understand and to describe the processes during the response of a smart gel on changes of enviromnental properties, a two-step mechanism can be assumed (Fig. 8). In a first step, the stimulus which triggers the swelling/shrinking must permeate the gel. Heat transfer for temperature-sensitive polymers or mass transfer (ions, organic solvents) determine the rate of the first step. 

The reversible nature of DNA hybridization in response to an external stimulus causes this type of DNA-polymer hydrogels to have special properties, such as sol-gel phase transition and responsive releasing capability [93]. Thus, potential applications of these materials as label-free DNA sensing device or for controlled drug delivery have been discussed. Just a small portion of DNA is needed to achieve hydrogel responsiveness. 

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