External Stimuli-Responsive Polymers

Delivery systems in which the drug release rates can be activated by an external stimuli are still largely experimental. The mechanisms include irradiation (photo], magnetism, and ultrasound. 

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Polymers which change their visible optical properties in response to external stimuli have aroused the growing interest of researchers. According to the external stimulus, these polymers are classified as ... 

The second section of the book deals with the use of synthetic polymers for the purpose of isolating biologicals (bioseparation). The chapter on affinity precipitation describes the use of stimulus-responsive polymers for this purpose. Upon the change of a certain external parameter like the temperature or the pH, such polymers change their behavior, e.g., their solubility in water, in a very abrupt manner. If the polymer is linked to an affinity mediator, any target molecule can be captured and co-precipitated. The issue of stimulus-responsive (sometimes also... 

Fundamentally, surfaces can interact with proteins in a nonspecific or a specific way. Although nonspecific interaction of proteins with surfaces is undesirable in most cases, it can be of interest to switch the capability of adsorption using an external stimulus. Alternatively, it might be advantageous to switch the specific interaction of a surface with a distinct protein, while keeping it repellent for all other biological species under all conditions. For either approach, polymers are important building blocks that can mediate the stimuli-responsive properties. 

The era of biomimetic peptide- and sugar-based polymer vesicles has just begun and seems very promising. Bioinspired vesicles are mainly applied for drug deliv-ery/release and the fabrication of composite materials, but could readily be used for biomimetic materials science, biomineralization, and so on. Especially interesting are smart vesicles changing properties in response to an external stimulus (temperature, pH, ions). 

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. 

Presently, the study of shape memory polyurethane (SMPU) has been widely conducted. For common elastic fibers, the elasticity should be defined as the instant recoverability of the length on release of the deforming stress. The recovery in shape memory fibers is the ability of deformed fibers to recover under external stimulus such as heat or chemicals. In this case, the external stimulus is a must. Figure 3.1 shows the net-points and switches which response to the stimulus (Hu et al., 2012). The elongation of shape memory polymer fiber are a little less than spandex, but mechanical... 

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. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

Piezoelectric polymers have been known for more than 40 years, but in recent years they have gained repute as a valuable class of smart materials. There is no standard definition for smart materials, and terms such as intelligent materials, smart materials, adaptive materials, active devices, and smart systems are often used interchangeably. The term smart material generally designates a material that changes one or more of its properties in response to an external stimulus. 

Stimuli-responsive polymers consist of a class of smart materials that exhibit a physical response to changes in external conditions. Such stimuli include changes in pH, ionic strength, solvent polarity, and temperature, as well as mechanical force or electric fields. On the basis of their ability to switch conformations, stimuli-responsive polymers are being applied as sensors, actuators, and transducers (e.g., mechano-electrical or mechano-optical). Nanoporous membranes can be functionalized with stimuli-responsive polymers to modify their permeability, that is, to reversibly open and close the pores upon a given stimulus. ... 

When stimuli-responsive polymers are attached to a surface, changes in the physicochemical properties of the surface layer that result from an external stimulus can be assessed by AFM in situ. Micropattemed thermoresponsive polymers, such as poly(A-isopropylacrylamide) (PNIPAm), were characterized via AFM height imaging and the brush adhesiveness measured by force spectroscopy during... 

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