Camouflage Through Colloidal Gold Nanoparticles

A gel can be defined as a “jelly-like” substance, one that is more or less a solid material, and is formed from a colloidal solution (Wikipedia). Gels exhibit many mechanical properties similar to that of natural rubbers - a high deformity and reversibility (Phase transitions of gels). Microscopically, gels consist of a three-dimensional flexible cross-linked polymer network with a solvent that fills in the gaps (Critical kinetics of volume phase transition of gels). Depending on the particular solvent used, the polymer chains can either repel each other and cause a more dense gel, or repel and cause a less dense gel (Phase transitions of gels, see figure as well).

If our goal is to change the dimensions of a particular gel, or equivalently, change the volume, we can say that we are looking for a phase transition. Phase transitions in gels are predominantly a result of competition among two types of forces in the gel: repulsive forces, which act to expand the gel, and attractive forces, which act to contract the gel (Collapse of gels in an electric field). The most overpowering repulsive force within the gel is the Coulombic interaction between elements of the polymer network having the same charge. This can be induced by ionizing the polymer chains via a potential difference. Also contributing to the expanding is the osmotic pressure created by the counterions. The attractive forces can vary greatly - some of the predominant forces are van der Waals, hydrophobic interaction, ion-ion interactions between opposite kinds of charge, and hydrogen bonding (Phase transitions of gels pg 250).

The specific kinetics of the gel expansion/contraction is directly proportional to the flexibility of the polymer network as well as the viscosity between the network and the solvent (Phase transitions of gels, refs 123-126). The time for a complete phase transition is proportional to the square of a linear size of the gel (Phase transitions of gels, ref 123), which has been confirmed experimentally (Phase transitions of gels, refs 123-125). One of the key features of phase transitions in gels is that it is isotropic - that is, the expansion is uniform to the proportions in all directions (Phase transitions of gels, pg 259).

The state of a gel at any point in time can be determined by three variables: the reduced temperature (tao), the osmotic pressure (pi), and the network density (phi) (Phase transitions of gels, pg 263, more info there).

Smart gels, or intelligent gels, or smart polymers (etc…) are defined by exhibiting a change in properties when introduced to a stimulus. The stimuli can vary greatly, as can the result it produces. Some common stimuli are temperature, pH, visible light, UV light, external pressure, fluid composition, antigens, electric fields, and magnetic fields (Preparation and Characterization of fast response microporous hydrogels).

The stimulus that I am currently investigating is electricity, which will be actuated into a change in volume. It has been shown that it is possible to initiate phase transitions subsequent to the application of an electric field across a gel. There are several ways in which this can occur - the most broad and encompassing explanation is that a stress gradient is created along the direction of the applied field, which is due to the electrical responsiveness of the charged sites of the gel (Collapse of gels in an electric field). This summarizes a point made previously - that the phase transition of the gel can be controlled via the Coulombic interaction between the polymer chains in the network, which can be changed via changing the degree of ionization across the constituent molecules (changing the ratio of positive and negative charges, and correspondingly resulting in the gel to expand or contract). This can be explained in terms of the energy gain from the formulation of ion pairs in the contracted state of low polarity (competing with the expanded state in which most of the counterions are dissociated). An increase in ionization increases the thermodynamic advantages of the collapsed state with ionomeric multiplet structure over the swollen polyelectrolyte state (Weakly charged polyelectrolytes).

Another way that we can imagine a phase transition from an electrical stimulus is as follows. Since the polymer network is negatively charged, the positively charged surfactant molecules tend to bond to its surface. This decreases the difference in osmotic pressure between the gel interior and the solution, causing localized contraction. We are able to focus the surfactant binding selectively - and therefore control the contraction - to one side of the gel by introducing an electric field (“A polymer gel with electrically...").

It is also possible to control the pH of the gel indirectly using an electric field through a process called electrodiffusion. This same process can be used to control the inter-membrane ionic strength. Furthermore, electrokinetic processes can cause mechanical deformations in the hydraulic permeability of polyelectrolyte gels subjected to DC electric fields ("Pulsed and self-regulated drug delivery" Pg 50).