(2015) 409-17) . Natural polymers have appeal because they display good biocompatibility, which makes them useful for biomedical applications. However, a disadvantage to natural polymers is that researchers can have more control over the structure of synthetic polymers and so can design those polymers for specific applications. Multi-stimuli polymers Polymers can be designed to respond to more than one external stimulus, such as pH and temperature. Often, these polymers are structured as a copolymer where each polymer displays one type of response. Structure pH sensitive polymers have been created with linear block copolymer, star, branched, dendrimer, brush, and comb architectures. Polymers of different architectures will self-assemble into different structures. This self-assembly can occur due to the nature of the polymer and the solvent, or due to a change in pH. pH changes can also cause the larger structure to swell or deswell. For example, block copolymers often form micelles, as will star polymers and branched polymers. However, star and branched polymers can form rod or worm-shaped micelles rather than the typical spheres. Brush polymers are usually used for modifying surfaces since their structure doesn’t allow them to form a larger structure like a micelle. Response to change in pH Often, the response to different pH values is swelling or deswelling. For example, polyacids release protons to become negatively charged at high pH. Since polymer chains are often in close proximity to other parts of the same chain or to other chains, like-charged parts of the polymer repel each other. This repulsion leads to a swelling of the polymer. Polymers can also form micelles (spheres) in response to a change in pH. This behavior can occur with linear block copolymers. If the different blocks of the copolymer have different properties, they can form micelles with one type of block on the inside and one type on the outside. For example, in water the hydrophobic blocks of a copolymer could end up on the inside of a micelle, with hydrophilic blocks on the outside. Additionally, a change in pH could cause micelles to swap their inner and outer molecules depending on the properties of the polymers involved. Responses other than simply swelling and deswelling with a change in pH are possible as well. Researchers have created polymers that undergo a sol-gel transition (from a solution to a gel) with a change in pH, but which also change from being a stiff gel