Polymers can be both isotropic and anisotropic, depending on their molecular arrangement and processing. Isotropic polymers exhibit uniform properties in all directions, typically seen in amorphous polymers where the molecular chains are arranged randomly. This random arrangement results in uniformity in mechanical, optical, and thermal properties, regardless of the direction of measurement. On the other hand, anisotropic polymers, such as crystalline or semi-crystalline polymers, display directional dependence in their properties. Their molecular chains are arranged in a more ordered fashion, leading to variations in characteristics based on the direction of observation. Factors like the rate of cooling from the melt, applied stress during processing, and the presence of additives can influence the degree of isotropy or anisotropy in polymers.

The amino acid most similar to histidine is arguably tryptophan, based on their molecular structure and function. Both contain a heterocyclic aromatic ring, imparting unique properties to these amino acids within proteins. Histidine is renowned for its imidazole side chain, enabling it to participate in enzyme catalysis and act as a proton donor or acceptor at physiological pH levels. Tryptophan, with its indole side chain, shares this capacity for ring-based interactions, although its role is more aligned with stabilizing protein structures and facilitating signaling pathways through its ability to absorb and emit UV light. Despite differences, their shared aromatic character and involvement in biochemical signaling render tryptophan the closest analogue to histidine in terms of side chain functionality and physicochemical properties.

Polyvinyl alcohol (PVA) is a synthetic polymer that is water-soluble and often used in a variety of applications, including textiles, adhesives, and paper treatments. Unlike some industrial chemicals, PVA itself is not considered corrosive to metals or skin. Its inert and non-toxic nature makes it suitable for use in packaging, pharmaceuticals, and even in some food applications. However, it's important to consider the context in which PVA is used. In formulations where PVA is mixed with other substances, the overall mixture's corrosiveness could be influenced by the additional components rather than the PVA itself. For direct application or handling, PVA does not pose the corrosive hazards associated with acids, bases, or other aggressive chemicals.