Nonfibrillar

Nonfibrillar is an adjective used in biology and materials science to describe substances or structures that do not form fibrils. In contrast, fibrillar components assemble into long, thread-like fibers. The term is often applied to proteins, polymers, and extracellular matrix components to indicate the absence of fibrous morphology in their organization or behavior.

In the biology of connective tissue, many collagens are fibrillar, forming organized fibrils that provide tensile strength. Other collagens are considered nonfibrillar, such as types IX, XII, and XIV, which are known as FACITs (fibril-associated collagens with interrupted triple helices). These nonfibrillar collagens do not form continuous fibrils themselves but attach to the surface of existing fibrils and modulate fibril diameter, cross-linking, and interactions with cells and other matrix components.

Nonfibrillar molecules also include various proteoglycans, glycoproteins, and small globular proteins that occupy spaces between fibrils and influence hydration, signaling, and matrix organization. In a materials science context, nonfibrillar materials describe substances that form dense networks or particulate phases rather than elongated fibers, such as certain colloidal gels or isotropic polymer matrices.

Usage of the term is context-dependent and may refer to structural morphology, assembly behavior, or functional role within a composite system. In research, distinguishing nonfibrillar from fibrillar components helps describe mechanical properties, interactions, and biological function.