Getting in line: Filler alignment strategies for anisotropic composite fabrication

The growing demand for high-performance materials in cutting-edge technologies has prompted intensive research into anisotropic composites, which exhibit properties that vary with direction. By aligning fillers within a matrix, it is possible to harness the distinct advantages of each filler with minimal loading, allowing for the production of lightweight, highly functional polymeric composites at a fraction of the cost of conventional engineering materials. This review examines fabrication techniques that induce anisotropy through controlled filler orientation and concentration. We categorize these methods by their principal alignment mechanisms: mechanically induced, field-induced, template-/scaffold-based, and self-assembly-driven, along with hybrid approaches. Each category offers unique benefits and faces distinct challenges in achieving targeted filler orientation and, consequently, in tuning the resultant anisotropic properties. This review also discusses the factors influencing filler alignment and how they enhance the mechanical and conductive performance of anisotropic composites.