For luxury or high-end segments, prioritize genuine sheepskin, premium suede, or top-grain leather to ensure breathability and durability. For mass-market retail, synthetic fibers like polyester faux fur, coral fleece, or felt are cost-effective and versatile. Ensure the material weight (GSM) is specified; for example, high-density plush (above 280 GSM) provides a more premium feel and better heat retention. If sourcing for outdoor-use slippers, verify that the upper material has been treated with water-resistant or stain-repellent coatings.