Sitka Gear Hunting Base Layer Shirt, Optifade Forest, X-Large. The Core Series is made of 100% polyester bi-component technology that actively moves moisture away from the skin. ATB-100 Silver Scent Inhibiting Technology. Practically essential to any and every Sitka System, the Core base layer series is designed purely for moisture wicking and comfort, utilizing 4-way stretch fabric. Polyester Bi-Component Fabric. Wicks moisture away from skin. Welded Bicep Pocket.