Cellular Rubber Technology is the manufacturing technology behind Lundhags cellular rubber bottom. Cellular rubber is a yeast rubber with numerous micro air bubbles that provide a light and flexible waterproof material. To describe the differences in rubber, each version is numbered 2.0–5.0, with the highest number representing a more stable and insulating design. The same construction is available in both Regular and Opti lasts. The Ws versions have slightly softer soles and thinner insulation for a more customized flex.