Strong breathable sheets.
U.S. Patent 9,138,690 (September 22, 2015), "Method for Producing Air-Permeable Composite Sheet," Hiroshi Manabe (W. L. Gore & Associates, Co., Ltd., Tokyo, Japan).
A porous PTFE sheet can be breathable, allowing the transport of water vapor and other gases but not liquid water. However, porous PTFE sheet is not sufficient in compressive resistance, mechanical strength, and abrasion resistance for many applications.
Manabe developed an air-permeable composite with excellent mechanical strength and compression resistance as well as air-permeability, water repellency, heat resistance, and chemical resistance. Pores of a PTFE sheet are filled with a hardenable material, and then the hardened sheet is expanded. A porous PTFE sheet with a porosity of 30 to 70% is filled with silicon alkoxide by vacuum impregnation, cured by a sol-gel reaction, and expanded two to ten times. The permeable composite sheet has two times or more strength, compared to the original porous PTFE sheet.