They clever guys at California Near Space Project, known for their successful trans Atlantic crossings, are currently developing super pressure balloons to aid large payload long distance HAB flights.
SPEED stands for Super Pressure Evolution Efficiency Design. CNSP say on their web site, “The primary goal of the SPEED Project is to create successful Super Pressure Balloons. The evolution of efficiency and design with a Super Pressure Balloon requires extensive testing of candidate films including, tensile strength, film weight, gas permeation, seaming techniques, solar heating and geometric design. The project leader of SPEED is Lee Meadows; he has designed and built several custom Super Pressure Balloons. Most recently, flight # CNSP-21 that circumnavigated the world. CNSP will continue to refine the techniques used in our SPEED Project to push the evolution of efficiency and design in the Super Pressure Balloons we build. “
So far this year CNSP have done 6 super pressure balloon flight.