Hybrid life support systems combining physicochemical and biological algae based subsystems are in great interest for the midterm future of manned spaceflight including extensive ground missions on Moon and Mars. Whereas many possible systems have been theorized and tested in laboratory conditions, no experiments on realistic system level have been performed in space. The DLR experiment PBR@LSR (Photobioreactor at the Life Support Rack, former name PBR@ACLS) is set to give a first technology and performance demonstration on board the ISS in the Destiny module in 2018 by combining an algae based photobioreactor with the carbon dioxide concentrator of ESA’s Advanced Closed Loop System built by Airbus DS. This paper shows the design process of the ongoing flight hardware development. The prototype design is detailed in conjunction with the operational requirements, safety limitations and scientific needs. The connection to the carbon dioxide concentrator enables the use of carbon dioxide from the cabin. The algae medium loop consists of different components of specific functionalities (pumping, illumination, nutrient supply, temperature control, etc.) to allow cultivation of the microalgae Chlorella vulgaris. The progress of ground experiments is presented and the derived decisions for the system design and setting parameters are explained. The paper concludes with an overview of reached and open milestones to the flight design of PBR@LSR.