In 2004 the United Kingdom opened the world's first stem cell bank. The UK Stem Cell Bank takes donations of ethically approved stem cell lines, tests them, grows larger stocks, and re-distributes the material internationally. As such the Bank has an important guardianship role in the international movement of human embryonic stem cell lines. It also enacts a particular future vision of stem cell science. Its strategies involve a complex temporal interplay: securing accounts of the past (both technical and social), while validating the regulatory legitimacy of the present. We analyse the centrality of trust, social networks, and wider public legitimacy in the Bank's work. It is important to recognize the ways in which the Bank makes these social relationships visible, and in some cases durable, through their embodiment in documentary form. These practices are essential to the Bank's particular vision of the future of stem cell science.