This week the work of Chinese artist Ai Weiwei, photographed by Hugo Tillman above, is on view on both sides of the Atlantic, from his bronze "Circle of Animals/Zodiac Heads" sculpture at New York's Pulitzer Fountain, to his nature-inspired "Rock" installation in Berlin's Neugerriemschneider gallery, to a new retrospective at London's Lisson Gallery. With the global tribute comes international concern for the social activist, who has been missing since being detained by Chinese authorities at Beijing Airport on April 3. Protests have been mounted around the world to demand the release of the artist, best known for co-designing the Bird's Nest Olympic Stadium in Beijing and for ambitious installations such as last year's "Sunflower Seeds" at the Tate Modern, which filled the museum's Turbine Hall with 100 million hand-painted porcelain seeds. "Weiwei has always maintained that each individual has to set an example in society. Your own acts tell the world who you are and at the same time what kind of society you think it should be," says the artist's friend and collaborator, Hong Kong–based independent curator Melissa Lam, who has been working with Ai on projects including a film about his life with French art and architecture duo MAP Office. "These convictions that intertwine with his art are what make his work resonate with us," she adds. Visit the Free Ai Weiwei Facebook page here.