DNA tandem repeats are frequently found in eukaryotic genomes . High-copy DNA repeats can serve as building blocks of complex DNA structures, but the in vitro synthesis of DNA repeats has been challenging due to complicated procedures and the high cost . Here, a new, simple method is developed using the strategy of blocking polymerase chain reaction for highly efficient DNA repeat expansion (BPRE). With BPRE, dsDNA fragments composed of more than 40 copies of the repeat sequence can be quickly produced, while the cost is reduced by at least 90% . As a typical application, reannealing of the dsDNA repeats generates elastic hydrogels, which shows a high capacity for doxycycline absorption and prolonged release.