Technology convergence network of PRIs.

(a) from 1997 to 2001, (b) from 2002 to 2006 and (c) from 2007 to 2011. Consult Table 3 for the symbols representing the technology fields. The node size is proportional to the number of patent applications corresponding to the field of technology; however, it should be noted that each node has the minimal size for the sake of visibility.