The following patents and publications are believed to represent the current state of the art:
Published PCT Application WO 01/59975A2; U.S. Pat. No. 6,266,048; Published European application EP 0 982 676 A1; Published European application EP 1 039 365 Aw; U.S. Pat. No. 4,468,694; U.S. Pat. No. 5,969,698; Published Japan application 2000029605; Published PCT application WO 00/39663; Published PCT application WO 01/54110 A1; U.S. Pat. No. 6,175,679; Published PCT application WO 99/13395 A1; U.S. Pat. No. 5,767,842; U.S. Pat. No. 6,043,805; U.S. Pat. No. 5,909,210; U.S. Pat. No. 5,786,810; U.S. Pat. No. 5,821,92; U.S. Pat. No. 5,864,334; Published PCT application WO 00/21024; U.S. Pat. No. 6,037,882; U.S. Pat. No. 6,121,960; U.S. Pat. No. 5,789,739; U.S. Pat. No. 6,031,519; U.S. Pat. No. 5,736,976.