The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 4,134,405; 5,282,824; 5,387,235; 6,740,121; 6,939,374; 6,946,002; 7,025,791; 7,122,058; 7,160,312; 7,175,669; 7,204,842; 7,267,694; 7,288,101; 7,329,285; 7,335,210; 7,347,875;
U.S. Patent Application publication Nos. 2004/0220682; 2004/0249362; 2005/0075622; 2005/0080395; 2005/0080431; 2005/0080491; 2005/0085923; 2005/0125020; 2005/0125075; 2006/0009858; 2006/0161187; 2006/0235448; 2007/0005147; and
International Patent Application publication WO 2008/030403.