I found this paper to be very helpful on the topic:

Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS)
elastomer
Jo, Byung-Ho (Univ of Illinois at Urbana-Champaign); Van Lerberghe, Linda M.;
Motsegood, Kathleen M.; Beebe, David J. Source: Journal of
Microelectromechanical Systems, v 9, n 1, 2000, p 76-81

They talk about bonding PDMS and aligment techniques.

Hope that helps,
Joe Grogan


Quoting Terry Barchfeld :

> First of all, thanks to everyone who has helped out
> with answering my questions about this so far. Below
> you'll find a more detailed question from the person
> I'm working with this summer. Maybe this will help
> give all of you a better idea about what we're working
> on.
>
> "I was wondering if you have worked with PDMS bonding
> of TWO patterned pieces (rather than bonding a channel
> patterned PDMS piece to a flat, non-patterned PDMS
> piece)? I'm looking for technique tricks. Times and
> temperatures of curing, and ratios, if you have it. I
> use the Dow Corning Sylgard 184 Elastomer and Curing
> Agent. Any tips with something better to use? Thanks
> so much for your work and any advice, take care,
> Andrea Wesser
> Bio-MEMS and Nano-Fabrication
> University of Central Florida"