Structure of the thermal gradient chip. (A) Thermally controlled silicon chips were produced with both 20- and 100-island structures. The spacing and arrangement for the 100-island chip are shown. (B) The location and size of the heating and thermal sensing regions within a single island are illustrated. (C) In cross section, the relative thickness of island and thermal-isolation membrane can be seen. (D) Photographic detail of an island as shown in B.