At least one surface of an implantable medical device is concave along at least one axis such that it substantially conforms to a surface within a patient, such as the cranium, when it is implanted on that surface. In some embodiments, the surface of the implantable medical device substantially conforms...http://www.google.com/patents/US20040176673?utm_source=gb-gplus-sharePatent US20040176673 - Concavity of an implantable medical device
