The present invention generally relates to devices and techniques associated with diagnostics, therapies, and other applications, including skin-associated applications, for example, devices for delivering and/or withdrawing fluid from subjects, e.g., through the skin. In some embodiments, the device includes a system for accessing an extractable medium from and/or through the skin of the subject at an access site, and a pressure regulator supported by a support structure, able to create a pressure differential across the skin at at least a portion of the access site. The device may also include, in some cases, a sensor supported by the support structure for determining at least one condition of the extractable medium from the subject, and optionally a signal generator supported by the support structure for generating a signal relating to the condition of the medium determined by the sensor.