An apparatus include an elastomeric barrier that circumscribes a PPG sensor on an underside of a housing of a wearable device. The elastomeric barrier is configured to maintain a position of the housing on the human's skin to fix the PPG sensor at a location on the human's skin to sense the function of a human organ at the location on the human's skin. The apparatus can include an enclosure that attaches to and at least partially encloses the housing of the wearable device. The apparatus can further include a band with tightly spaced ridges, and a ratchet configured to apply micro-sized adjustments for applying tension by the band to secure the wearable device to the location on the human's skin.
