the present invention provides novel filled silicone compositions comprising a nano - particulate metal oxide filler selected from nano - particulate monoclinic alpha - phase bismuth oxide , nano - particulate erbium oxide , and mixtures thereof . the new composite compositions exhibit a combination of outstanding performance characteristics in both the cured and uncured states . thus , the uncured formulations are typically free flowing liquids which are self - leveling , which de - gas readily under vacuum , are readily colorable through the addition of pigments , and which cure at temperatures of about 60 ° c . the cured compositions possess outstanding acoustic characteristics which make them ideally suited for use in applications such as acoustic lenses for medical imaging and therapy . for example , the acoustic impedance of compositions provided by the present invention is closely matched to the acoustic impedance of human tissue . this may result in an enhancement of ultrasound transmission efficiency between the lens of an ultrasound probe comprising such a composition and the patient .