Partial substitution is quite possible and rather non-toxic: a 70 kg person contains on average 0.36 g of rubidium, and an increase in this value by 50 to 100 times did not show negative effects in test persons.
pˈɑːɹʃəl sˌʌbstɪtˈuːʃən ɪz kwˈaɪt pˈɑːsᵻbəl ænd ɹˈæðɚ nˈɑːntˈɑːksɪk: ɐ sˈɛvənti kˌeɪdʒˈiː pˈɜːsən kəntˈeɪnz ˌɔn ˈævɹɪdʒ zˈiəɹoʊ.θˈɜːɾi sˈɪks dʒˈiː ʌv ɹuːbˈɪdiəm, ænd ɐn ˈɪŋkɹiːs ɪn ðɪs vˈæljuː baɪ fˈɪfti tə wˈʌnhˈʌndɹɪd tˈaɪmz dɪdnˌɑːt ʃˈoʊ nˈɛɡətˌɪv ɪfˈɛkts ɪn tˈɛst pˈɜːsənz.