Effect of shell material on micromotor behavior. (a) Motion of Mg-TiO2 and Mg-parylene micromotors and (b) their corresponding speeds. (c) In the mid-lifetime of the Mg-parylene micromotor, the time required for Gear 1 bubble ejection is very slow (more than 30 times that of a Mg-TiO2 micromotor) and the generated bubbles are typically larger than the micromotor. (d) The Mg surface inside the micromotor is concave (i) and the bubble have more space to grow, thus (ii) the ejected bubble can grow larger than the motor size. (e) Circle fitting to a bubble (i) fully inside and (ii) partly outside the micromotor.