Can I use relays for heating element?
A PWM transistor switch can regulate the average current, or you can choose a better uC such as a PIC12 that supports max. current out of low 200 mA  or 150 mA high.
100mW @ 71mA = 1.4V  or 20 Ohms  choose duty cycle from 1.4/Vdd supply voltage ratio and add a flyback diode for wire inductance.
ZTX450 NPN. 200~300 Ohm base drive from 3V, PWM 40%.