Syringes can be acquired by sending requests to your friends. After you have entered the hospital, just click on the "Ask For More" button below the number of your syringes to send out requests.
It is recommended to ask for syringes beforehand, as the special and powerful units tend to be resurrected with only syringes or cash, such as most of the mech units.
- Once level 15 and 20 are reached, the Field Hospital and Mechanical Garage will be unlocked respectively. These buildings will allow you to access units (Medic and Engineer) which enable you to heal your damaged units.