Drones are unmanned aerial vehicles (UAVs) with a wide range of applications, including product delivery, asset inspection, search and rescue, law enforcement and military services, disaster management, and emergency medical services [50,51,52,53,54,55]. Drones are emerging as safe alternatives to humans in applications involving inaccessible environments or dangerous scenarios [56]. In cases of medical emergencies, when a person is stuck in remote locations and an ambulance is unable to reach a patient in time, medical drones are being used for emergency medical services [53,57]. Door-opening drones are unmanned aerial vehicles (UAVs) equipped with the capability to open doors.

The Lemur-2 drone by BRINC has a blade attachment to break glass and is able to push doors that are ajar [58]. The drone could be user-operated or in autonomous mode. The onboard 3D LIDAR sensor makes real-time floor plans. Lemur-2 was developed for use by law enforcement personnel and is National Defense Authorization Act (NDAA) compliant. Although Lemur-2 is able to break glass panes to access buildings, it lacks the ability to turn doorknobs.

A team of researchers at Purdue University has developed a drone called the BoomCopter that is able to open and close doors, flip switches, and attach sensors to walls. The BoomCopter was designed to perform tasks in environments that would be considered dangerous or inaccessible to humans. The BoomCopter (see the illustration in Figure 1) utilizes a tri-rotor design, allowing it to hover in the same way as the more common quad-rotor design [59,60].

The BoomCopter drone features a horizontal robotic arm with a propeller mounted at 90 degrees to move the drone back and forth as it hovers. The robotic arm is equipped with different end-effectors to accomplish specific tasks. The BoomCopter can be operated either remotely by a user or autonomously using on-board sensors, cameras, and processors. For autonomous mode operation [61], the BoomCopter drone could be trained to recognize items such as door handles using computer vision techniques and be able to open doors. Another group demonstrated the capability of a drone to push a rolling cart and a door [62]. In a related study, a team from ETH Zurich demonstrated a planning and control method that enables drones to manipulate articulated objects with the least amount of operator-provided priors [63]. Recently, the group presented a reinforcement learning method for drones to learn motion behaviors for a manipulation task such as a door-opening task [64].

The tiny drones known as FlyCroTugs provide an intriguing example of drone technology in action. Researchers from Stanford University and Ecole Polytechnique Federale de Lausanne (EPFL) created the FlyCroTugs, which are tiny drones that collaborate to carry out activities in hazardous or unreachable places for humans. By coordinating their movements, the quadrotor FlyCroTugs drones can lift or transport heavy objects up to 40 times their own weight. These tiny drones can attach to different surfaces and apply force via the winches and replaceable adhesives on their bases. The ability of two FlyCroTugs to coordinate their activities in order to open doors was demonstrated [55,65,66,67]. While the second tiny drone fixed itself to the ground and pulled open the door, the other tiny drone turned the doorknob. FlyCroTugs’ ability to anchor is made possible by the drone’s base using replaceable adhesives. The adhesives could contain ridged silicone (inspired by Gecko feet) to cling onto glass and microspines for slicing through tough surfaces such as stucco, carpet, or rubble [66].

Table 1 lists the technical specifications of the door-opening drones. Table 2 summarizes the door-opening drones. The Lemur-2 drone is commercially available and costs around USD 10,000 to 20,000 [68]. The BoomCopter, FlyCroTug, or other drones discussed here are still in the research and development phase and are not available on the market.