Table 1 Summary of advantages and disadvantages of MNMs based on different propulsion mechanisms

Chemical propulsion

Gastric acid, l-arginine, H₂O₂, glucose, urea, etc

Bubble propulsion, self-diffusiophoresis and self-electrophoresis

Chemical-propelled MNMs are simple to operate, do not require external actuation systems and can produce high movement speeds. Adapted for specific diseases such as gastrointestinal and urinary system diseases, because they can respond to special ingredients in the disease environment (such as gastric acid, urea, etc.)

Chemical-propelled MNMs usually exhibit random locomotion and a lack of directionality. In addition, continuous fuel requirements and potential risks of gas generation limit in vivo applications

[48, 49, 57, 58, 68, 98, 100, 104]

External physical fields propulsion

Light energy

Local thermophoresis

By changing the intensity or lighting direction, both velocity and direction of MNMs can be manipulated. Moreover, Photoactive MNMs can absorb light energy and trigger PDT, PTT and PCT

The tissue penetration depth of NIR is low, only 1–2 cm, which is not suitable for deep tissue application. Prolonged exposure may result in potential skin tissue damage

[55, 62, 81, 88, 110]

Ultrasound waves

Ultrasonic forces

Ultrasonic driven MNMs exhibits good directionality, strong penetration ability and outstanding biocompatibility. Furthermore, with tunable acoustic parameters (e.g., frequency, voltage), these MNMs usually demonstrate powerful propulsion

Compared with photoactivated nanomaterials, the types of acoustic activated nanomaterials are relatively few and need to be further developed

[89]

Magnetic fields

Magnetic force

Magnetically driven MNMs has good controllability and navigation, which can achieve remote, precise, and multi-degree of freedom motion control. Topical application can be recycled to avoid internal accumulation

Driving magnetic MNMs at scales of several microns is difficult owing to scaling laws. In addition, the use of magnetic MNMs will limit some clinical examinations, such as MRI

[80]