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Table 1. Comparison of preparation methods
Category Techniques Mechanisms Advantages Disadvantages

Stirring Simple
Physical dispersion Performance degradation
Grinding Low-cost
Mechanical methods
Rolling Scalable Surface oxidation
Adhesive oxide coating
Sonication Universal Low stability
Acid/base treatment Oxide removal Controllable Complex procedures
Chemical methods Electrochemical treatment Reactive wetting Stable & rapid Limited scalability
Metal coating Redox reactions Versatile Potential byproducts
Integration Physical dispersion Function enhancement
(e.g., coating, mixing, and packaging)
Other methods Complex procedures
Patterning Adhesion Good designability
(e.g., stencil and suspension printing) Shaping

final device [Figure 4E] . Additionally, LMs can also be injected into tubular molds and directly shaped into
[83]
2D or 3D coil structures . Moreover, the 3D LM coil can be fabricated by suspension printing [Figure 4F] [86]
[85]
or direct shaping of LMs at a solid state . A wide array of patterning techniques is available . However,
[19]
[24]
since these contents fall outside the main scope of our review, we will not elaborate on them here.
In summary, the fabrication of MLMs involves distinct strategies with inherent trade-offs. While mechanical
methods offer simplicity and scalability, they often compromise the fluidic and conductive properties of the
LM due to oxidation. Conversely, chemical methods provide superior stability and precision through
intricate interfacial engineering but require more complex setups. To provide a clear overview and facilitate
understanding of progress in this field, Table 1 presents a detailed comparison of these preparation methods,
highlighting their specific techniques, underlying mechanisms, key advantages, and limitations.

MAGNETIC MANIPULATION OF LMs
Based on their underlying mechanisms, magnetic manipulation strategies can be broadly classified into two
categories: magnet manipulation and Lorentz-force manipulation. The former is suited for MLMs containing
embedded magnetic particles, which can directly respond to external magnetic fields. The latter applies to
current-carrying LMs, in which the magnetic response arises from the Lorentz force acting on the moving
electrons within the LMs (i.e., electromagnetic induction). In addition to magnetic fields, LMs can be
manipulated through various other fields, including thermal, optical, and electric. The integration of these
multi-physical fields may greatly enrich the functionality of LM soft robots and substantially broaden their
scope of potential applications, which is discussed in the following section on multi-field manipulation.

Magnet manipulation
The magnetic particles embedded within LM can be broadly categorized into two types: soft magnets and
hard (permanent) magnets . Soft magnets, such as Fe or Ni, exhibit high magnetic permeability and low
[1]
coercivity. They can be easily magnetized and demagnetized by an external field. In contrast, hard magnets,
including NdFeB or strontium ferrite microparticles, possess high remanence and coercivity. Once
magnetized, they retain a strong permanent magnetic moment. When a magnetic field is applied to the MLM
entity, the field acts directly on the magnetic particles rather than the LM. However, the coupling between
particles and the surrounding LM mediates the transfer of magnetic forces to the entire MLM. This coupling
arises from viscous drag, particle aggregation, and surface tension modulation at the LM interface [42,46] . As a
result, localized particle responses to the magnetic field are transduced into motion or morphological
changes of the whole MLM entity . Moreover, magnetic fields commonly employed to manipulate MLMs
[50]
are typically generated by permanent magnets and electromagnets.