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Page 2 of 26 Blewitt et al. Soft Sci 2024;4:13 https://dx.doi.org/10.20517/ss.2023.49

humans, and thus, a robotic solution is required. In recent years, considerable research has delved into the
[1]
use of in-pipe inspection robots (IPIRs) to aid in this maintenance . For large-diameter pipelines,
(> 75 mm Ø) solutions such as pipe inspection gauges exist and have demonstrated their use in industrial
[2]
environments . However, smaller ones tend to be more difficult to access and require more manoeuvrable
inspection devices. This has led to the development of small and more agile pipe inspection robots with
smaller actuators and streamlined bodies . Nevertheless, the miniaturisation of rigid robots remains
[3]
challenging due to the bulkiness of traditional actuators such as motors; thus, research is looking towards
[4]
smart and softer materials to create such devices . Soft robotic design has taken a lot of its inspiration for
structures and actuators from biological examples, and soft IPIRs are no exception.

Snakes and annelids use a wriggling or worming motion to shift their body through soil or uneven terrain
and can form complex shapes to fit their surroundings. Hence, they are the common inspiration for soft
robots designed to move in constrained environments such as pipe structures . Another example of
[5-9]
bioinspiration applicable to constrained environments is vine or everting robots, which can travel into
confined spaces by growing through the tip . Earthworms create forward movement by contracting and
[10]
[8]
extending their body segments to create a wave-like motion [Figure 1]. This pattern of contraction and
extension from front to rear is called peristalsis and can be seen in other annelids and legless insects . On
[11]
the other hand, inchworms use an inching motion to propel themselves forward by anchoring the surface
with the front of their body whilst contracting the middle and then extending their middle whilst anchoring
the surface with the back of the body .
[12]

Both methods of worm motion rely on patterned anchoring and extension to create propulsion. To travel
up a vertical pipe, robots must apply a traction force to the pipe to prevent falling or slippage, something
that lends itself naturally to worm-like robotics. Furthermore, soft actuators, which tend to be made from
high-friction materials such as rubber, are a natural choice for these worm-like robots . Smart materials
[13]
are also often used as constituents within these devices, such as shape memory alloys and dielectric
actuators, or alternatively, they may use linear solenoids that bend the structures to create a force on the
[15]
pipes [13,14] . Whilst worm-like robots have been developed for other uses such as exploration and
gastrointestinal inspection [16,17] , this review will focus on their applications within pipework.

To analyse the applicability of worm-like robots to pipe inspection, we first must define what is important
for a robot to carry out pipe inspection.
1. Locomotion: The robot must be able to effectively move through pipework. For most applications, this
requires that a robot can move in any orientation around swept bends and tight turns.
2. Wall-press: The robot must be able to grip the pipe walls with enough force to hold its own weight and
the addition of any sensors or loads required for inspection.
3. Steering: The robot must be able to turn left or right at junctions so that entire pipe networks can be
explored.
4. Navigation and mapping: Thus, any areas of damaged pipe network or contamination can be located.

This paper will first discuss fabrication, wall-press ability and motion or earthworm mechanisms and
inchworm mechanisms separately as their difference in locomotive strategy affects these areas. Then,
control, navigation and mapping of worm-like robots will be discussed.

EARTHWORM MECHANISMS
Fabrication
Earthworm mechanisms are robots that move using peristaltic motion. These platforms are made from a