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Kulkarni et al. Soft Sci. 2025, 5, 12 https://dx.doi.org/10.20517/ss.2023.51 Page 7 of 35

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homeostasis . Contamination of body fluids upon contact with heavy metals or other harmful materials
[112]
can result in infections and illnesses such as heavy metal poisoning . Additionally, the internal
[113]
temperature of the human body must be regulated at 36.1 to 37.8 °C . The gut microenvironment consists
[114]
of microbiomes and bacteria used for digestion and preventing diseases . Gut bacteria help digest dietary
fiber and cellulose, provide vitamins, and destroy toxins . However, unwanted bacteria can cause disease
[115]
[115]
due to dysbiosis after antibiotic treatment or surgery . Maintaining the biological microenvironment
surrounding implanted devices helps prevent negative impacts on homeostasis.
[117]
Implantable robotic devices include assistive devices , minimally invasive surgical tools , and drug
[116]
delivery vehicles . They are designed to (1) avoid inflammatory responses due to contact between the
[118]
device and biological fluids, which can lead to thromboembolism; (2) leverage biocompatible and
biodegradable materials; and (3) have a high safety factor by being reliable and durable for long periods .
[104]
Soft robots can largely meet these requirements as they are often designed to mimic biological functions
using biocompatible materials with stiffnesses similar to human tissues . While the human body is a
[104]
highly complex environment, we see (1) infection reduction; (2) compliance matching; and (3)
biocompatibility as ways in which soft robots can address engineering challenges in the body.

Embodiments of soft actuators in vivo
Shifting material composition from metals to soft elastomers and hydrogels is the most distinct difference
between traditional rigid and soft robots. Silicone rubbers are commonly used to build soft robots given
[119]
their robust elastomeric material properties . Silicone is stable at low and high operating temperatures (>
150 °C ), in different oxidation states, in water and chemicals, and is an electrical insulator . Silicone
[120]
[121]
elastomers have similar mechanical properties to human muscles . However, silicone can elicit a harmful
[122]
[123]
immune response when implanted for long periods . Biocompatibility and biodegradability of soft
material chemistry are vital for the long-term stability of implantable devices.
Hydrogel actuators are widely used for building biomedical devices . Hydrogels have high water content
[124]
leading to swelling and deswelling responses which is useful for soft actuator function . Hydrogel
[125]
materials can be stimulated by various inputs including environmental parameters such as pH, temperature,
light, humidity, and electricity . These materials have low thermal conductivities and can be used for
[126]
devices where thermal insulation and regulation of body temperatures are required . For instance,
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polyacrylamide hydrogels composed of 88 wt% water have a thermal conductivity of 0.57 ± 0.04 Wm ·K -1[128] .
-1
Hydrogel-based actuators can be biodegradable or have biocompatible properties that may be ideal for
[19]
[125]
[129]
some applications inside the human body . Material chemistry can be tuned to achieve stiffnesses and
degradation rates to enhance biocompatibility. The properties of elastomer and hydrogel materials make
[130]
them good candidates for engineering actuators to build medical devices in vivo.
Silicone fluid pressure-driven soft actuators have long been used in implantable assistive devices and
wearables . Movements induced by pneumatic actuation, such as bending, twisting, and expansion, can be
[131]
achieved by mechanical programming using fiber reinforcements . PAM actuators have a large specific
[132]
[133]
[133]
power of about 10 kW/kg and similar energy efficiency to human muscle . Specifically, muscles have
an efficiency of about 40% while PAMs have an efficiency of about 49%. PAMs can be used to mimic the
contractile motion of the heart in indirect cardiac compression [Figure 2A] and ventricular assistive
[89]
[134]
devices [Figure 2B]. Hu et al. designed a PAM-based device to support diaphragm function using
[135]
biocompatible polyurethane composites [Figure 2C] . As a pneumatic actuator, PAMs usually have high
[136]
[137]
payloads but may suffer from noise and poor power consumption performance . However, they can
recapitulate biological functions allowing soft robotic devices to safely integrate with the body by