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Materials Science in Additive Manufacturing Functional materials for AM

and 3D printer technology, which allows for on-demand 3.2.1. Soft sensors embedded with metallic nanoparticles
production. Polymer-based sensors printed using 3D printing
Metal nanoparticle fillers have a direct impact on the technology can achieve excellent shape restoration and
electrical and mechanical characteristics of the printed perform various functions depending on the embedded
products. 52-54 To determine the suitability of metal conductive material. Particularly, polymer composites
nanoparticles, factors such as electrical conductivity, embedded with conductive nanoparticles are known
oxidation stability, and electrical properties can be to exhibit a sensitive resistance response to strain and
considered based on the required performance of possess excellent electrical conductivity. 67-69 An ideal
individual soft electronics. The most commonly used metal elastic conductor maintains constant high conductivity
nanoparticles are those of single elements, such as silver, over a wide range of strain rates. In particular, strain and
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gold, and copper nanoparticles. 52-56 Among them, silver is tactile sensors made of metals with excellent conductivity
the most widely used conductive filler due to its outstanding are actively being researched. The material, fabrication
electrical conductivity, mechanical rigidity, and high method, and performance of the polymer-based sensor
corrosion resistance among metals. 55-59 Moreover, silver embedded with metallic particles are summarized in
nanoparticles find extensive use in medical applications, Table 3.
as they can be employed for selective laser photothermal Taking advantage of the fact that extrusion-based
treatment, leveraging the surface plasmon resonance effect DIW 3D printing allows for multi-material printing, a
and the ability to convert strongly absorbed light into local tactile sensor was demonstrated through a single process
heat. 60-62 Copper nanoparticles are relatively inexpensive 71
compared to silver and gold and possess similar electrical with four different independently addressable nozzles.
conductivity and a low electron transfer effect as silver. 53,54 The tactile sensor comprises two electrode layers, one
However, when forming copper nanoparticles in air, an insulating layer, a support layer, a sensor layer, and a
oxide layer is generated on the surface for thermodynamic base layer. The sensor layer and electrode layer contained
stability, leading to a reduction in electrical conductivity silver nanoparticles embedded in silicone elastomer, and
the completed sensor exhibited high flexibility, electrical
and an increase in sintering temperature. 63-66 The conductivity, and sensitivity. It is also possible to expand it
formation of an oxide layer renders the sintering of copper
nanoparticle inks technically challenging, which is one into an array form.
of the main reasons why copper nanoparticles are used Strain sensors and capacitive sensors fabricated using
less frequently than the relatively more expensive silver a new method called hybrid 3D printing have been
nanoparticles as conductive fillers. reported. Advanced soft sensors are accomplished
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Table 3. Comparison of materials, fabrication methods, and performance of soft sensors embedded with metallic nanoparticles
3D printing method Material composition Applications Performances References
Multi-material Submicrometer-sized silver Tactile sensor As the pressure applied rose from 100 to 71
extrusion (DIW) particles+silicone elastomers 500 kPa, the device’s resistance decreased
(Dragon Skin 10) approximately twelvefold, dropping from 1.14
kΩ to 95 Ω; gauge factor: about 180
Material extrusion Silver micro-flakes+thermoplastic Microcontroller Electrical conductivity; initial: 10 ×S/cm; at 72
4
(DIW) with automated elastomers (thermoplastic device and strain of 240%: 0.1 S/cm; gauge factor 13.3
pick-and-place urethane) wearable device
Feedback-controlled Silver micro-flakes+poly (ethylene Inductive coil Electrical conductivity (1.38±0.0814) × 10 73
4
material extrusion oxide) (PEO) (also suitable for S/cm (one order of magnitude lower than bulk
(DIW) moisture sensing), silver)
wearable device
Coaxially material Galinstan+silicone elastomers (737 Strain sensor Maximum tensile strain of 100%; can be bent 74
extrusion (DIW) neutral cure sealant) up to 180 degrees
Integrating vat (i) DLP elastomer: acrylate-based Strain gauge Gauge factor: 251 75
photopolymerization (ii) DLP plastic: acrylate-thiol
(DLP) and material based
extrusion (DIW) (iii) DIW ink: photosensitive ink,
conductive silver ink, and
LCE ink
Abbreviations: DIW: Direct ink writing; DLP: Digital light processing; LCE: Liquid crystal elastomers.

Volume 3 Issue 2 (2024) 9 doi: 10.36922/msam.3323