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Review Article

A Review of Printable Flexible and Stretchable Tactile Sensors

Table 1

Features of the additive manufacturing techniques frequently used for the fabrication of flexible/stretchable tactile sensors.

Fused filament fabrication (FFF) [18, 233]Aerosol jet printingInkjet printing [33]Direct ink writing (DIW)Electrohydrodynamic printing (E-jet) [44, 234, 235]


Technology
Heated nozzle by thermal energyAerodynamic focussingThermal or piezoelectricPneumatic or syringe nozzleDriven by electric field

Flexibility of printed element
★★★☆☆★★★★☆★★★★☆★★★★★★★★★★

Stretchability of printed element
★★☆☆☆★★★★☆★★★☆☆★★★★★★★★★★

Flexible/stretchable element (FSE) material
Thermoplastic, composites made into filamentsConductive inks, dielectrics of viscosity (1–1000 cP)Any low viscosity inks (1–20 cP)Any type of flowable inkPolymer, nanoparticle inks

Dimensional accuracy
50–500 μm10–250 μm20-100 μm250 nm–100 μmFew hundred nm-μm

Advantages
(i) Inexpensive machine and materials
(ii) Possibility of multimaterial printing
(iii) Adjustable temperature of nozzle and build platform
(iv) Mild usage and maintenance
(i) Printability complex nonplanar surfaces
(ii) High resolution
(iii) Clog-resistant nozzle
(iv) Continuous stream
(v) Highly focussed
(vi) Low processing temperature
(i) Cost-effective
(ii) High throughput
(iii) Wide range of materials
(iv) Low material wastage
(v) Drop-wise material deposition
(i) Highly versatile
(ii) Wide range of materials
(iii) Possibility of multimaterial printing
(i) Noncontact printing
(ii) High resolution
(iii) Broad range of materials
(iv) Multimode printing
(v) Ability to control jet emission

Disadvantages
(i) Nozzle too close to the substrate
(ii) Heating effect may damage the printed trace
(iii) Rough surface finish of printed pattern
(iv) Unable to build sharp features
(v) Limited dimensional accuracy
(vi) Materials need to be made into filaments
(i) Droplet carrier creates a cloud of powder at the printed spot
(ii) Sheath gas inhibits the local bonding of printed trace by solidifying or crystallizing it locally
(i) Nozzle clogging
(ii) Slow speed compared to the rest
(iii) High contact angle produces bulging of printed traces
(iv) Potential coffee-ring effect due to unequal distribution
(v) Random directionality of drops
(i) Nozzle too close to the substrate
(ii) Might require postprocessing for some materials
(iii) Challenge to maintain mechanical integrity and shape during printing
(i) High electrical forces during ejection cause printed feature to be much smaller than nozzle diameter
(ii) Insulating materials affect the intensity of the electric field
(iii)