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International Journal of Bioprinting 3D printing and 3D-printed electronics in smart drug delivery devices

electronics fail to meet the increasing requirements For FFF technique, the process involves the use of
of these devices in terms of the material softness and a heated nozzle to melt the filament into molten state.
stretchability, biocompatibility, and freeform fabrication. The high temperature of the extruded material aids in
So far, the more common electronic fabrication techniques forming bonds between the newly deposited materials
that are being used and compatible with the fabrication of and the previous layer (Figure 3a). For 3D electronic
conventional electronics for soft and flexible drug delivery printing, conductive filaments in the form of polymeric
devices (DDD) are the microfabrication [10,49,54] , selective nanocomposite are often used. Thus far, the nanocomposite
electrodeposition , and screen printing techniques [56,57] . filaments that have been developed for 3D printing of
[55]
These fabrication methods have several drawbacks such physiological sensors include graphene–polylactic acid
as the tedious fabrication process, the need for stencil or (PLA) [60,61] , graphene nanorods-PLA , carbon nanotube
[62]
mask, and being limited to 2D patterning. For the case of (CNT)-PLA , and carbon black (CB)-PLA , just to
[64]
[63]
microfabrication and electrodeposition, these methods are name a few. The sensors that were fabricated through
also incompatible to process advanced materials such as FFF printing technique include electrochemical sensor ,
[64]
composite material incorporated with novel nanomaterials pressure sensor , and temperature sensor [62,63] , and
[63]
like graphene and MXene . biosensors [60,61] . The FFF techniques are suitable for the
[58]
fabrication of 3D and freestanding electronic structures,
In contrast, 3D electronic printing techniques which but FFF-printed devices usually suffer from poor electrical
often utilize a layer-by-layer fabrication approach offer properties due to the nature of nanocomposite materials.
added benefits, such as low processing temperature,
compatibility with various kinds of soft and flexible DIW techniques, on the other hand, typically involve
substrate, ability to process various kinds of functional inks, the extrusion of the functional inks through a nozzle by
and greater design flexibility to the electronic designers. In means of screw, pneumatic, and microvalve dispensing
[58]
addition, 3D electronic printing techniques produces lesser methods (Figure 3b) . The functional inks can exist in
waste compared to conventional electronic fabrication the form of nanomaterial dispersions or nanocomposite
methods as the material and energy are used to deposit resins. The former usually forms high-purity structure
materials where necessary and do not require stencil or with good electrical properties after the solvent completely
tooling. The simple fabrication process of 3D printing evaporates, but these materials are used to manufacture
techniques makes it more convenient for rapid prototyping thin film structures that are attached to a substrate. The
of smart DDD and it also opens new opportunities for the latter, on the other hand, can be used to manufacture
development of highly customized DDDs are individual- freeform and freestanding electronic structures but suffers
specific. The following section discusses the more common in terms of the electrical properties due to the presence of
3D electronic printing techniques that have been used for the matrix materials similar to the materials used for FFF
the fabrication of smart DDDs. technique. Some examples of functional inks for DIW
include silver-based ink , CNT-polydimethylsiloxane
[58]
In general, 3D electronic printing techniques can be (PDMS) nanocomposite ink , graphene–PDMS , and
[66]
[65]
categorized as material extrusion printing techniques, PEDOT:PSS/HPU hydrogel , just to name a few. For
[67]
droplet-jetting printing techniques, vat polymerization photosensitive resin-based functional inks, a UV curing
techniques, and powder-bed fusion techniques . laser or light source will be used to cure the resin in situ to
[59]
Interestingly, it was found that majority of the works that ensure that the extruded filament can retain its geometry
are related to smart DDDs have used the extrusion-based immediately after being extruded from the nozzle to
printing techniques and droplet-jetting printing techniques. prevent the structure from collapsing. Typically, the DIW-
As such, the discussion in this paper will mainly focus on printed electronics are fabricated on a substrate and then
these two techniques and briefly cover some works on vat subsequently removed from the substrate for use. In certain
polymerization techniques for 3D-printed electronics for case, a solution support bath may be used to achieve more
use in smart DDDs. complex or freestanding electronic structures . To date,
[68]
Material extrusion 3D printing techniques deposit DIW technology has been reported to be used for printing
materials layer-by-layer through a coordinate-controlled heaters, temperature sensors, pH sensors, sweat sensors,
[65-70]
nozzle. The feedstock materials mainly come in two forms, UV sensors, etc.
namely filament and viscous liquid. The printing technique Droplet-jetting techniques are a class of 3D printing
for the former is usually called fused filament fabrication techniques that deposit materials layer-by-layer in the
(FFF) technique, whereas the printing technique for the form of liquid droplets. In general, there are two main
latter is commonly known as direct ink writing (DIW) types of droplet-jetting techniques for electronic printing,
technique. namely inkjet printing (IJP) and aerosol jet printing (AJP)

Volume 9 Issue 4 (2023) 150 https://doi.org/10.18063/ijb.725