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

Figure 4. (a and b) Optical images showing the PDMS-CNT-based microheater fabricated using DIW technique. (c and d) Graphs showing the heating
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performance of the microheater. (e and f) Comparison of the drug release with and without the microheater. (Reprinted with permission from . Copyright
(2019) Wiley, John & Sons, Inc.)
mechanism is achieved by Joule heating in which the Temperature sensor is essential for temperature
electrical energy is converted into heat energy when it monitoring applications for regulating the microheater to
passes through an electrical resistive element. Therefore, it stimulate drug release, and for monitoring purposes [108] .
is imperative that the materials used for the microheaters Generally, there four main types of temperature sensors
must be electrical conducting. Although any electrical that have been manufactured through 3D printing
conducting materials can be used as the heating element technologies, namely resistance temperature detector
for microheater, the performance and durability differ (RTD), thermistor, thermocouple, and fiber grating
depending on the material properties. Usually, the resistive sensor [109,110] . RTDs and thermistors work similarly, and
materials should have properties such as high resistivity for they detect the temperature change based on the change
low energy consumption and good oxidation property for in its resistance at different temperature. The main
longer working lifespan [105] . Conventionally, these resistive difference is the type of materials that they are made of;
elements can be fabricated on substrates using sputtering, RTDs are usually made of pure conducting metal, whereas
plasma vapor deposition, etching, direct laser writing, and thermistors are typically made of a mixture of metal
selective deposition [55,105] . More recently, 3D printing has oxides. Thermocouple, on the other hand, is formed by
also been explored for the fabrication of microheaters, joining two dissimilar electrically conducting materials
but there are not many of them. For instance, Cai et al. to form electrical junctions that are used for temperature
developed a technique that combines DIW technique with detection [111] . The temperature is detected via the electrical
laser sintering technique to process, print, and sinter the voltages that are generated across the hot and cold ends
platinum (Pt) ink on a ceramic substrate for the fabrication of the thermocouples. In general, it is observed that the
of microheaters [106] . They have demonstrated using different more common 3D-printed temperature sensors are based
laser parameters to produce different microstructure of the on RTDs and thermistors [109] . So far, researchers have
Pt material to optimize the heating performance. Similarly, attempted various materials as the temperature-sensitive
Vasiliev et al. demonstrated the fabrication of microheaters materials for 3D-printed temperature sensors, including
via AJP, and they showed that the printed Pt microheaters various types of nanocomposite materials such as
can operate at a temperature as high as 450°C [107] . PEDOT:PSS-CNT [112] , PDMS-graphene , PLA-CNT ,
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Other than printing pure metallic materials, Yin et al. etc. It was found that the thermal sensitivity of the sensing
demonstrated the use of nanocomposite ink (PDMS-CNT) element can be improved through the use of nanocomposite
as the resistive materials of the microheater (Figure 4) . inks [112] . Interestingly, it was also found that raw feedstock
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The DIW-printed microheater exhibits high stretchability printing material and the as-printed materials can have
and conformity that can be worn on human skin for drug- different thermal responsiveness due to the residual stress
encapsulated patch system for pain management. induced by the printing technique . Other than that,
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Volume 9 Issue 4 (2023) 153 https://doi.org/10.18063/ijb.725