Page 197 - IJB-9-6

International Journal of Bioprinting Attractiveness of 4D printing in medical field

Figure 1. (a) Shape change effect. (b) Shape memory effect. (Adapted from Subeshan et al. (2021) )
[18]
have been produced, such as shape memory gels biological stimuli. Physical stimuli are classified
(SMG), shape memory hybrids (SMH), shape memory into temperature, liquid/moisture, light, magnetic
alloys (SMA), shape memory metals (SMMet), shape field, and electrical field. Temperature, light, and
memory polymers (SMP), and shape memory ceramics moisture stimuli are the most frequently used in
(SMC). Among these, SMP and SMA are the most literature . Chemical stimuli are identified as
[26]
investigated in the literature . The main advantages variations in physiological conditions, for instance,
[19]
of SMP include high strain recovery, low density, pH levels or ionic concentrations. The pH stimulus
low cost of raw material, fabrication and processing, is frequently used in drug delivery systems,
biocompatibility, and biodegradability , and they can especially for diseases like cancer, infections, and
[20]
respond to temperature, light, and chemical stimuli . degenerative and cardiovascular diseases where
[12]
On the other hand, the main characteristics of SMA there is an imbalance of pH levels . Finally,
[6]
are elasticity, high strength and low elastic modulus, biological stimuli refer to variations in molecules,
high corrosion resistance, biocompatibility, and non- such as glucose, enzymes, proteins, nucleic acids,
ferromagnetic properties , and they can respond to and peptides. The SRMs develop shape-shifting
[21]
load and temperature stimuli and are typically used behaviors, such as folding, curling, twisting,
in biomedical and engineering fields . Regarding expansion, and contraction .
[27]
[6]
SCM, the most common materials are semi-crystalline
networks (SCNs), interpenetrating network (IPN) , (iv) Mathematical models: These are divided into
[22]
electro-active polymers, and piezoelectrical materials . two categories. The first is backward prediction,
[20]
The difference between SMM and SCM is the ability of which focuses on the printing profile (printing
SCM to return instantaneously to their original form paths and nozzle size) given the required final
after the applied stimulus is removed (Figure 1b); this form and its properties. The second category is
is the shape change effect (SCE) mechanism. LCE are forward prediction, which aims to determine the
attractive to researchers because of their ability to have transformation process of the structure to achieve
[28]
large, reversible, and fast dimension deformations . the final shape . The mathematical models connect
[23]
They are used in applications such as soft robots, four fundamental elements: material structure
artificial muscles, and sensors . Hydrogels are widely (filament size, orientation, interfilament spacing,
[24]
used in the literature due to their ability to drastically etc.), desired final shape (bending and twisting angle,
increase their volume and are frequently printed length, volume, etc.), material properties (shear
along other polymers to increase their strength. These modulus, elastic modulus), and stimulus properties.
materials are particularly useful in biomedical and (v) Interaction mechanisms: This refers to the order of
engineering applications . the steps required to obtain the desired final shape .
[25]
[29]
An example of this is the SME for SMA and SMMet,
(iii) External stimuli: As mentioned, SRMs can respond which includes mechanisms such as one-way shape
to external stimuli such as physical, chemical, and memory effect (OWSME), two-way memory effect

Volume 9 Issue 6 (2023) 189 https://doi.org/10.36922/ijb.1112