Page 325 - IJB-9-5

International Journal of Bioprinting Review of 4D-printed smart medical implants

Figure 2. Design of printing ink in 4D structures. (A) 4D scaffolds made by single-component ink printing, a) self-deploying process of the peripheral
vascular stent based on PLA . Copyright 2022, Royal Society of Chemistry. b) Deforming process by simple pattern design via a mask film in a single layer
[60]
of PNIPAM hydrogel . Copyright 2022, MDPI. (B) Adding different materials into a poly network to realize multiple and complex responsive behaviors.
[61]
CNTs: carbon nanotubes; metal-NPs: metal-nanoparticles; MNPs: magnetic nanoparticles; PDA: polydopamine; PLA: polylactic acid; PNIPAM: poly
(N-isopropylacrylamide).
stress when the ambient temperature is above T deformation modes, and mechanical behavior of SMP-
g
and keeps stable at a cooled temperature. Once the based scaffolds in order to better cater to body desire.
temperature increases above T , the shape recovers. For example, some structures introduce active substances
g
Correspondingly, shape transformation is the result of into SMPs polymer networks (such as carbon nanotubes
molecular chain conformation changing at the molecular (CNTs) , magnetic nanoparticles (MNPs) , etc.) to
[39]
[38]
level . This process is reversible and can be easily realize 4D dynamic functionalities under various non-
[26]
predicted and repeated by accurate measurement of the contact stimuli, and this will be described at length in the
pre-programming scheme . Direct thermal activation later section.
[27]
of SMPs to initiate macroscopic deformation is the most
common method in 4D programming. Due to their 3.2. Stimuli-responsive hydrogels
flexible shape memory effects, good biodegradability, and Hydrogels, defined as hydrophilic polymeric materials,
good biocompatibility, SMPs have been widely applied are attractive for biomedical implants for their similarity
in the fabrication of bioscaffolds [28-31] . SMPs commonly to biological tissues in structures and characteristics [23,40] .
used in the fabrication of smart implants include PU, In general, they form insoluble networks by inner covalent
polycaprolactone (PCL), PLA, and recently discovered or physical crosslinking of hydrophilic polymers [41] .
soybean oil epoxidized acrylate (SOEA) [32-34] . Medical The stimuli-responsive ability of hydrogels depends
implants prepared from SMPs can be temporarily fixed in on their volume changes caused by water absorption
a contracted or folded state to facilitate passage through or desorption that are influenced by pH, temperature,
minimally invasive surgical wounds. Its shape restores ion concentration, and some biomedical signals [23,42,43] .
to its initial shape after being placed in the specific site. Combined with 4D printing, they are endowed with
During this process, the mechanical damage to the more application potential in tissue reconstruction, drug
wound is greatly reduced. By identifying the shape of delivery, etc. Among them, poly (N-isopropylacrylamide)
defects in specific sites in advance, setting as the initial (PNIPAM) is a typical thermo-sensitive hydrogel for its
shape of printed SMPs, then pre-programming so as to lower critical solution temperature (LCST) at ~32°C,
implant, the shape can be restored after implantation to which is close to human body temperature [44] . It is
repair defects seamlessly, which is more conducive to the hydrophobic when placed in an environment with a
subsequent renovation of tissues . Furthermore, many temperature above its LCST; on the contrary, it becomes
[35]
kinds of fillers can be incorporated into the SMP matrix hydrophilic [45] . This phenomenon appears as its volume
to form shape memory polymer composites (SMPCs) variation with temperature. However, weak mechanical
[26,36,37] to optimize responsive temperature, responsive strength limits the use of hydrogels. Secondary polymeric

Volume 9 Issue 5 (2023) 317 https://doi.org/10.18063/ijb.764