Page 327 - IJB-9-5

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

responsive behaviors. Adding MNPs into the printing have two basic components: a driving layer and a passive
ink is one of the methods to fabricate 4D constructs that layer. The anisotropy of swelling or shrinkage behaviors
can be actuated by external magnetic fields. Magnetite between these layers is a basic driving mechanism.
ferroferric oxide (Fe O ) microparticles are the most As hydrophilic polymer networks, the crosslinking
3
4
commonly used [39,62-64] . In this way, contactless control density and solvent type can affect the swelling property
of product deformation under a magnetic field can be of hydrogels. The swelling difference between hydrogel
realized. By combining SMPs with MNPs, the composite layers in a solvent can lead the entirety of construction
simultaneously presents thermo- and magnetic-responsive to self-bend [73-75] . For instance, bilayer hydrogels based
shape-morphing behavior . Printed implants possess on different concentrations of silk fibroin bioink were
[39]
electroactive shape memory performance by adding CNTs prepared, and shape bending was obtained due to
into printing ink [65,66] , and implants that incorporate SMPs anisotropic volume change of different layers in an aqueous
with CNTs have dual stimulus responsiveness similarly. solution. Moreover, a reversible de-bending process was
Some studies embedded particular substances in the discovered in saltwater because of the trigger of osmotic
printed SMP/SMA-based shape memory composites to action by salt. This reversible deformation process is
convert other forms of energy into thermal energy and thus precisely dependent on the volume change induced by
deform the structure. Materials like the CNTs mentioned water absorption and dehydration behavior of the driving
earlier, polydopamine (PDA), and some metal nanoparticles layer in different osmotic pressure solutions (Figure 3Aa)
are provided with photothermal conversion properties. [76] . Beyond this, some materials show pH-dependent
Some studies reported shape memory frameworks with swelling behavior as a result of the electrostatic repulsion of
near-infrared light (NIR) or other light responsiveness by the charged unit at the molecular level. On this basis, pH-
the addition of CNTs , PDA [68,69] , gold nanoparticles , or sensitive bilayer structures whose geometric changes were
[70]
[67]
copper sulfide nanoparticles (CuSNPs) in the precursor actuated by imbalanced swelling behavior under different
[71]
ink. In addition, Koh et al. presented ferromagnetic pH conditions can be fabricated (Figure 3Ab) . Some
[77]
PLA actuated by microwave radiation. The iron powder hydrogels, such as sodium alginate, show ion responsiveness
functioned as heater elements and triggered non-contact and exhibit 4D deformation in ionic solution by design .
[78]
and localized deformation by placing strategically . More impressively, stimuli-responsive material in a recent
[72]
study was generated and actuated by volume change in
We discovered that multiple materials printing
could potentially acquire leapfrogging from contact the driving layer, which integrated genetically engineered
yeast that only proliferated with the presence of particular
to contactless deformation controlling, from single to biomolecules. This method provided for the production
multiple stimulus response, that extends their application of bioactive scaffolds and devices responsive to biological
prospects (Figure 2B). Non-contact stimulus response is stimulation .
[79]
also necessary for the design of smart implants. In vivo
deformation can be realized under in vitro stimulation The volume change differences under particular
by remote contactless control to trigger the subsequent conditions also exist in elastomers and other materials. The
deformation process, which broadens application scenarios high swellable driving layer can be designed by embedding
of 4D implants. At the same time, the biocompatibility of low boiling point microliquid chambers [80,81] or thermally
composite structures should also be taken into account. expanding microspheres (TEM) in elastomers. In
[82]
the former, liquid–vapor-phase change of microliquid
4.2. Design of printed structures chambers such as ethanol when heated or cooled causes
Various smart designs in structures represent significant volume change of the driving layer and thus actuates bilayer
breakthroughs for 4D printing methods. Numerous structures to curve reversibly (Figure 3Ac). The process is
design schemes have sprung up recently. Proceeding from reversible. Moreover, extra filling of thermally conductive
the basic properties of materials, these designs recreate material particles such as liquid metal fillers provides faster
dynamic biomimetic processes and functions. We will thermal response speed [80,81] . In contrast, the deformation
make an integrated discussion from three aspects: (i) bi-/ caused by the thermal expansion coefficient with a large
multi-layer structure design; (ii) gradient structure design; difference in two layers due to the existence of TEMs in the
(iii) origami structure design. latter is irreversible . Another study innovatively presented
[82]
a bilayer structure (one layer of elastomer and another layer
4.2.1. Bi-/multi-layer structure design of transition material) actuated by combined stimuli of heat
The manufacture of bi-/multi-layer structures is a popular and ethanol. The structure was programmed to curve in 60°C
design in 4D printing, which has been demonstrated ethanol because the elastomer swelled via ethanol diffusion,
effective in dynamic structure building. These constructs

Volume 9 Issue 5 (2023) 319 https://doi.org/10.18063/ijb.764