Page 326 - IJB-9-5

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

networks are introduced to combat this limitation implants through particular programming designs will be
in the fabrication of smart scaffolds [22] . For instance, elaborated on in the next section.
interpenetrating network hydrogels consisting of ionic
and covalently bonded crosslinked polymer networks 4. Deformation design
are designed to increase application potential [46] . Besides, The 4D deformation of biomedical implants presents as
many strategies have been proposed to design hydrogel- self-rolling, self-bending, self-expanding, and self-folding.
based deforming scaffolds on the basis of their natural Designs of printed ink composition and structures allow the
properties, and we will make a detailed elaboration in the implementation of various complex deformations under
following section. different physical, chemical, or physiological stimulation.
3.3. Shape memory alloys (SMAs) These, in turn, facilitate a wide range of applications of
Shape memory alloys (SMAs) are materials with the 4D-printed implants for different organizational structures
shape memory effect that transforms thermal energy into under different physiological conditions. In this section,
mechanical energy . They can return to their original we will expound on mechanisms of dynamic deformations
[47]
shapes even after strong deformation, which attributes from the aspects of ink composition design and structural
to their phase change under the external stimulus. design. These design methods can unite with each other to
This superelastic behavior as well as their satisfactory create complex multi-responsive structures.
biocompatibility and corrosion resistance make SMAs an 4.1. Design of ink composition
ideal choice for biomedical uses [48,49] . Particularly, nickel- The ink components play an important role in the design of
titanium (NiTi) alloy performs best in shape recovery and 4D structures. Constructs composed of single or multiple
superelastic strain, resulting in extensive application in materials can both demonstrate shape transformation due
clinic orthopedics [47,50] . However, the restricted flexibility to different mechanisms, and we will detail the two aspects.
of alloy materials, which leads to compliance mismatch,
limits their application in implantation for soft tissue 4.1.1. Single-component ink
engineering. Single-component ink printing (SMPs, hydrogels, SMAs,
LCEs, etc.) can achieve the expected 4D deformation
3.4. Liquid crystal elastomers (LCEs) effect through a simple program. The shape change mainly
Liquid crystal elastomers (LCEs) are polymer networks depends on the temperature-response property of the
with anisotropic liquid crystalline properties while materials applied. For example, 4D-printed peripheral
maintaining the properties of elastomers . They are vascular stents based on PLA via an FDM printer were
[51]
another representative intelligent material that shows proposed in Wang’s study, and its shape memory effect
large and reversible shape changes under external stimuli was based on the transition between the glassy state and
(heat, light, electricity, magnetism, pH, solvent, etc.) . rubbery state of SMPs (Figure 2Aa) . The findings show
[23]
[60]
They shift phase state or molecular structure that causes a that 4D implants can be prepared as required using single-
change in the arrangement order of liquid crystal elements shape memory material.
when heated above their nematic-to-isotropic transition
temperature (T ). The removal of external stimuli causes Apart from the simple single-material printing,
NI
LCEs to return to their original shape reversibly. This process some ingenious pattern designs have been introduced
reflects the deformation of materials macroscopically. into single-component printing based on the former.
Huge advances have been achieved in LCEs for medical For example, a single-layer 4D-printed object based on
application recently [52-54] . They have shown a great individual PNIPAM hydrogel was obtained by different
promising application in artificial muscles , actuators, UV-focusing times of static and shape-morphing parts with
[55]
and sensors due to their excellent driving performance, the help of a mask film. This led to different coefficients
mechanical properties, and biocompatibility . of thermal expansion in different parts and subsequent
[56]
[61]
thermal-induced deformation (Figure 2Ab) . Through
3.5. Other materials the printing of single-component ink and surface pattern
Apart from the above materials that react to external design, 4D scaffolds can be generated quite simply, and
stimuli directly, a variety of other active, inactive, or the subsequent shape-morphing process can be actuated
multi-component materials have also displayed their immediately.
dynamic deformability that conduces to the generation
of 4D constructs. For example, certain ceramics possess 4.1.2. Multiple components ink
high energy output and high-temperature usage, and can The responsive modes of single-material-based 4D printing
serve as a possible class of smart materials [57-59] . More are usually simplistic. Some other substances are added
relevant materials contributing to the generation of 4D into polymer networks to realize multiple and complex

Volume 9 Issue 5 (2023) 318 https://doi.org/10.18063/ijb.764