Page 323 - IJB-9-5

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

Three-dimensional (3D) printing, widely known as materials of 4D-printed scaffolds. Then programming
additive manufacturing, provides technical support for designs and applications of these scaffolds are highlighted.
effectively generating medical implants with complex Finally, we propose the prospects and outlook of 4D-printed
morphology or fulfilling individual requirements . It has shape-morphing scaffolds (Figure 1).
[1]
been widely applied in healthcare due to its advantage in
developing manufacturing methods for many specific 2. 4D printing technologies
products. It simplifies production processes, saves time,
reduces costs, and promotes the innovation of medical Three-dimensional (3D) printing is a general technology
models . The remarkable applications of 3D printing that manufactures 3D objects layer by layer by steadily
[5]
in tissue engineering with the aid of clinical imaging adding materials or inks in accordance with the imported
[15]
information are the generation of scaffolds for the repair pre-defined digital models . 4D printing is based on
and replacement of body defects or diseasedparts [6,7] . its core technology, considered as an extension of 3D
printing. The printing processes are mainly extrusion-
Most biological tissues present more complicated based printing (fused deposition modeling (FDM), direct
forms and possess unique functions through dynamic ink writing (DIW)), and light-assisted printing (digital
changes , posing a new challenge to medical implants. light processing (DLP), stereolithography (SLA), selective
[8]
Meanwhile, the widespread use of minimally invasive laser sintering (SLS), selective laser melting (SLM), and
surgery also has placed demands on implants. The static inkjet printing) . An overview of various technologies is
[16]
structures produced by 3D printing may not satisfy the given in this section.
growing expectation of implants. Under this request,
four-dimensional (4D) printing has emerged at the right One of the most often used techniques is extrusion-
moment, along with the diversity of materials and the based printing, which prints objects along pre-determined
development of processing methods, incorporating the horizontal and vertical pathways as materials flow through
fourth dimension, “time.” In detail, it has the capacity to print nozzles. FDM melts solid filaments by heating
present conformational changes under one or more specific the nozzle to the melting temperature of materials and
stimuli such as temperature, pH, light, water, and others . then squeezes the materials out on the platform along
[9]
It overcomes the weaknesses of 3D printing that only the planned path. Thermoplastic materials, including
considers the initial status of printed objects and is limited polyurethane (PU), polylactic acid (PLA), polyamide (PA),
to creating static products [10,11] . Based on this superiority and acrylonitrile butadiene styrene (ABS), are generally
of bionic capacity, it has great application prospects to used in FDM to fabricate thermo-responsive biological
[1]
fabricate dynamic scaffolds to satisfy specific demands of implants with good mechanical properties . Running
implantation sites or approaches. Specifically, 4D-printed FDM is cheap and easy to operate; however, the printing
scaffolds show great advantages in adapting to the dynamic resolution is relatively low. On the other hand, DIW
structure of human organs and tissues [12,13] and applying to extrudes ink through its nozzle to build complex geometric
minimally invasive surgery. More interestingly, they have objects layer by layer. It works mainly for liquid inks with
ability to respond to specific external or physiological shear-thinning behavior, thermosetting, or light-curing
conditions (solvent, temperature, pH, etc.) by design to property, which is also friendly to cell-laden bioinks. It can
[6]
realize time-dependent physical changes or replicate the print one or more materials simultaneously and has been
dynamic biological behavior of native tissues for better in extensive use to build bionic heterogeneous and gradient
adaptation to the body environment. Thus, they play a structures, such as human skin and musculoskeletal
[17]
role in repairing tissue defects intelligently. Furthermore, scaffolds .
[18]
4D dynamic supports are more beneficial for cell The light-assisted printing is another widely used
attachment and subsequent orientation, proliferation, and printing process, where a laser beam or a UV light
differentiation . On this account, biomimetic cellular performs as a light source to initiate photocuring to
[14]
scaffolds can be made for tissue repair and functional solidify photocrosslinkable liquid polymer or resin layer by
tissue substitutes.
layer to form a structure (the former is called SLA, and the
Several challenges present during the manufacturing latter is called DLP) . It is also popular in the fabrication
[19]
process of 4D dynamic medical implants: (i) selection of bioscaffolds for tissue engineering, and its improved
of stimuli-responsive materials that possess printability, resolution makes the formation of subtle and complex
biocompatibility, and biodegradability; (ii) application of structures possible . SLS/SLM sinters powdered materials
[20]
printing technologies; (iii) design of deformation strategies; under laser, and a new layer is laid after one layer is cured,
(iv) aimed at specific application scenarios. Therefore, this so the process is repeated to obtain specific structures.
review systematically reports typical technologies and Common materials used in SLS/SLM include polymers

Volume 9 Issue 5 (2023) 315 https://doi.org/10.18063/ijb.764