4D Printing: A Review on Recent Progresses

Additive manufacturing landscape and materials perspective in 4D printing

4D printing is inspired by embedded product designs to produce stimuli-responsive consumables fabricated by available commercial 3D printers. Although significant progress on smart material performance has been made and different studies have focused on new strategies and process improvements in typical additive manufacturing. Herein, the proposed review article discusses material arrangements for 4D printing, highlighting the structural evolvement mechanism, the behavior of deformation, and their prospective implementation with respect. Starting from a generalized idea, and fundamental workflow, together with a graphical manifestation of the 4D printing concept, and 4D printing for shape-memory materials (SMMs), self-fitting wearables based on shape memory alloys (SMAs) are reviewed exclusively. Furthermore, the capabilities of single and multiple materials mechanisms for shape-shifting behavior are summarized. Finally, we explored the future application potential under succeeding context: SMA-based knitted garments, transforming food, and relevant sectors wise development and proceedings with the advancement in smart materials. We determined our review by aiming our future directions such as the "dream it and make it feasible" technology.

GRAPHICAL ABSTRACT

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

3D bioprinting of cardiac tissue: current challenges and perspectives

Demand for donor hearts has increased globally due to cardiovascular diseases. Recently, three-dimensional (3D) bioprinting technology has been aimed at creating clinically viable cardiac constructs for the management of myocardial infarction (MI) and associated complications. Advances in 3D bioprinting show promise in aiding cardiac tissue repair following injury/infarction and offer an alternative to organ transplantation. This article summarizes the basic principles of 3D bioprinting and recent attempts at reconstructing functional adult native cardiac tissue with a focus on current challenges and prospective strategies.

Polylactic Acid (PLA) Biocomposite: Processing, Additive Manufacturing and Advanced Applications

Over recent years, enthusiasm towards the manufacturing of biopolymers has attracted considerable attention due to the rising concern about depleting resources and worsening pollution. Among the biopolymers available in the world, polylactic acid (PLA) is one of the highest biopolymers produced globally and thus, making it suitable for product commercialisation. Therefore, the effectiveness of natural fibre reinforced PLA composite as an alternative material to substitute the non-renewable petroleum-based materials has been examined by researchers. The type of fibre used in fibre/matrix adhesion is very important because it influences the biocomposites' mechanical properties. Besides that, an outline of the present circumstance of natural fibre-reinforced PLA 3D printing, as well as its functions in 4D printing for applications of stimuli-responsive polymers were also discussed. This research paper aims to present the development and conducted studies on PLA-based natural fibre bio-composites over the last decade. This work reviews recent PLA-derived bio-composite research related to PLA synthesis and biodegradation, its properties, processes, challenges and prospects.

Recent advances in bioprinting technologies for engineering different cartilage-based tissues

Inadequate self-repair and regenerative efficiency of the cartilage tissues has motivated the researchers to devise advanced and effective strategies to resolve this issue. Introduction of bioprinting to tissue engineering has paved the way for fabricating complex biomimetic engineered constructs. In this context, the current review gears off with the discussion of standard and advanced 3D/4D printing technologies and their implications for the repair of different cartilage tissues, namely, articular, meniscal, nasoseptal, auricular, costal, and tracheal cartilage. The review is then directed towards highlighting the current stem cell opportunities. On a concluding note, associated critical issues and prospects for future developments, particularly in this sphere of personalized medicines have been discussed.

Fused Filament Fabrication-4D-Printed Shape Memory Polymers: A Review

Additive manufacturing (AM) is the process through which components/structures are produced layer-by-layer. In this context, 4D printing combines 3D printing with time so that this combination results in additively manufactured components that respond to external stimuli and, consequently, change their shape/volume or modify their mechanical properties. Therefore, 4D printing uses shape-memory materials that react to external stimuli such as pH, humidity, and temperature. Among the possible materials with shape memory effect (SME), the most suitable for additive manufacturing are shape memory polymers (SMPs). However, due to their weaknesses, shape memory polymer compounds (SMPCs) prove to be an effective alternative. On the other hand, out of all the additive manufacturing techniques, the most widely used is fused filament fabrication (FFF). In this context, the present paper aims to critically review all studies related to the mechanical properties of 4D-FFF materials. The paper provides an update state of the art showing the potential of 4D-FFF printing for different engineering applications, maintaining the focus on the structural integrity of the final structure/component.

Investigation on spontaneous 4D changes in color and flavor of healthy 3D printed food materials over time in response to external or internal pH stimulus

This study aimed to investigate 4D changes in colors and flavors of 3D-printed healthy food products in response to an external or internal pH stimulus. The formulations obtained by 3D printing of multi-smart materials, comprised of the combination of red cabbage juice, vanillin powder, potato starch and different fruit juices were used. 3D printing ability of red cabbage juice and vanillin powder affected by different potato starch concentrations was first studied. Then, changes in color, texture, flavor (by E-nose) and taste (by E-tongue) induced by the stimulus were determined. Results revealed that the color of the 3D-printed product changed from blue (control sample) to red, purple, violet, blue, blue-green, and green-yellow colors when sprayed with pH solutions of 2, 3-4, 5-6, 7, 8-9, and 10, respectively. In addition, clear differences in aroma and taste profiles among pH samples were detected. Moreover, dried 4D product samples exhibited color and anthocyanins stability when stored in ambient temperature for three weeks. This study is important for manufacturing new healthy 3D-printed food products with desired and attractive sensory characteristics, which can be particularly significant to people with poor appetite.

Four-Dimensional (Bio-)printing: A Review on Stimuli-Responsive Mechanisms and Their Biomedical Suitability

The applications of tissue engineered constructs have witnessed great advances in the last few years, as advanced fabrication techniques have enabled promising approaches to develop structures and devices for biomedical uses. (Bio-)printing, including both plain material and cell/material printing, offers remarkable advantages and versatility to produce multilateral and cell-laden tissue constructs; however, it has often revealed to be insufficient to fulfill clinical needs. Indeed, three-dimensional (3D) (bio-)printing does not provide one critical element, fundamental to mimic native live tissues, i.e., the ability to change shape/properties with time to respond to microenvironmental stimuli in a personalized manner. This capability is in charge of the so-called “smart materials”; thus, 3D (bio-)printing these biomaterials is a possible way to reach four-dimensional (4D) (bio-)printing. We present a comprehensive review on stimuli-responsive materials to produce scaffolds and constructs via additive manufacturing techniques, aiming to obtain constructs that closely mimic the dynamics of native tissues. Our work deploys the advantages and drawbacks of the mechanisms used to produce stimuli-responsive constructs, using a classification based on the target stimulus: humidity, temperature, electricity, magnetism, light, pH, among others. A deep understanding of biomaterial properties, the scaffolding technologies, and the implant site microenvironment would help the design of innovative devices suitable and valuable for many biomedical applications.