Modular 4D Printing Assisted by Dynamic Chemical Bonds

Abstract

Modular 4D printing is achieved by assembling and welding the modules of printed self-folding structures containing dynamic chemical bonds, enabling multifunctional shape-shifting structures and devices. Modular 4D printing is achieved by assembling and welding the modules of printed self-folding structures containing dynamic chemical bonds, enabling multifunctional shape-shifting structures and devices. Nature abounds many plants with delicate geometries that can change their shapes under external stimuli. Inspired by this, many human-made shape-shifting materials, including shape memory polymers (SMPs), hydrogels, and liquid crystal elastomers (LCEs), have been developed for on-demand shape programming for multifunctionality.1Lendlein A. Trask R.S. Multifunctional materials: concepts, function-structure relationships, knowledge-based design, translational materials research.Multifunctional Materials. 2018; 1: 010201Crossref Scopus (63) Google Scholar Recently, the emerging technique of additive manufacturing (or 3D printing) has offered unprecedented opportunities for sophisticated shape control. The merging of 3D printing and shape-shifting materials gives birth to the concept of 4D printing—the as-printed structure can change its shape under external stimuli with time (the fourth dimension).2Ge Q. Qi H.J. Dunn M.L. Active materials by four-dimension printing.Appl. Phys. Lett. 2013; 103: 131901Crossref Scopus (517) Google Scholar,3Raviv D. Zhao W. McKnelly C. Papadopoulou A. Kadambi A. Shi B. Hirsch S. Dikovsky D. Zyracki M. Olguin C. et al.Active printed materials for complex self-evolving deformations.Sci. Rep. 2014; 4: 7422Crossref PubMed Scopus (376) Google Scholar The interdisciplinary research, including the development of smart/functional materials and design strategies for 3D printing, has driven 4D printing toward even faster, more complex geometry control and enhanced functionalities. In this issue of Matter, Xie and colleagues report a 4D printing method called modular 4D printing, which is done via the assembly of the 3D-printed self-folding SMPs through dynamic exchange reaction-based interface welding toward geometry-dictated functions.4Fang Z. Song H. Zhang Y. Jin B. Wu J. Zhao Q. Xie T. Modular 4D Printing via Interfacial Welding of Digital Light-Controllable Dynamic Covalent Polymer Networks.Matter. 2020; 2 (this issue): 1187-1197Scopus (54) Google Scholar Among the various 3D printing methods, the digital light processing (DLP) method, based on digital micro-mirror devices (DMDs), emerges as a fast printing approach.5Tumbleston J.R. Shirvanyants D. Ermoshkin N. Janusziewicz R. Johnson A.R. Kelly D. et al.Continuous liquid interface production of 3D objects.Science. 2015; 347: 1349-1352Crossref PubMed Scopus (1347) Google Scholar More recently, grayscale DLP (gDLP) has been developed to print shape-shifting multi-material for 4D printing. For example, a two-stage cure resin has been developed for gDLP printing of graded multi-materials with widely tunable mechanical property gradients and thermomechanical properties for 4D printing.6Kuang X. Wu J. Chen K. Zhao Z. Ding Z. Hu F. et al.Grayscale digital light processing 3D printing for highly functionally graded materials.Sci. Adv. 2019; 5: v5790Crossref Scopus (201) Google Scholar However, direct 3D printing of shape-shifting structures with on-demand shapes usually requires the sophisticated design of functional resins. It is also time-consuming for large-size object printing because of the intrinsic layer-by-layer fabrication process. An alternative approach to access 3D structures is by self-folding from 2D precursors, which is also one type of 4D printing. Much progress in the digitally controlled 2D-to-3D transformation has been reported by using different shape-shifting mechanisms, including non-uniform swelling/deswelling and local stress relaxation/generation. However, shape shifting from flat films suffers from a narrow range of accessible 3D configurations. Xie and colleagues introduced a concept of modular 4D printing by using the DLP-printed self-folding structures as modules for devices assembly/welding assisted by dynamic chemical bonds.4Fang Z. Song H. Zhang Y. Jin B. Wu J. Zhao Q. Xie T. Modular 4D Printing via Interfacial Welding of Digital Light-Controllable Dynamic Covalent Polymer Networks.Matter. 2020; 2 (this issue): 1187-1197Scopus (54) Google Scholar To achieve this goal, they first synthesized a diacrylate crosslinker with a semi-crystalline polycaprolactone (PCL) backbone and dynamic hindered urea linkages. Then they formulated various photopolymers for DLP printing. The location-specific exposure using digital light pattern and light attenuation across the thickness by photo absorbers allows precise heterogeneous curing in terms of in-plane and thickness direction, respectively. Upon removal of the residual monomers via dissolution treatment, the cured 2D films could bend and fold into predetermined 3D geometries. Because the cured material is an SMP, a smart material that can be programmed into a temporary shape and recover the initial shape in the presence of external stimuli, post-shape programming of the 4D-printed self-folding structures can also be achieved. Owing to the thermally reversible exchange reaction by the dynamic hindered urea linkage, the printed materials possess the attributes of robust interface weldability and reshaping capability at elevated temperatures. Xie and colleagues could, therefore, use the printed structures as modules for complex shape manipulation by a two-step assembly and welding approach (Figure 1). For example, four pieces of printed SMP films were first transformed into a Miura-ori module by monomer removal and then were assembled and welded into a folding-based lattice cylinder. Moreover, the obtained cylindercould be further reprogrammed into different meta-structures and permanent shape via the shape memory effect and dynamic exchange reaction, respectively. It was demonstrated that assembled structures could be programmed into different permanent configurations for shape-shifting devices with tunable Poisson’s ratio. Besides, multimaterial modular assembly can be achieved by using modules with distinct mechanical and thermomechanical properties. One of the advantages of the current approach is the versatile monomer choice to enable tailorable material properties. Xie and colleagues were able to prepare an elastomer and two semi-crystalline plastics with different glassy transition temperatures by altering monomer ratios and types. As an example, a Kresling-patterned model was then assembled into a three-layer multi-material cylinder using different materials in each layer. This modular cylinder shows functional properties, including rotation and sequential deformation upon tension and compression. The work of Xie and colleagues provide a versatile approach to access functional shape-shifting structures and devices with on-demand complex geometry. Notably, the vibrant interdisciplinary research of developing multi-stimuli-responsive materials, including various shape-programmable materials and the emerging covalent adaptive network polymers containing dynamic chemical bonds,7Podgórski M. Fairbanks B.D. Kirkpatrick B.E. McBride M. Martinez A. Dobson A. Bongiardina N.J. Bowman C.N. Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs).Adv. Mater. 2020; : e1906876Crossref Scopus (169) Google Scholar will play an increasingly critical role in 4D printing. Toward dedicated geometries and multifunctional properties of 4D printing, a parallel advance in stimuli-responsive functional materials, 3D printing techniques, and design across multiple length scales are under development.8Kuang X. Roach D.J. Wu J. Hamel C.M. Ding Z. Wang T. Dunn M.L. Qi H.J. Advances in 4D Printing: Materials and Applications.Adv. Funct. Mater. 2018; 0: 1805290Google Scholar This invocation is shifting 4D printing from simple shape changing to the next generation of integrated structure-functionality evolution with enhanced sustainability. In addition to welding, the merit of dynamic chemical bonds also allows other properties, including self-healing, recycling, and reprocessing, for 4D printing.9Zhang B. Kowsari K. Serjouei A. Dunn M.L. Ge Q. Reprocessable thermosets for sustainable three-dimensional printing.Nat. Commun. 2018; 9: 1831Crossref PubMed Scopus (200) Google Scholar Moreover, we believe multifunctional properties, including mechanical, optical, electrical, and magnetic properties evaluation, can also be accommodated and integrated into 4D printing by using the modular concept. This rapid progress is laying the foundation for this exciting field, which can find broad applications in aerospace, electronics, biomedical devices, and more. Modular 4D Printing via Interfacial Welding of Digital Light-Controllable Dynamic Covalent Polymer NetworksFang et al.MatterMarch 9, 2020In BriefThe concept of modular 4D printing is put forward with the introduction of dynamic covalent bonds. The activation of dynamic bonds at a contact interface permits the assembly of different modules with tailorable materials properties. Intriguingly, the intrinsic dynamic nature of modules allows further manipulation of the permanent shapes. With the integration of multiple materials and geometric structures, we demonstrate the fabrication of shape-shifting devices with geometry-dictated functions. Full-Text PDF Open Archive