4D printing: Technological developments in robotics applications

Abstract

The idea of four-dimensional (4D) printing is the formation of intricate stimuli-responsive 3D architectures that transform into different forms and shapes upon exposure to environmental stimuli. 4D printing (4DP) of smart/intelligent materials is a promising and novel approach to generate intricate structures for biomedical, food, electronics, textile, and agricultural fields. Nowadays, soft robotics is a growing research field focusing on developing micro/nanoscale 4D-printed robots using intelligent materials. Herein, recent advancements in 4DP of soft robotics, actuators, and grippers are summarized. This review also highlights some recent developments in novel robotics technologies and materials including multi-material printing, electro-, and magneto-active soft materials (MASMs), and metamaterials. It also sheds light on different modeling mechanisms including numerical models and machine learning (ML) models for fabricating highly precise and efficient micro/macro-scaled robots. The applications of shape-memory polymers (SMPs), hydrogels, and liquid crystal elastomers (LCEs)-based 4D-printed soft and intelligent robots in different engineering fields are highlighted. Lastly, this review incorporates current challenges which are hindering the actual utilization of 4D-printed soft robotics and their possible remedies.

Introduction

In the contemporary era, additive manufacturing (AM), or three-dimensional (3D) printing, has drawn a tremendous attraction from engineers and scientists due to its excellent adaptability and ability to print intricate shapes [1], [2], [3]. This technique incrementally adds different materials including polymers, metals, composites, cermets, ceramics, and metamaterials to develop complex and precise geometries [4], [5], [6]. Consequently, 3D printing (3DP) technology is extensively applied in a wide range of industries including automotive, aerospace, biomedical, food, construction, and electronics [7], [8], [9], [10]. According to the American Society for Testing and Materials (ASTM) international, fused filament fabrication (FFF)/fused deposition modeling (FDM), inkjet printing (IJP), digital light processing (DLP), stereolithography (SLA), selective laser sintering (SLS), selective laser melting (SLM), direct ink writing (DIW), and continuous liquid interface production (CLIP) are different processes employed for the 3DP of a variety of materials [11], [12], [13], [14]. Multi-photon lithography or direct laser writing (DLW) is another high-resolution 3DP technology, which is used for the nano-/micro scaled printing of intricate 3D objects [15], [16], [17]. This approach depends upon the multi-photon technique using near-infrared femtosecond laser pulses firmly concentrated within the volume of the photoresist, thus, permitting the development of 3D-printed architectures using sub-micrometer resolution [18], [19], [20].

Despite the significant development in the 3DP field, the production of static solid objects is currently limiting its commercial utilization [21], [22], [23]. Therefore, novel materials that are programmed to change their properties, functionalities as well as their shapes are developed through newly emerged four-dimensional (4D) printing technology [24], [25], [26]. In other words, incorporation of life into 3D-printed objects through time dimension helps in developing 4D-printed adaptive and dynamic products [27], [28], [29]. These dynamic products are made-up of different smart materials including shape memory polymers (SMPs), hydrogels, and liquid crystal elastomers (LCEs) [30], [31], [32], [33]. Compared to conventional subtractive manufacturing, 4DP technology possess myriad advantages like rapid prototyping, cost-effectiveness, accessibility, lower material consumption, excellent design flexibility and geometric complexity [34], [35], [36]. The shape transformation of dynamic products is mainly triggered by external energy inputs like ultraviolet (UV) light, heat, pH, or other sources [37], [38], [39]. Besides one-way change, reversibility is another feature of 4D printing (4DP) technology [40], [41], [42]. It is mandatory to select appropriate stimuli-responsive materials for developing intricate and reversible 4D-printed architectures [43], [44], [45]. Additionally, these materials are triggered through internal and external stimuli. External stimuli incorporate light, electric field, acoustic waves, and magnetic field [46]. On the other hand, chemical stimuli including pH and humidity are the internal stimuli [47], [48], [49]. The stimuli-responsive behavior of smart materials and endless shape possibilities upon exposure to stimuli expands their utilization in smart textile, mechatronics, self-folding food packaging, electronics, automotive, deployable structures, and healthcare systems [50], [51], [52], [53], [54]. Recently, 4DP technology is extensively applied for developing macro/micro-scaled soft robots for different engineering applications.

Even in this modern world, classical manufacturing techniques are still used to fabricate conventional large robots for numerous industrial applications [55]. The wastage of materials and high energy consumption rates are some of the prominent limitations of these techniques. Thus, the world is continuously searching for innovative technologies for the development of soft robotics and other smart devices [56]. 3DP/4DP technology can also be applied to manufacture micro-/nano-scaled robots [57], [58], [59]. Conventional micro-fabrication processes exhibit certain limitations in terms of geometries, design, and material selection [60]. 4DP technology at a small scale has shown tremendous potential in developing soft robotics and actuators for mechanical engineering, material science, and biomedical engineering fields [61], [62], [63], [64]. These robots use advanced integrated technology incorporating control systems, smart/intelligent materials, acoustics, and chemistry at micro-/nano levels [65]. This technology incorporates main features such as repeatability, reproducibility, and controllability of 3DP along with supporting technical developments including modeling of novel 3D-structured soft robots [66], [67], [68]. The primary objective of this review article is to highlight research advancements and current challenges in 4D-printed soft robotics. This will be helpful for developing a variety of robots for a wide range of applications.

Fig. 1 shows three different steps involved in the transformation of a 3D-printed product into a 4D-printed intelligent object. Smart/intelligent/programmable materials upon exposure to certain stimuli change their shapes, aesthetics, or color through bending, twisting, swelling, and deswelling [69], [70], [71]. These soft structures developed through smart materials have tremendous potential for developing sensors, actuators, and other smart devices for biomedical, haptics, adaptive optics, and microfluidics applications [72]. 4DP technology can impart different properties such as in-homogeneous, homogeneous, and functionally-graded as well as develop mono- or multi-material printed architectures [73]. 4DP uses the same 3DP processes to print stimuli-responsive materials (SRMs).