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Materials Science in Additive Manufacturing Adhesion study for multi-material 3D printing

tissue engineering, and aerospace and defense. to conduct electricity, and TPU for its elasticity and
1-5
Fused filament fabrication (FFF) stands out among durability. 19
the array of multi-material 3D printing technologies The fusion of these materials within a single print
(Figure 1A) for its versatility, accessibility, and efficiency could lead to the creation of objects with both structural
in creating complex geometries and the availability support and flexible characteristics, offering myriad
6-8
of a wide range of materials, such as polylactic acid possibilities such as actuators and electronic components
(PLA), thermoplastic polyurethane (TPU), acrylonitrile integrated into structural frameworks. This capacity
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butadiene styrene (ABS), polyethylene terephthalate for in situ fabrication of structures combining multiple
glycol, nylon, and polyether ether ketone (PEEK). To material allows for more integrated system with fewer
date, various material combinations have been attempted, parts and reduces the need for manual assembly. In recent
such as PLA/ABS, PLA/TPU, conductive polylactic acid research, Goh et al. explored the multifaceted capabilities
(cPLA)/ABS, PLA/TPU, chlorinated polyethylene (CPE)/ of multi-material 3D printing by synergizing the distinct
TPU, CPE/PLA, PLA/conductive-PLA, ABS/conductive- properties of cPLA and TPU. Their innovative approach
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PLA, and ABS/TPU, just to name a few. 7,9-14 One of the led to the fabrication of a multifunctional robotic gripper,
promising areas of interest within multi-material FFF is which integrates the rigid features of cPLA as a variable-
the printing of soft and rigid materials, which promises stiffness backbone, along with the inherent softness of
to revolutionize the manufacturing of multifunctional TPU to impart a gentle gripping action. This integration
applications, from structural electronics to soft robotic exemplifies the potential of combining soft and rigid
devices. 15-18 Among the materials of interest, cPLA and materials to create adaptive, responsive structures that
TPU have garnered significant attention due to their leverage the best attributes of each constituent material
unique properties – cPLA for its rigidity and ability for advanced robotic applications. However, a critical
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challenge that impedes the broad implementation of
A this technology is the issue of interlaminar adhesion
between distinct material types. Typically, the adhesion
mechanisms between dissimilar polymeric materials are
adsorption, mechanical interlocking, polarity, diffusion,
etc., as illustrated in Figure 1B. Effective adhesion between
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cPLA and TPU is pivotal to ensure mechanical integrity
and functional performance. Nevertheless, the disparity in
the thermal, mechanical, and chemical properties between
PLA and TPU creates a complex interface that is prone
to delamination and structural weakness under stress.
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Moreover, the different melting temperatures and rates
of thermal contraction during the cooling phase further
complicate the bonding process, resulting in a lackluster
interfacial adhesion.
Existing approaches to mitigate these adhesion issues
B
are as diverse as the applications they serve. Strategies
range from plasma surface treatments to increase the
surface energy of the materials, to physical methods
such as the inclusion of mechanical interlocks or using a
gradient transition between dissimilar materials. 7,10,11,14,23-26
From a mechanical perspective, enhanced adhesion
between 3D-printed filaments can be achieved by
increasing the contact surface area at the interface. This
can be accomplished through the implementation of
Figure 1. Schematics illustrating (A) the fabrication of a multi-material a skewed toolpath configuration and by elevating the
part through a multi-nozzle fused filament fabrication process and (B) material extrusion rate, both of which serve to augment
the possible adhesion mechanisms at the interface of a fused filament the interfacial bonding area between the filaments. 27,28
fabricated multi-material part. Adapted from Watschke et al. (licensed
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under CC BY 4.0). Abbreviations: cPLA: conductive polylactic acid; TPU: From a materials science perspective, researchers have
Thermoplastic polyurethane. experimented with the introduction of chemicals or

Volume 3 Issue 1 (2024) 2 https://doi.org/10.36922/msam.2672