Towards 3D-printed organic electronics: Planarization and spray-deposition of functional layers onto 3D-printed objects

Highlights

• Comparison of different planarization methods for FDM 3D-printing.

• Realization of a novel planarization process based on the spray-deposition of ABS.

• Spray-deposition of PEDOT:PSS, AgNW and CNTs onto printed objects.

• Realization and characterization of a fully-printed semitransparent heating chamber.

Abstract

The last years have seen a surge in the interest for “smart objects” obtained with the integration of electronics and custom designed structures. One of the possible approaches for the fabrication of such devices is 3D printing supporting elements and attach to them discrete electronic components. Opposed to this approach, we demonstrate the facile integration of conformal organic electronics devices in 3D printed structures, where the device is directly fabricated on or in the printed object. To obtain cost-effective and easy-to-scale devices, the substrates are fabricated via a Fused Deposition Modeling (FDM) procedure which leads to inherently rough layers. In order to enable the fabrication of functional layers few hundreds of nm thick, we develop a process, based on spray-deposition, for the planarization of the structure and the in situ deposition a functional layer. The realization of conductive and semi-transparent CNTs, AgNW and PEDOT:PSS thin films on a 3D-printed substrate is demonstrated and the films are characterized in terms of transmittance and sheet resistance. The technique is finally applied for the realization of a semi-transparent heating chamber with an arbitrary shape with potential applications in the field of bioelectronics and consumer applications.

Introduction

With the generic term 3D printing, or additive manufacturing, have been designated all those technologies aimed at the fabrication of solid objects through the deposition of a material, starting from a computer-based 3D model. This technology has recently attracted a paramount interest of research, industry and consumers, leading to the attainment of important results in the fields of bio-engineering [1], [2], [3], electronics [4], [5], [6] and structural engineering [7]. Given the current significance of the technology, it is surprising that although the first registered patent related to a 3D printing machine dates back to 1986 [8], its use for the “fast prototyping” and research has been restricted to the big enterprise business until mid of the first decade of the 2000s. The motivations behind this phenomenon are numerous but they are mostly related to the high prices of the printing apparatuses and materials and the “user unfriendly” modeling software tools [9]. One of the causes for the high prices of the equipment is the complex system behind the classic approaches for the additive manufacturing, namely the Stereolithography (SLA) and the Selective Laser Sintering (SLS). Both methods, in fact, rely on techniques which are difficult to scale down, such as motorized high power light sources. A totally different approach to the production of 3D printed objects is constituted by the so-called Fused Deposition Modeling (FDM), where a thin filament of the wanted material is led through a hot nozzle with a small orifice. The molten plastic is driven over a substrate plate where it cools down and the 3D object is constructed layer by layer. This approach led to the fabrication of more cost-effective printing machines, which can be bought as a finite product or in form of a mountable kit for less than 1000$ [9]. Asides from their limited cost, the FDM simple working principle leads to two other significant advantages. On one hand, these printers can be easily modified to be integrated into complex processes, while, on the other hand, the choice of materials is wide, since it is extended to most of the thermoplastics. Particularly, the latter characteristic can be profitably exploited to directly 3D print functional materials, such as conducting or semiconducting polymers, onto plastic structures [4], [10], [11]. However, in order to produce integrated electronic devices, the use of stand-alone 3D printing technologies does not suffice. In fact, albeit the technology is mature and able to produce layers as thin as few tens of microns, which can guarantee the fabrication of electric lines and simple electronic devices, the manual insertion of discrete components is still needed, and the realization of stacked thin film devices into 3D printed scaffolds is prevented. A possibility to overcome this problem is given by the integration of established deposition techniques, such as ink-jet printing, spin coating or spray coating into the additive manufacturing apparatus. In principle (Fig. 1), an arbitrary substrate could be printed, then, with means of the chosen fabrication method, the functional layers could be developed and, as last step, the mechanical structure of the object could be completed. Furthermore, with the correct choice of materials, the obtained device could be encapsulated and packaged, leading to a fully-integrated production system.

Although for the discussed simplicity of modification and the wide choice of materials, FDM seems to be the most natural choice to realize this kind of complex systems, it presents a major drawback which has prevented so far the obtainment of such solutions. In fact, while for SLA and SLS, the direct sintering or curing of the materials leads to accurate features and to smooth finish, the juxtaposition of extruded lines needed to obtain a single layer through fused deposition molding renders wavy and rough structures [5], [6]. Such uneven substrates are not suitable for conventional deposition techniques (e.g. spin-coating), but they could be profitably exploited through the use of less conventional and more robust deposition methods, such as spray-deposition. This technique has been revealed in the recent years as a promising candidate for the realization of thin film devices with arbitrary shapes [12], on various substrates [13], [14], [15] and integrated with more conventional electronic circuits [16].

The objective of this work is to demonstrate the full integration of spray-deposition and 3D printing. Firstly, we will show that spray-deposition allows the fabrication of thin films on 3D-printed substrates, following the approach used in the past for the deposition on conventional substrates of conducting materials, such as Carbon Nanotubes (CNTs) [17], [18], [19], Silver Nanowires (AgNW) [20] and polymers [21], [22]. In order to achieve this goal, and to overcome the limitations posed by the uneven substrates, different planarization methods are compared in terms of surface roughness, peak-to-trough height and resistance of the fabricated conducting films. Subsequently, we introduce a planarization method based on spray-deposition, which proved to be the most effective and the easiest to integrate. This breakthrough could be the cornerstone for the cost-effective production of thin film electronics on 3D-printed structures and the realization of new classes of functional devices and deposition apparatus.