Liquid Phase 3D Printing: How This New Technology Can Help Bring 3D Printing to the Operating Room

Abstract

Purpose of Review

The purpose of this review is to discuss the state of technology in liquid phase three-dimensional (3D) metal printing, how this has affected the field of orthopedic surgery, and changes that we can expect in the future with the rise of this printing technology. We will also discuss how liquid phase metal printing can possibly bring three-dimensional printing to the operating room.

Recent Findings

The use of liquid phase 3D metal printing may become commonplace for manufacturing orthopedic implants and devices. Traditional metal printing involved powder-based metals and high-energy beam technologies that are expensive, time-consuming, and potentially wasteful. This unfortunately leaves them out of reach for most end consumers such as orthopedic surgeons. Liquid phase metal printing is less expensive and faster. However, there is still major work required to bring this technology to the operating room and benefit patients.

Summary

While major strides have been made in the field of liquid phase metal three-dimensional printing, there are still significant developments in the pipeline. These could lead to future production of personalized orthopedic implants and devices with optimal material properties for patients.

Introduction

Three-dimensional (3D) printing is a process of making solid objects using digital design data [1•]. Objects are printed in an additive process whereby material is deposited in successive layers that form an object’s cross-section [1•]. For this reason, 3D printing is also referred to as additive manufacturing [1•]. These methods contrast modern subtractive manufacturing wherein a block of material is cut or hollowed out, for instance, with a milling machine into the final design [1•]. 3D printing enables the production of complex shapes using less material than traditional manufacturing methods [2••].

There are several uses of 3D printing in orthopedic surgery including anatomical models for surgical planning, guided jigs to avoid important structures, and metallic implants for areas of bone loss or replacement [3]. 3D printing has improved preoperative surgical planning as patient-specific models can now be printed to understand complex anatomy [2••, 4]. Models are useful in challenging trauma cases, such as pelvic or acetabular fractures, as well as adult reconstruction cases, such as revision hip and knee arthroplasty with severe bone loss. This has allowed surgeons to adjust their surgical plan and reduce their time in the operating room [5]. Furthermore, personalized 3D-printed jigs now enable surgeons to be precise and accurate during procedures especially for rotational and angular deformity cases [1•].

The advent of 3D printing of metal objects introduces the possibility of personalized orthopedic implants with improved mechanical properties and osseointegration. There are already examples of 3D-printed orthopedic implants being put in patients including knee prostheses, interbody fusion cages, hip prostheses, and acetabular cups [3, 5]. Figures 1 and 2 show examples of modern 3D-printed implants. Not all metal 3D printing, however, is the same. Most commercially available metal 3D printers today perform additive manufacturing (AM) using traditional metal powder-based methods. These are limited by high costs of manufacturing equipment and materials as well as extensive post-printing processing. However, new methods of liquid metal printing bypass the limitations of powder-based metal printing and may become an emerging technology that changes the face of implant manufacturing in orthopedic surgery.

Fig. 1.

3D-printed femoral cone implantation

Fig. 2.

3D-printed acetabular augment

Powder-Based 3D Printing

There are many types of 3D printing AM methods based on the Standard Terminology for Additive Manufacturing (ASTM) including beam-based methods like directed energy deposition or powder bed fusion and numerous beamless methods such as nanoparticle jetting and binder jetting [6]. But the most common commercially used techniques are beam-based printing methods that use powder bed fusion [7]. These methods include selective laser sintering (SLS), selective laser melting (SLM), and electron beam melting (EBM) [7]. These all use a high-energy source, whether it be an electron beam (EBM) or laser (SLM, SLS), to melt (SLM, EBM) or sinter (SLS) metal powders layer by layer in a vacuum or inert gas–protected chamber [7]. The printer receives instructions from a computer-aided design (CAD) file to create the 3D object. These techniques can be used to print many metals into novel designs at a high resolution, and there are many companies that use 3D metal printing methods today [2••]. For example, orthopedic implant developers LimaCorporate collaborated with Arcam to create the Delta-TT acetabular cup using these powder-based printing methods [8]. These cups have a hexagonal microstructure that mimics trabecular bone leading to improved osseointegration [2••]. With almost 100,000 cups printed and implanted into patients, the Delta-TT is an example of what metal 3D printing has to offer to orthopedics.

There are several challenges to this method of 3D AM metal printing at scale. The first is cost. Current metal printers that use beam technology are large and expensive and therefore are usually not directly accessible to many consumers. [7] Part of this cost comes from large amounts of unused metal powder at the end of the printing process, which can be recycled but may affect component quality from metal powder oxidation [9••]. Furthermore, each finished component needs post-processing that is laborious and time-consuming [7]. Structural integrity is another challenge. The printers have high heating and cooling rates that can lead to cracking with certain alloys [10]. A new alternative technology is liquid phase metal 3D printing.

Liquid Phase Metal 3D Printing

3D printing using liquid metal is considered a type of beamless AM as defined by the ASTM [6]. This methodology extends the printing mechanism found in home and office printers to metal printing, making it more scalable than the beam-based powder fusion methods that require high-energy lasers, inert environments, and substantial post-processing. Liquid phase metal 3D printing prints objects that would take hours to days with beam-based powder fusion techniques in seconds to minutes. There are two main categories of liquid metal printing processes—continuous inkjet printing (CIJ) and drop-on-demand (DOD) printing [9••, 11]. In CIJ printing, metal droplets are continuously released from a small nozzle by vibrations from a piezoelectric transducer. Electrodes near the nozzle charge the metal droplets, which then become deflected by high-voltage deflection plates. Individual droplets eventually generate the metal object. Figure 3 shows a schematic of CIJ printing. On the other hand, DOD printers involve ejecting metal droplets only when needed and thus do not require the components present in CIJ printers. Since the droplets of metal are created individually in DOD-type printers, this process can take longer than CIJ-type printers [12]. CIJ printers, however, can have significant waste of the liquid metal unless complex mechanisms are created to recycle the unused stream of liquid metal.

Fig. 3.

Schematic of continuous inkjet printing. A drop generator continuously releases ink droplets, which become charged after passing the charge electrodes. High-voltage deflection plates aim the droplets onto the paper. A recirculation system returns excess droplets to the ink supply so they are not wasted

The first 3D-printed metal was as early as 1969 using CIJ techniques in which a Bi-Pb–based metal alloy was used as the ink [9••]. This process was further developed over later decades using other piezoelectric processes and other metals [12–16]. While some printers use CIJ-type techniques [13], DOD printing has been more popular due to the simplicity of its design. There are several different DOD processes for ejecting the liquid metal. Piezoelectric DOD techniques utilize actuators that eject liquid metal through squeeze, bending, or push processes when a voltage is applied [17, 18]. Electrohydrodynamic, magnetohydrodynamic, and electromagnetic printing all use electric fields, magnetic fields, or their combination to eject liquid metal particles. In electrohydrodynamic DOD printing, an electric field affects the surface tension of the liquid metal into the shape of a cone, allowing for droplet generation from the tip of the cone. With the help of electric fields, ejected liquid particles can be orders of magnitude smaller than the nozzle. Thus, the main benefit of these techniques is the ability to print high-resolution parts [19]. Figure 4 shows a schematic of this process. Magnetohydrodynamic DOD liquid phase metal printing involves the use of a magnetic field to eject liquid metal drops. Two products use this technology including MetalJet (Oce-Technologies BV, Netherlands) [20] and MagnetoJet (Vader Systems, Xerox) [17]. These promising new technologies offer metal printing of complex parts at much less the cost than traditional powder printers. In electromagnetic DOD printing, both an electric and magnetic fields are used to eject the metal droplets, and magnetic field intensity could be used to change droplet diameter [21]. These designs are complex and require electricity either for a piezoelectric material or for electric and magnetic field generation. More recently, there have been several research groups using pistons and pumps or gas pressure to eject liquid metal droplets to make metal wires of various sizes and shapes [22]. Another group used a unique liquid phase Bi₃₅In_48.6Sn_15.9Zn_0.4 metal alloy that has a melting temperature just above room temperature [23]. The syringe with the metal is heated with a coil to above room temperature to melt the metal. When the metal is ejected, it rapidly cools within ethanol or water at room temperature, allowing for faster turnover to the next layer of printing [23].

Fig. 4.

Electrohydrodynamic drop-on-demand printing. The droplet at the tip of the nozzle gets stretched to a fine point. An electric field causes charges to migrate (A). The surface stretches (B) into a cone (C). This allows even finer drops to be released

Many of the methods presented here use liquid metals with relatively low melting points. Printing liquid phase metals with high melting points is more challenging as the printer must be able to sustain these high melting points. One system, the Starjet system, uses silicon parts and can therefore print metals such as aluminum and tin [24]. Printing metals such as steel and copper, however, is more challenging. Several new technologies have shown success in this endeavor. One technique suspends these high melting point metals in using graphite and aluminum crucibles [25, 26]. Another is impact-driven DOD printing. This method utilizes an impactor driven by a solenoid to strike a vibrating rod that then moves a piston to eject metal droplets within a temperature-controlled crucible that was tested at temperatures high enough to melt silver and copper (1227°C) [27]. Lastly, laser technology that melts solid metals into liquid droplets in a process called laser-assisted forward transfer is still being developed [9••]. Figure 5 illustrates a schematic of this process. This methodology is very promising as the melting point of the metal is less of a concern in creating the metal droplets. The challenge here instead, however, is the cooling of the liquid metal in a fashion that allows the objects to be generated. The droplets must cool enough before they hit the platform on which the 3D object is being built as to not damage this platform but also cannot be cooled too fast that they do not adhere to the other droplets being ejected to create the 3D object [9••]. In summary, there are numerous discoveries in droplet generation in CIJ and DOD printing. Further research is required to develop manufacturing standards with these techniques, but active interest in this field and its rapid progression show promise for liquid metal 3D printing.

Fig. 5.

Schematic showing laser-induced droplet generation from metal film. a A microscope focuses a laser beam onto a metal sheet, shown in orange. b The laser turns off while the metal is melted into a droplet. c The droplet falls onto the substrate below, shown in green

Liquid Phase Metal Printing Benefits

While the technology is still relatively new, liquid phase metal printing may eventually become a simpler and more scalable method of 3D metal printing. Many of the current commercially available 3D printing options rely on high-power lasers, inert environments, and/or complex processing after printing. Additionally, metal alloys may be printed with liquid phase metal printing as opposed to selective laser sintering/melting which use powder methods [20]. In contrast to powder-based fusion printing, the only requirement for liquid metal printing is that the material can be melted; this may expand the range of materials that can be printed. This contrasts with melting or fusing powder in more traditional 3D printing methods. Furthermore, the technology has the potential to be less expensive as some DOD methods are less wasteful than powder-based AM methods [9••, 28]. If liquid printing can be performed at lower cost, this has the potential to be accessible by end users and customers such as hospitals and orthopedic surgeons.

Challenges of Liquid Phase Printing

There are still several challenges to liquid phase 3D metal printing before this technology becomes more commercially available. Currently, Vader Systems (now Xerox) is only one liquid metal printed to our knowledge on the market. In contrast, there are many different traditional powder-based printers being used today [2••]. First are the melting points of the metals that can be used for liquid metals that can be used for most of these printers. Many of these printers require metal or metal alloys with low melting points. From an orthopedic standpoint, stainless steel, titanium, and cobalt-chromium are common in orthopedic implants and have very high melting temperatures. Liquid phase metal printers must be able to handle these high melting temperatures, and some of them, such as laser DOD printers, have shown significant promise.

Another challenge is the resolution of the objected being printed. Creating intricate objects, such as those used in surgery, usually requires high-resolution 3D products. Creating this from droplets of metals is no easy feat. There are many aspects of these new printers that will need to be adjusted and experimented with in order to bring a high-resolution such printer to market. Some of these include, but are not limited to, the type of metal being printed, metal temperature, cooling methods, droplet size, droplet shape, printing environments, speed, and distance between nozzle and the platform on which the object is being printed. Furthermore, these prints may still need significant post-processing features to give a final product.

Another specific challenge for application in orthopedic surgery is the material properties of the 3D-printed objects. In most fields and especially in orthopedic surgery, the material properties of the objects used in surgery are essential. Currently, most of metals being printed with this technology are not the standard metals, such as cobalt-chromium, titanium, and stainless steel, being used in orthopedic surgery. In addition, the strength of any of the materials being printed by various techniques is yet to be determined [9••, 13, 29]. These studies are needed before we can bring this technology to the orthopedic surgery operating room.

Clinical Application

Much of the potential of the clinical applications of liquid phase 3D metal printing stems from the ability of putting this technology in the hands of the end users, namely orthopedic surgeons and their teams. There are vast uses for this technology in all orthopedic specialties. The holy grail of 3D printing is the production of personalized implants on-demand for patients. With 3D printing techniques, these implants could have intricate internal architecture to enhance osseointegration, excellent fit in the patient, and improved material properties that better match native bone resulting in better patient outcomes and longer implant life.

Conclusions

While major strides have been made in the field of liquid phase metal 3D printing, there are still significant developments in the pipeline. These could lead to future 3D printing capabilities for orthopedic surgeons.

Declarations

Competing Interests

The authors declare no competing interests.