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Research

My Research

As part of the third industrial revolution, 3D printing has been continue to evolve including cost reduction, higher resolution, multimaterial integration, and multifunctional devices. I have been working on the development of advanced 3D printing technologies for more than 9 years. The extraordinary flexibility of 3D printing makes this a rich topic with many possible avenues of investigation. In particular, my research is not limited to single ink development, but encompasses multi-material and multifunctional systems including metallic and ceramic materials, high-performance polymer composites (thermoplastic and thermoset), nano materials (metals and semiconductors), smart stimuli-responsive materials, and biomaterials (hydrogels and silicone rubber). The printed devices could impact various fields such as translational medical devices, airplane or automotive industry, and human-machine interfaces.

Project I: Multi-material 3D printing system

A multi-material 3D printing system with high resolution and versatility has been invented. The system is composed of three subsystems, including motion control subsystem, ink dispensing subsytem and monitoring camera subsystem. These subsystems are integrated to provides the whole system with high-performance, non-clogging, noncontact, high speed, high resolution, and high accuracy precision. The printed objects can be made from a wide variety of materials inks, such as thermoset epoxy, thermoplastic polymer, rubber, hydrogel, bioinks or metallic materials. An US patent US20190072439A1 was issued.

One of the multi-material 3D printing systems

Four ink dispensers are employed to print electronics

Project II: Hybrid 3D bioprinting-Neural regeneration scaffold

A bioengineered spinal cord is fabricated via extrusion-based multi-material 3D bioprinting, in which clusters of induced pluripotent stem cell (iPSC)-derived spinal neuronal progenitor cells (sNPCs) and oligodendrocyte progenitor cells (OPCs) are placed in precise positions within 3D printed biocompatible scaffolds during assembly. The bioprinted sNPCs differentiate and extend axons throughout microscale scaffold channels, and functional activity of these 'living' neuronal networks are confirmed by physiological spontaneous calcium flux studies. This work has been published on Advanced Functional Materials, and highlited as back cover and in Science Transnational Medicine.

Schematic of the hybrid 3D bioprinting

Comparison of a transected rat spinal cord and the printed scaffold

Axon was regenerated from the bioprinted stem cells after four-day culture

Project III: 3D printed stretchable electronics

The wearable devices usually require the electronics of both structures and materials that can stretch, bend, fold, twist and generate signals to monitor the body motion or the physiolocial change of the wearer. Such properties are limited with traditional silicon/metal-based electronics. However, 3D printing of multi-materials including semiconducting nanoparticles, elastomer, metal/polymer composites, solid and liquid metal leads are ideal to be explored for future wearable devices. I developed a highly conductive and stretchable ink which can be printed under ambient conditions. Afterwards, the inks were demonstrated by 3D tactile sensors.This work has been published on Advanced Materials and highlighted in over 50 news outlets.

Optical images of the 3D tactile sensors

The printed tactile sensor can be stretched to three times of the origianl length.

Then sensing behivor of the tactile sensor

Project IV: Solvent-cast 3D printing technique & Freeform printing

During my Ph.D. study, I invented the solvent-cast 3D printing technique to fabricate various microstructures using thermoplastic polymer polylactide (PLA) and their nanocomposites solutions. The method consists of the robotically controlled micro-extrusion of a concentrated polymer solution ink filament, combined with rapid solvent evaporation after the filament exits the micro-nozzle. The effects of printing parameters including polymer content, applied pressure, nozzle diameter and robot velocity on the ink viscosity, filament evaporation rate, microstructure geometry and crystallinity were characterized. By carefully controlling the ink viscosity and the solvent evaporation speed, 3D freeform structures without any supporting layer can be directly printed. This work has been published on Small as the inside cover and Langmuir.

The schematic of the solvent-cast 3D printing of freeform structures

3D printed structures varying from 1D to freeform 3D.

Solvent-cast printing processing map showing the parameter ranges for different microstructures fabrication

Zone I 1D filament

Zone II 2D network or 3D lbl structure

Zone III 3D freeform structure

Zone IV Broken or rough filament

Zone V Insoluble PLA solutions

Zone VI Dilute PLA solutions

Video showing the solvent-cast 3D printing of freeform squire spiral

Video showing the unstable solvent-cast 3D printing of bionic high-toughness fiber

Project V: 3D microfluidic channels

3D printing could significantly change the field of microfluidics with the ability to fabricate a complex 3D microfluidic device in a single step from a computer model using integrated multimaterials. These printed 3D microfluidic chips could provide new microfluidic capability that is challenging, which can find wide applications in the sensors, lab on a chip, translational medicine, and self-healing structures. Part of my research about 3D printed microfluidic channels was published on Small and more work is ongoing.

Fluorescent top and side view images of a 3D spiral microchannel embedded in an epoxy matrix, which can be used as cell or particle sorting and seperation.

3D printed microfludic chip can be used to print water-in-oil droplet or core-shell microspheres

Project VI: 3D printed sensors

3D printing technique offer us a high degree of freedom to design and fabricate various complex 3D sensors and actuators, which are difficult to achieve for the tranditional methods. During my Ph.D. study, a multifunctional 3D liquid sensor made of a PLA/MWCNT nanocomposite and shaped as a freeform helical structure was fabricated by solvent-cast 3D printing. The 3D liquid sensor featured a relatively high electrical conductivity, the functionality of liquid trapping due to its helical configuration, and an excellent sensitivity and selectivity even for a short immersion into solvents. This work was published on Nanoscale. Recently, a highly stretchable 3D tactile sensor was directly printed on a fingertip using four different inks and this work was published on Advanced Materials. Besides, a sweat sensor and a photo sensor were printed and published in another two Advanced Materials.

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