July 08 2017 by Drew Haser
Implanting organs grown in the lab into patients is a grand challenge, which first came to reality in 1999. However, after two decades of additional research very few of these lab grown organ transplants have been attempted, and more than 10,000 people still die every year waiting on organs. Needless to say, promise remains large for advances in tissue engineering.
Now, plasmonic nanoparticles - specifically gold nanorods (AuNRs) - are providing new advances towards an ultra precise, high-throughput, low cost ability to assemble and grow tissues.
APPROACHES TO TISSUE ENGINEERING
The majority of transplanted tissues grown in labs today use a ‘top-down’ approach. This involves starting with a biocompatible (often biodegradable) scaffold upon which cells are seeded and allowed to grow and differentiate.
With the top-down approach comes limitations due to
- low-density cell growth,
- uneven or slow cellular diffusion,
- and imprecise control of the cellular microenvironment.
Control of the microenvironment is notable for its importance in forming microvasculature, cell-to-cell signaling, and the physiological detail of cellular niches. While top-down advances continue, more attention is being generated towards novel ‘bottom-up’ methods.
As opposed to its counterpart, this approach seeks to build (or in some cases print) microscale features that will eventually assemble into tissues. Bottom-up is appealing due to the ability for greater control over the microenvironment. Additionally, some argue a biological advantage since it mimics the natural construction process of tissues from biological building blocks.
Bottom-Up methods to assemble biological tissue components include:
- Patterning [acoustic, magnetic, liquid template, etc.]
- Advanced Fabrication [microfluidic guidance, DNA targeting, electrowetting, etc.]
- 3D Bioprinting
- Light-Directed [optical/optoelectronic tweezers, etc.]
No single method can claim dominance, and each has unique limitations.
- Patterning lacks microscale structural control;
- Fabrication is too slow and costly;
- 3D printing is not achievable for all cell types and is a harsher process;
- Optical tweezers cannot move objects greater than ~100 µm in diameter.
However, new research using nanoparticles and light-directed assembly suggests the possibility of rapid, controlled assembly of cellular structures. Like advances following the introduction of stem cells, AuNRs may be a key to the continued evolution of tissue engineering and regenerative medicine.
PLASMONIC GOLD CONTROLLED TISSUE ASSEMBLY
Scientists from the National University of Singapore and the University of Houston have demonstrated a new bottom-up approach utilizing the plasmonic properties of gold nanorods.
In a recent paper published in Small, the researchers constructed tissue patterns using stem cell seeded microgels, spheroids, and other microparticles. With AuNRs suspended in media and near infrared (NIR) light, they achieved rapid and precise assembly. At 30 seconds of irradiation a visible 1 mm diameter particle was formed, and in 2 minutes they had organized up to 1000 microparticles (~ 2 cm).
Light-directed assembly alone is unable to move and manipulate large microparticles. But by adding plasmonic AuNRs the authors were able to induce a controlled convection flow capable of directing movements.
HOW IT WORKS
By matching the laser wavelength to the local surface plasmon resonance (LSPR) of the AuNRs a strong conversion from light energy to heat energy occurs, creating so-called thermoplasmonic convection. The localized heat gradients produced by the interaction of AuNRs and NIR light were shown to generate up to 50 pN of force with less than 1 W/cm2 of laser energy; enough to quickly move objects approaching 300 µm in diameter. In their design, the convection flow vectors move and trap particles towards the center of the light spot. Traditional direct optical trapping methods often require close to 500 kW/cm2 of energy to trap a single 80 µm particle.
Furthermore, the researchers constructed a motorized XYZ stage which enabled various patterns to be constructed by altering the location of the incident light. Using this capability tissues were constructed in precise patterns including 3 parallel straight lines, a ‘N’ shape, and a clock-key configuration. Cell viability remained high at approximately 80% because of the use of low energy NIR light, and after 2-3 weeks of incubation post-assembly, formed 3D microtissues and maintained healthy rates of proliferation.
The results of the study are exciting since they point to the feasibility of precisely controlled, high-throughput, and robust tissue assembly. With this capability, “a broad array of application is expected” according to the authors, “ranging from 3D bioprinting to regenerative medicine, tissue engineering, bottom-up manufacturing and biofabrication.”
Will Gold Nanoparticles Lead to
Greater Control of Tissue Engineering?