An international research team has reported an organic pigment with semiconducting properties  adhering to a human kidney cell for low-cost wireless physiology. As reported in a recent issue of Nature Communications, this discovery brings the researchers one step closer to achieve photostimulation of cells. It could be used for implants to stimulate healthy neuron growth such as in the case of traumatic injury or to stimulate blind retinas that will restore partial sight to blind patients.

Organic pigments with semiconducting properties are commonly used for coloration purposes because they are chemically stable, low cost, and environmentally friendly. They can be applied in health monitoring and medical treatments because of their non-toxicity and biocompatibility. Currently, expensive materials such as silicon, gold, and platinum dominate these applications; however, organic pigments with ease in solution processing bring the materials cost down significantly.

Organic semiconductors react to light that could be used for photostimulation of cells, but this is not as simple as it sounds. An organic semiconductor, to be employed as a photo-responsive material in medical applications such as to stimulate blind retinas or healthy neuron growth, should have high absorption coefficient, high conductivity, biocompatibility, and high mechanical strength. In this research, quinacridone was employed as an organic semiconductor. According to co-researcher Eric Daniel Głowacki of Johannes Kepler University and Linköpings Universitet, “The first step in this is to achieve a close and intimate interface between semiconductor and cell in a way which preserves both the properties of the semiconductor and the viability of the cells.”

The synthesized hierarchical nanostructures of quinacridone show the crystalline nature with four polymorphs (α₁, α₂, β, γ) with high reproducibility, confirmed through x-ray diffraction. Different biological-inspired nanostructures were the result of different ligand species used during the synthesis. In order to synthesize nanostructures resembling hedgehogs, coral fungi, houseleek, and agave, ligands used were oleylamine, methylamine, butylamine, and a mixture of butylamine and di-methylaminopyridine, respectively. The size of these nanostructures is controlled through the concentration of the solution mixture prepared for the synthesis. The colloidal synthesis route with all its benefits of synthesizing different shapes and sizes of nanostructures avoids the expensive vacuum deposition approach.   

Rat basophilic leukemia (RBL) and human embryonic kidney cells were chosen to test the attachment with quinacridone. The researchers showed that RBL cells grew on planar vacuum deposited films of quinacridone and formed a good attachment, but human embryonic kidney cells did not show signs of a good interface. However, the hedgehog-shaped nanostructured organic pigment was demonstrated to have superior adherence to the human embryonic kidney cell than the planar thin film of the same pigment. This highlights the importance of nanostructured organic semiconductor rather than its planar thin film, according to the researchers.   

Transient receptor potential vanilloid (TRPV1), well known for its role in the transduction of pain caused by heat, transfected human embryonic kidney cells grown on hedgehog-shaped nanostructure provides rapid and reversible photoinduced stimulation of cation (K⁺) influx when measured at the cell resting potential (−60 mV) during voltage-ramp measurements. This is a successful demonstration of temperature-gated channels that can be directly and rapidly photostimulated in cells under ambient physiological conditions. The nanostructures prepared in the research shows not only an intimate interface with the human embryonic kidney cells but also demonstrate excellent photothermal behavior.  

Sahika Inal of the Biological and Environmental Science and Engineering Division at King Abdullah University of Science & Technology (KAUST) says that the work of Głowacki and colleagues is remarkable in addressing the challenge of intimate interface and excellent electrical communication. “Their semiconducting nanocrystals make high surface area contact points with the cell membrane, but they do so without comprising cell viability and semiconductor performance,” she says. Inal appreciates that the crystal adopts its shape according to the spatial needs of growing cells. She adds, “Therefore, I think the work makes us think about this new set of materials to build artificial retinas and to get insight into retinal neuronal signaling.”

Głowacki says the future direction of this work would be to translate these findings of cell excitation from the solid substrate to an environment inside a living tissue.  

Read the article in Nature Communications.