(Nanowerk Spotlight) The emergence of living materials that combine biological components with man-made structures is opening up new possibilities across fields as diverse as medicine, energy, and computing (see our previous Nanowerk Spotlight: “Engineering sustainable living materials for a greener future“). One particularly promising area is the integration of engineered living materials (ELMs) in optoelectronics, where living cells are used to create advanced lighting systems.
The latest developments in this field bring together biology and materials science in novel ways, particularly in applications where light must be manipulated, controlled, or converted. One of the major challenges in this area has been finding sustainable and cost-effective alternatives to rare-earth-based and toxic materials traditionally used in light-emitting diodes (LEDs).
White LEDs, widely used in everyday lighting, often rely on phosphors, substances that convert blue or ultraviolet light into visible light. These phosphors typically contain rare earth elements, which are expensive to extract and process, leading to high production costs and environmental concerns. Additionally, many current alternatives, such as quantum dots, contain toxic heavy metals like cadmium. For years, researchers have explored ways to replace these phosphors with more sustainable materials, including fluorescent proteins, which are naturally occurring molecules that glow under specific light conditions. However, the challenge has been finding efficient, non-toxic, and affordable ways to incorporate these proteins into LED devices while maintaining the necessary stability and brightness.
The concept of engineered living materials brings together the adaptability and functionality of living cells with the structural advantages of polymer matrices, potentially addressing these issues. These materials have been used in applications such as drug delivery, tissue engineering, and environmental sensing, but their integration into lighting systems has been limited by several obstacles. Chief among these is the natural autofluorescence – unwanted background light emission – and the scattering properties of living cells, which interfere with the precise control of light. Additionally, the instability of biological components in these hybrid systems has made their use in practical lighting applications difficult.
In a recent study (Advanced Materials, “Bacterial Hybrid Light-Emitting Diodes”), researchers introduced a significant step forward in overcoming these limitations by creating bacterial hybrid light-emitting diodes (BaHLEDs) that integrate ELMs for photon conversion. This new technology presents a novel approach to sustainable lighting by using bacteria as a medium for photon manipulation. The key innovation lies in the development of spheroplasts—bacteria that have had their cell walls removed—enabling better light transmission and reducing unwanted scattering. These spheroplasts are engineered to express fluorescent proteins, such as enhanced green fluorescent protein (EGFP) and monomeric Green Lantern (mGL), both of which are used to convert blue LED light into visible light.
Scheme of the BaHLED concept: Spheroplasts formation followed by the encapsulation in PVA, resulting in a BaHLED containing the living phosphor. (Image: Reprinted from DOI:10.1002/adma.202402851, CC BY)
One of the major advancements in this research was the reduction of light scattering by more than 90%. Traditional methods to remove bacterial cell walls, using substances like sucrose, could only reduce scattering by around 40%. The team instead employed maltodextrin, a carbohydrate that induces higher osmotic pressure, effectively shrinking the bacteria and reducing scattering to levels that make these BaHLEDs feasible for lighting applications. This reduction in scattering, paired with the preservation of the bacteria’s ability to produce fluorescent proteins, represents a breakthrough in using living materials for photon conversion.
The process begins with creating spheroplasts from Escherichia coli (E. coli), which are then encapsulated in a polymer, such as polyvinyl alcohol (PVA), to form a stable material capable of converting light. The resulting bacterial-polymer hybrid structures are stable for over a year under ambient conditions, making them practical for long-term use in LED devices. The encapsulation in PVA also provides an added benefit: it allows the bacteria to survive, making it possible to recycle the fluorescent proteins through recultivation after the device has reached the end of its life cycle.
These bacterial hybrid LEDs offer several advantages over traditional fluorescent protein-based systems. First, they eliminate the need for costly protein purification, as the bacteria themselves produce the fluorescent proteins in situ. Second, the bacterial spheroplasts can be reused, further reducing costs and making the process more sustainable. Moreover, the stability of these hybrid LEDs is comparable to that of devices using purified fluorescent proteins, with lifetimes of up to 330 hours at high current levels. Under lower power conditions, these devices can last even longer – up to 1,500 hours – without significant degradation in performance.
The researchers demonstrated that these BaHLEDs could match the performance of LEDs using traditional materials, with conversion efficiencies and light stability that are competitive. For example, devices using EGFP showed a conversion efficiency of up to 80%, while maintaining their structural and optical integrity over time. The spheroplasts also exhibited good compatibility with the polymer matrix, which helped prevent cytolysis, or cell rupture, that could otherwise reduce the effectiveness of the device. This was a crucial factor in the success of the BaHLEDs, as fully intact bacterial cells are prone to breaking down under the thermal and electrical stresses present in LED systems.
The study marks a significant advancement in the field of living light-emitting devices, introducing a new approach to sustainable and recyclable lighting. By using bacteria to generate the fluorescent proteins directly within the device, this approach reduces the environmental and economic costs associated with current lighting technologies. The successful development of these BaHLEDs opens the door to further innovations in the field of bio-hybrid optoelectronics, where living organisms can be integrated into devices for more efficient and eco-friendly lighting solutions.
Moving forward, the researchers plan to optimize the genetic engineering of these bacteria to enhance the production of fluorescent proteins without the need for antibiotics, further increasing the sustainability of the system. Additionally, they aim to explore the use of other living organisms that may be more resistant to the high levels of light and heat generated in LED devices, which could lead to even longer-lasting and more efficient lighting technologies. Comparative cost analyses and life cycle assessments will be essential in determining the feasibility of these devices for commercial use, but the initial findings are promising.
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