Description
Imagine the extraordinary pyramids of Giza, the emblematic Parthenon in Athens, or the iconic Empire State Building.
What do they all have in common?
The material they are made of: Limestone.
Background
From ancient civilisations to modern days, limestone has been widely used and continues to prove its practicality and relevance. As a Maastricht team, we recognize the importance of preserving the «Mergel», as it is also known as in the province of Limburg. This stone has been an integral part of the city's architectural history since Roman times, with its extraction starting in the 13th century from the limestone quarries found in the region. Notably, during the excavations a remarkable event in history was witnessed: the discovery of the Mosasaur, now exposed in the Natural Science Museum of the city. Furthermore, during World War II, the network of tunnels created by the quarrying served as shelters for people and secret hiding places for valuable paintings.
The historical significance of the city lies in its association with limestone production: this compound was instrumental in the construction of landmarks such as the St. Servaas Church, the red tower of Sint-Janskerk or even the city wall. Our city contains 1677 national heritage buildings, the second highest amount in the Netherlands after Amsterdam (Rijksmonumenten voor her Cultureel Erfgoed, 2011)[1]. Therefore, by preserving this stone, we aim to safeguard our rich cultural heritage and continue to appreciate the monuments that make our city unique.
As a carbonate sedimentary rock, limestone is particularly affected by the processes of weathering and erosion (Adham & Kobayashi, 2009)[2]. With the escalation of climate change, this natural deterioration has been accelerated, owing mainly to the detrimental impacts of acid rain, elongated rain seasons and air pollutants. These agents gradually change the microstructure of stone by causing the dissolution of the mineral matrix, which in turn increases porosity and ultimately leads to the structural weakening of the material (Tiano et al., 1999)[3]. This progressive decay not only poses immediate risks to structural stability but also constitutes a threat to our cultural heritage.
The problem
Currently, crack healing systems for limestone restoration present various challenges. Common physical and chemical procedures used are labour-intensive and expensive. One example is the widely utilised technique of replacing historic stone with alternative rock types, which does not fully address cracks and only cleans the surface. Protective structures only provide temporary relief but fail to resolve underlying issues, while incurring high costs (Sanders & Keenan, 2005; Campbell & DeRosa, 2014)[4][5].
Manual injection of adhesives into fissures is sometimes employed but time-consuming. Drawbacks of traditional consolidants include poor performance, limited penetration, low adhesion, and lack of flexibility. Additionally, the use of resins and epoxies alter the structure's appearance, requires ongoing maintenance and is often harmful to the environment as well as human health (Chowaniec-Michalak et al., 2022; Xiao et al., 2020).[6][7]
Limestone presents a significant challenge for restoration due to its diverse physical and mechanical properties, including variations in porosity, water absorption, and hardness. With no standard characteristic defining limestone, the restoration process becomes highly case-specific, depending on the origin and type of limestone. Addressing this issue, the development of a universal, "one-size-fits-all" system for renovating limestone and restoring its damaged structure is much needed within the field of architectural restoration.
Microbially induced calcite precipitation
Another challenge lies in the absence of robust scaffolding within the cracks, which is vital for maintaining the structural integrity of the affected structure. By leveraging synthetic biology techniques, we aim to engineer bacteria capable of repairing cracks in limestone by generating a biologically derived lattice within the damaged areas which will act as a nucleation site for calcite crystals produced by a second bacteria.
This approach offers significant advantages over conventional methods, as it allows for a universal restoration strategy independent of the specific requirements of different limestone types. This technology has the potential to streamline and enhance restoration efforts, saving time and resources while preserving the structural integrity and aesthetic value of limestone architectural masterpieces. Moreover, our biologically inspired approach promotes sustainable practices by utilising naturally occurring processes and reducing the need for environmentally harmful restoration techniques.
Our solution
Our project aims to engineer bacteria capable of promoting the restoration of cracks in limestone. To achieve this, we propose a two-phase system involving the engineering of two distinct strains of E. coli specifically designed to address these cracks. The first strain will produce ssDNA, which will self-assemble through complementary base pairing into robust DNA origami octahedron nanostructures, providing a scaffold within the crack.
The second strain will display carbonic anhydrase on the surface of our E. coli, which will increase its enzymatic activity. By utilizing and , the enzyme will enable the production of calcium carbonate, ultimately leading to the deposition of calcite. Importantly, the carbonic anhydrase enzyme operates without generating any toxic byproducts, and it even contributes to the sequestration of . The calcite will then precipitate along the negative phosphate backbone of the DNA origami scaffold, which acts as a strong nucleation site, facilitating the mineralization of calcite. The resulting calcite crystals will improve the structural stability of the limestone, providing a protective layer and effectively repairing cracks.
In the future, we believe we could implement this technology for other materials. Our highly programmable DNA structure can be adapted to numerous structures, expanding the use of our engineered bacteria.
References
Rijksmonumenten voor het Cultureel Erfgoed (2011). ↩︎
Adham, A. K. M., & Kobayashi, A. (2009, July). Effect of Intensity and pH of Rain on the Dissolution of Limestone. In ISOPE International Ocean and Polar Engineering Conference (pp. ISOPE-I). ISOPE. ↩︎
Tiano, P. et al. (1999). Bacterial bio-mediated calcite precipitation for monumental stones conservation: methods of evaluation. Journal of microbiological methods, 36(1-2), 139-145. ↩︎
Sanders, A.L. & Keenan, L.E. (January 2005). Repair and Maintenance of Historic Marble and Limestone Structures. ↩︎
Campbell, E.A. & DeRosa, C. (February 2014). Historic Stone Masonry Restoration. ↩︎
Chowaniec-Michalak, A. et al. (2022). Recycling of waste limestone powders for the cleaner production of epoxy coatings: Fundamental understanding of the mechanical and microstructural properties. Journal of Cleaner Production, 372, 133828. ↩︎
Xiao, C., et al. (2020). Hazards of bisphenol A (BPA) exposure: A systematic review of plant toxicology studies. Journal of hazardous materials, 384, 121488. ↩︎