Environment-controlled water adsorption at hydroxyapatite/collagen interfaces

Bone mineral tessellation: Atomic force microscopy of the volume-filling mineralization pattern in hydrated and dehydrated states

Bone is a specialized hard connective tissue with a hierarchical organization of its components. At the micrometer scale, mineral entities of roughly uniform shape tessellate in 3D within an organized, crosslinked and hydrated scaffold of mostly type I collagen. Here we report on the visualization by atomic force microscopy (AFM) of the volume-filling mineralization pattern of tesselles in lamellar bone, in hydrated and dehydrated conditions (for human, bovine, porcine and ovine bone). Microscale mineral tessellation was clearly visible when bulk lamellar bone is hydrated, whereas dry bone showed submicron nanogranularity instead of tesselle boundaries. Time-lapse AFM experiments of gradual passive dehydration of bone revealed topographical changes for all bone species with the tessellation appearance vanishing after two weeks of dehydration. AFM adhesion forces dropped within the first days of dehydration in all bone species, indicating that surface stickiness is more sensitive to passive dehydration than is stiffness. Irrespective of the bone species, AFM stiffness measurements found that hydrated bone was more compliant than dehydrated bone. AFM Young's modulus measurements of more recently formed osteonal lamellae intersecting with older interstitial lamellae found that the modulus in both hydrated and dehydrated states was lower in the osteonal lamellae. Modelling of water sorption to the surface of stochiometric hydroxyapatite showed that the presence of rigid hydration shells delineates the tesselle boundaries and smoothens the nanogranularity, confirming the AFM observations. This study highlights the importance of regarding water as a fundamental architecting component of bone. STATEMENT OF SIGNIFICANCE: Here we report on visualization of the mineral tessellation pattern in lamellar bone by atomic force microscopy (AFM) in hydrated and dehydrated conditions. We show that lamellar bone (human, bovine, porcine and ovine) contains a universal volume-filling mineral tessellation. The visibility of the tessellation pattern by AFM strongly depends on the state of bone hydration. Modelling water sorption to the surface of stochiometric hydroxyapatite indicated that mechanical and morphological characteristics of lamellar bone (e.g., stiffness, adhesion, contours of tesselle boundaries) can be attributed to the presence of rigid hydration shells. This study highlights the importance of water incorporation as a fundamental component of bone, on par with the mineral and the organic extracellular matrix.

Water and Collagen: A Mystery Yet to Unfold

Collagen is the most abundant protein in the human body and plays an essential role in determining the mechanical properties of the tissues. Both as a monomeric protein and in fibrous assemblies, collagen interacts with its surrounding molecules, in particular with water. Interestingly, while it is well established that the interaction with water strongly influences the molecular and mechanical properties of collagen and its assemblies, the underlying mechanisms remain largely unknown. Here, we review the research conducted over the past 30 years on the interplay between water and collagen and its relevance for tissue properties. We discuss the water-collagen interaction on relevant time- and length scales, ranging from the vital role of water in stabilizing the characteristic triple helix structure to the negative impact of dehydration on the mechanical properties of tissues. A better understanding of the water-collagen interaction will help to unravel the effect of mutations and defective collagen production in collagen-related diseases and to pinpoint the key design features required to synthesize collagen-based biomimetic tissues with tailored mechanical properties.

Bone fragility during the COVID-19 pandemic: the role of macro- and micronutrients

Bone fragility is the susceptibility to fracture due to poor bone strength. This condition is usually associated with aging, comorbidities, disability, poor quality of life, and increased mortality. International guidelines for the management of patients with bone fragility include a nutritional approach, mainly aiming at optimal protein, calcium, and vitamin D intakes. Several biomechanical features of the skeleton, such as bone mineral density (BMD), trabecular and cortical microarchitecture, seem to be positively influenced by micro- and macronutrient intake. Patients with major fragility fractures are usually poor consumers of dairy products, fruit, and vegetables as well as of nutrients modulating gut microbiota. The COVID-19 pandemic has further aggravated the health status of patients with skeletal fragility, also in terms of unhealthy dietary patterns that might adversely affect bone health. In this narrative review, we discuss the role of macro- and micronutrients in patients with bone fragility during the COVID-19 pandemic.

Molecular Dynamics Simulation of Biomimetic Biphasic Calcium Phosphate Nanoparticles

Biphasic calcium phosphate (BCP) is used as a bone substitute and bone tissue repair material due to its better control over bioactivity and biodegradability. It is crucial to stabilize the implanted biomaterial while promoting bone ingrowth. However, a lack of standard experimental and theoretical protocols to characterize the physicochemical properties of BCP limits the optimization of its composition and properties. Computational simulations can help us better to learn BCP at a nanoscale level. Here, the Voronoi tessellation method was combined with simulated annealing molecular dynamics to construct BCP nanoparticle models of different sizes, which were used to understand the physicochemical properties of BCP (e.g., melting point, infrared spectrum, and mechanical properties). We observed a ∼20 to 30 Å layer of calcium-deficient hydroxyapatite at the HAP/β-TCP interface due to particle migration, which may contribute to BCP stability. The BCP model may stimulate further research into BCP ceramics and multiphasic ceramics. Moreover, our study may facilitate the optimization of compositions of BCP-based biomaterials.

Bone hydration: How we can evaluate it, what can it tell us, and is it an effective therapeutic target?

Water constitutes roughly a quarter of the cortical bone by volume yet can greatly influence mechanical properties and tissue quality. There is a growing appreciation for how water can dynamically change due to age, disease, and treatment. A key emerging area related to bone mechanical and tissue properties lies in differentiating the role of water in its four different compartments, including free/pore water, water loosely bound at the collagen/mineral interfaces, water tightly bound within collagen triple helices, and structural water within the mineral. This review summarizes our current knowledge of bone water across the four functional compartments and discusses how alterations in each compartment relate to mechanical changes. It provides an overview on the advent of- and improvements to- imaging and spectroscopic techniques able to probe nano-and molecular scales of bone water. These technical advances have led to an emerging understanding of how bone water changes in various conditions, of which aging, chronic kidney disease, diabetes, osteoporosis, and osteogenesis imperfecta are reviewed. Finally, it summarizes work focused on therapeutically targeting water to improve mechanical properties.