Repurposing of waste PET by microbial biotransformation to functionalized materials for additive manufacturing.

Plastic waste is an outstanding environmental thread. Poly(ethylene terephthalate) (PET) is one of the most abundantly produced single-use plastics worldwide, but its recycling rates are low. In parallel, additive manufacturing is a rapidly evolving technology with wide-ranging applications. Thus, there is a need for a broad spectrum of polymers to meet the demands of this growing industry and address post-use waste materials. This perspective article highlights the potential of designing microbial cell factories to upcycle PET into functionalized chemical building blocks for additive manufacturing. We present the leveraging of PET hydrolyzing enzymes and rewiring the bacterial C2 and aromatic catabolic pathways to obtain high-value chemicals and polymers. Since PET mechanical recycling back to original materials is cost-prohibitive, the biochemical technology is a viable alternative to upcycle PET into novel 3D printing materials, such as replacements for acrylonitrile butadiene styrene. The presented hybrid chemo-bio approaches potentially enable the manufacturing of environmentally friendly degradable or higher-value high-performance polymers and composites and their reuse for a circular economy. ONE-SENTENCE SUMMARY: Biotransformation of waste PET to high-value platform chemicals for additive manufacturing.

Modeling of Stiffness and Strength of Bone at Nanoscale.

Two distinct geometrical models of bone at the nanoscale (collagen fibril and mineral platelets) are analyzed computationally. In the first model (model I), minerals are periodically distributed in a staggered manner in a collagen matrix while in the second model (model II), minerals form continuous layers outside the collagen fibril. Elastic modulus and strength of bone at the nanoscale, represented by these two models under longitudinal tensile loading, are studied using a finite element (FE) software abaqus. The analysis employs a traction-separation law (cohesive surface modeling) at various interfaces in the models to account for interfacial delaminations. Plane stress, plane strain, and axisymmetric versions of the two models are considered. Model II is found to have a higher stiffness than model I for all cases. For strength, the two models alternate the superiority of performance depending on the inputs and assumptions used. For model II, the axisymmetric case gives higher results than the plane stress and plane strain cases while an opposite trend is observed for model I. For axisymmetric case, model II shows greater strength and stiffness compared to model I. The collagen-mineral arrangement of bone at nanoscale forms a basic building block of bone. Thus, knowledge of its mechanical properties is of high scientific and clinical interests.

Employing the biology of successful fracture repair to heal critical size bone defects.

Bone has the natural ability to remodel and repair. Fractures and small noncritical size bone defects undergo regenerative healing via coordinated concurrent development of skeletal and vascular elements in a soft cartilage callus environment. Within this environment bone regeneration recapitulates many of the same cellular and molecular mechanisms that form embryonic bone. Angiogenesis is intimately involved with embryonic bone formation and with both endochondral and intramembranous bone formation in differentiated bone. During bone regeneration osteogenic cells are first associated with vascular tissue in the adjacent periosteal space or the adjacent injured marrow cavity that houses endosteal blood vessels. Critical size bone defects cannot heal without the assistance of therapeutic aids or materials designed to encourage bone regeneration. We discuss the prospects for using synthetic hydrogels in a bioengineering approach to repair critical size bone defects. Hydrogel scaffolds can be designed and fabricated to potentially trigger the same bone morphogenetic cascade that heals bone fractures and noncritical size defects naturally. Lastly, we introduce adult Xenopus laevis hind limb as a novel small animal model system for bone regeneration research. Xenopus hind limbs have been used successfully to screen promising scaffolds designed to heal critical size bone defects.

Bone resorption markers and dual-energy x-ray absorptiometry in dogs with avascular necrosis, degenerative joint disease, and trauma of the coxofemoral joint.

OBJECTIVES: To compare the ability of N-terminal telopeptide (NTx) assays and dual-energy x-ray absorptiometry (DEXA) to detect bone resorption in dogs with nonneoplastic bone lysis and evaluate the correlation between these diagnostic tools. STUDY DESIGN: Prospective, cross-sectional clinical study. ANIMALS: Dogs (n = 35; 39 femoral heads) that had femoral head and neck ostectomy and 6 cadaver specimens from healthy immature small dogs. METHODS: Small dogs with avascular necrosis (n = 12), a reference group of small dogs (7), large dogs with degenerative joint disease (DJD; 10), and large dogs with trauma (10) were studied in addition to 6 femoral heads harvested from 6 small immature and healthy dogs euthanatized for reasons unrelated to this study. Densitometric measurements of femoral heads, urine NTx excretion, and serum NTx concentration were compared between groups. RESULTS: Avascular necrosis resulted in a decrease in bone mineral density (BMD) (0.18 ± 0.01 g/cm(2;) P < .01) of the femoral head and elevation of serum NTx (159.3 ± 59.4 nM; P = .03) compared to small dog controls (0.28 ± 0.02 g/cm(2) ; 18.7 ± 1.83 nM, respectively), but did not seem to affect urine NTx. DJD in large dogs did not seem to affect any of the densitometric parameters evaluated. BMD (P = .03) and serum NTx (P = .04) were lower in small compared to large dogs. Serum NTx and densitometric measurements correlate inversely with each other (P = .001) but neither test correlated with urine NTx (P = .8-.9). CONCLUSION: Serum NTx levels vary with dog size but seem to correlate better with BMD better than urine NTx excretion in dogs with nonneoplastic bone resorption.

Avalanche criticality during compression of porcine cortical bone of different ages.

Crack events developed during uniaxial compression of cortical bones cut from femurs of developing pigs of several ages (4, 12, and 20 weeks) generate avalanches. These avalanches have been investigated by acoustic emission analysis techniques. The avalanche energies are power-law distributed over more than four decades. Such behavior indicates the absence of characteristic scales and suggests avalanche criticality. The statistical distributions of energies and waiting times depend on the pig age and indicate that bones become stronger, but less ductile, with increasing age. Crack propagation is equally age-dependent. Older pigs show, on average, larger cracks with a time distribution similar to those of aftershocks in earthquakes, while younger pigs show only statistically independent failure events.

Numerical modeling of long bone adaptation due to mechanical loading: correlation with experiments.

The process of external bone adaptation in cortical bone is modeled mathematically using finite element (FE) stress analysis coupled with an evolution model, in which adaptation response is triggered by mechanical stimulus represented by strain energy density. The model is applied to experiments in which a rat ulna is subjected to cyclic loading, and the results demonstrate the ability of the model to predict the bone adaptation response. The FE mesh is generated from micro-computed tomography (microCT) images of the rat ulna, and the stress analysis is carried out using boundary and loading conditions on the rat ulna obtained from the experiments [Robling, A. G., F. M. Hinant, D. B. Burr, and C. H. Turner. J. Bone Miner. Res. 17:1545-1554, 2002]. The external adaptation process is implemented in the model by moving the surface nodes of the FE mesh based on an evolution law characterized by two parameters: one that captures the rate of the adaptation process (referred to as gain); and the other characterizing the threshold value of the mechanical stimulus required for adaptation (referred to as threshold-sensitivity). A parametric study is carried out to evaluate the effect of these two parameters on the adaptation response. We show, following comparison of results from the simulations to the experimental observations of Robling et al. (J. Bone Miner. Res. 17:1545-1554, 2002), that splitting the loading cycles into different number of bouts affects the threshold-sensitivity but not the rate of adaptation. We also show that the threshold-sensitivity parameter can quantify the mechanosensitivity of the osteocytes.