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Multifractal theory provides an elegant statistical characterization of many complex dynamical variations in Nature and engineering. It is conceivable that it may enrich characterization of the sun's magnetic activity and its dynamical modeling. Recently, on applying Fourier truncation to remove the 11-year cycle and carrying out multifractal detrended fluctuation analysis of the filtered sunspot time series, Movahed et al reported that sunspot data are characterized by multifractal scaling laws with the exponent for the second-order moment, h(2), being 1.12. Moreover, they think the filtered sunspot data are like a fractional Brownian motion process with anti-persistent long-range correlations characterized by the Hurst parameter H = h(2) − 1 = 0.12. By designing an adaptive detrending algorithm and critically assessing the effectiveness of Fourier truncation in eliminating the 11-year cycle, we show that the values of the fractal scaling exponents obtained by Movahed et al are artifacts of the filtering algorithm that they used. Instead, sunspot data with the 11-year cycle properly filtered are characterized by a different type of multifractal with persistent longrange correlations characterized by H ≈ 0.74.

Hydrogels, consisting of hydrophilic polymers, can be used as films, scaffolds, nanoparticles and drug carriers. They are one of the hot research topics in material science and tissue engineering and are widely used in the field of biomedical and biological sciences. Researchers are seeking for a type of material that is similar to human tissues and can partially replace human tissues or organs. The hydrogel has brought possibility to solve this problem. It has good biocompatibility and biodegradability. After entering the body, it does not cause immune and toxic reactions. The degradation time can be controlled, and the degradation products are nontoxic and nonimmunogenic; the final metabolites can be excreted outside the body. Owing to the lack of blood vessels and poor migration ability of chondrocytes, the self-healing ability of damaged cartilage is limited. Tissue engineering has brought a new direction for the regeneration of cartilage. Drug carriers and scaffolds made of hydrogels are widely used in cartilage tissue engineering. The present review introduces the natural hydrogels, which are often used for cartilage tissue engineering with respect to synthesis, modification and application methods. The translational potential of this article This review introduces the natural hydrogels that are often used in cartilage tissue engineering with respect to synthesis, modification and application methods. Furthermore, the essential concepts and recent discoveries were demonstrated to illustrate the achievable goals and the current limitations. In addition, we propose the putative challenges and directions for the use of natural hydrogels in cartilage regeneration.

Three-dimensional (3D) printing is a rapidly emerging technology that promises to transform tissue engineering into a commercially successful biomedical industry. However, the use of robotic bioprinters alone is not sufficient for disease treatment. This study aimed to report the combined application of 3D scanning and 3D printing for treating bone and cartilage defects. Three different kinds of defect models were created to mimic three orthopedic diseases: large segmental defects of long bones, free-form fracture of femoral condyle, and International Cartilage Repair Society grade IV chondral lesion. Feasibility of in situ 3D bioprinting for these diseases was explored. The 3D digital models of samples with defects and corresponding healthy parts were obtained using high-resolution 3D scanning. The Boolean operation was used to achieve the shape of the defects, and then the target geometries were imported in a 3D bioprinter. Two kinds of photopolymerized hydrogels were synthesized as bioinks. Finally, the defects of bone and cartilage were restored perfectly in situ using 3D bioprinting. The results of this study suggested that 3D scanning and 3D bioprinting could provide another strategy for tissue engineering and regenerative medicine.

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