Letter to the editor regarding: Bio-3D printing of scaffold-free ADSC-derived cartilage constructs comparable to natural cartilage in vitro

To the Editor,

The recent study by Nonaka et al. [1] titled “Bio-3D printing of scaffold-free ADSC-derived cartilage constructs comparable to natural cartilage in vitro” represents a significant advancement in cartilage tissue engineering. The authors demonstrate that adipose-derived stem cells (ADSCs) can be engineered into scaffold-free, bio-3D-printed constructs exhibiting histological and biochemical properties resembling native cartilage after extended in vitro culture. While these findings are promising, several methodological and interpretive aspects warrant deeper discussion to contextualize the study’s impact within the evolving landscape of biomimetic cartilage regeneration.

The study’s core innovation lies in its scaffold-free approach, leveraging spheroid-based bio-3D printing to create macroscale structures. This strategy circumvents challenges associated with exogenous biomaterials, such as inflammatory responses and mechanical mismatch [1]. The authors report robust chondrogenic differentiation, evidenced by glycosaminoglycan (GAG) deposition and collagen type II expression, achieving compressive strength up to 6.1 MPa under optimized conditions (50 ng/mL BMP2). However, the claim of “comparability to natural cartilage” requires careful scrutiny. Native cartilage functionality derives from its zonal organization, depth-dependent mechanical gradients, and long-term stability—features not fully recapitulated here. Although histological staining shows ECM composition similar to hyaline cartilage, quantitative biomechanical data (e.g., dynamic compressive modulus, tensile properties) are absent. Such metrics are critical for evaluating functional parity, as emphasized in recent reviews of bioprinted osteochondral tissues [2].

Moreover, the study’s reliance on in vitro outcomes limits translational inference. Long-term stability of the chondrogenic phenotype remains uncertain, particularly given ADSCs’ propensity for hypertrophy and endochondral ossification. While collagen type X was undetected, extended culture or in vivo implantation might reveal divergent differentiation. Recent work by Chen et al. [3] highlights the necessity of multi-axial mechanical testing and metabolic profiling to ensure phenotypic stability in stem cell-derived cartilage. Additionally, the omission of gene expression data (e.g., SOX9, ACAN, RUNX2) represents a missed opportunity to mechanistically validate chondrogenic progression.

The study’s scalability also merits discussion. Bio-3D printing of cellular spheroids demands precise control over spheroid uniformity and fusion kinetics to ensure volumetric consistency and cell viability in thick constructs. Nonaka et al. [1] report construct thicknesses up to 2 mm, but higher BMP2 concentrations (250–500 ng/mL) correlated with apoptosis, likely due to diffusion limitations. This aligns with broader challenges in bioprinting, where vascularization strategies—such as embedded vasculature or angiogenic factor delivery—are increasingly critical for scaling tissues beyond diffusion limits [4].

Finally, while the study optimizes ADSC chondrogenesis using bFGF and BMP2, comparative analysis with other cell sources (e.g., iPSC-derived chondrocytes or articular chondrocytes) could strengthen its rationale. Recent advances in decellularized ECM bioinks and mechanical conditioning suggest that combinatorial strategies may enhance functional outcomes [5].

In conclusion, Nonaka et al. [1] provide valuable insights into scaffold-free biofabrication of cartilage-like tissues. However, to truly claim “comparability to natural cartilage,” future work must integrate:Multi-axial biomechanical testing under physiological loading conditions [3]; Long-term in vivo assessments of integration, stability, and hypertrophy risk [4]; Scalability solutions for clinical-sized defects [5].

This study aptly sets the stage for these essential investigations.

Sincerely,

DuJiang Yang, MD

Guoyou Wang, MD, PhD

President and Party Secretary of the Southwest Medical University Hospital of traditional Chinese medicine

Author contributions

YDJ, GGW: conceptualization, methodology, writing—original draft preparation, supervision. WS, LQ.: data curation, formal analysis, investigation, visualization. TXX: validation, resources, writing—review & editing. CJJ, WGY: project administration, funding acquisition, supervision. All authors have read and agreed to the final version of the manuscript.

Funding

Biomechanical research on cartilage transplantation based on the theory of “equal emphasis on muscle and bone” 2023ZYQJ02.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publications

Not applicable.

Competing interests

The authors declare no competing interests.