RESEARCH NEWS CURRENT SCIENCE, VOL. 118, NO. 3, 10 FEBRUARY 2020 345 Exploring North East India’s non-mulberry silk based bioinks for three dimensional bioprinting Rocktotpal Konwarh Three-dimensional bioprinting has catapulted research in the domain of tissue engineering and regenerative medicine to newer heights in recent years. In this milieu, silk-based bioink has garnered tremendous re- search thrust. Mulberry silkworm silk fibroin has found profound impetus for formulating biocompatible and mechanically robust bioink with high-cell loading potency, used in the fabrication of 3D bio-constructs for prospective clinical applications. Pertinently, North East India's non-mulberry silk varieties ‘muga’ and ‘eri’, endowed with specific cell-binding RGD sequence, exhibit special biomaterial-attributes including better mechanical resilience than their mulberry counterparts. The recent exploitation of the former for the formulation of novel bioinks to fabricate 3D constructs for prospective meniscus, cartilage and osteochon- dral tissue repair merits special mention. Their self-gelling attribute permits the evasion of the use of con- ventional toxic chemical cross-linkers. With prospective application in the niche of regenerative medicine, these non-mulberry silk varieties seem to have seeded new anticipations vis-à-vis increasing cases of dege- nerative diseases and associated morbidity. Three-dimensional bioprinting, repre- senting a nexus of interdisciplinary expertise, involves the fabrication of 3D structures via layer-by-layer specific positioning of biological materials, bio- molecules/biochemicals and living cells, the placement of the functional constitu- ents being spatially controlled 1,2 . Appro- aches of biomimicry, mini-tissue building blocks and autonomous self-assemblage have been adroitly employed for the fabrication of 3D functional human con- structs (e.g. multi-layered skin, tracheal splints, heart tissue, etc.), exhibiting bio- logical and mechanical attributes, befit- ting the clinical restoration of tissue/ organ function 3 . These seem to be highly pertinent in the context of incessantly in- creasing morbidity due to degenerative diseases. Albeit highly potential, the selection of appropriate materials, cell types, growth and differentiation factors as well as sensitivity of the cells and overall complicacy involved in the fabri- cation of tissues are some of the practical inconveniencies of the technology 2 . Prin- tability, biocompatibility, appropriate structural and mechanical attributes, degradation kinetics adjustable to the capacity of the cells to generate their extracellular matrix (ECM), nontoxic de- gradation by-products, and most impor- tantly, potency to mimic the tissue- specific endogenenous material proper- ties are the hallmark signatures of an ideal biomaterial to function as a bioink, the core-material for 3D bioprinting. In recent years, researchers have resorted to silkworm protein for formulating bio- ink 4,5 . The impetus received by silk in the domain of tissue engineering and allied biotechnological applications needs no further elaboration in the backdrop of its biocompatibility, amenability to pro- cessing into multiple formats (scaffolds, nanoparticles, hydrogels, electrospun membranes, etc.), tunable biodegrada- bility and mechanical robustness 6–10 . In the realm of 3D bioprinting, exploration of silk, in particular that extracted from Bombyx mori (domesticated, mulberry silk variety), has been a recent research thrust 4,5 . Pertinently, biospinning of the silkworm cocoon, in accordance to the dictates of microfluidics, perhaps could be projected as quintessential evidence of nature’s self-endeavours in the domain of 3D bioprinting 11 . Silkworm silk consists of the fibroin (SF) core (used in bioprinting), encased by the glue-like sericin (SS; that is de- gummed using approaches like alkali- degradation, autoclaving, etc.) prior to formulation of bioink 4,12 . However, low viscosity of the extracted SF solution is a serious practical problem and as such, rheological attributes must be adjusted to ensure printability 1 . Post SF purification through dialysis, the regenerated SF solution is subjected to evaporation or re-dissolution in organic solvents (1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), formic acid, etc.) to augment the visco- sity 5 . However, such solvents could usher in catastrophic effect on cell viability and may even lead to further scissoring of the fibroin protein. As an alternative, augmentation of free-standing attributes, plasticity and viscosity of SF-based bioinks could be facilitated through blending with other high-viscosity bio- materials like gelatin, chitosan, alginate, etc. Protein conformational transition (induction of beta-sheet structure) in re- generated SF, mediated by approaches involving biocatalysts, sonication, mod- ulations in temperature and pH values, salting, photo-crosslinking as well as (toxic) cross-linkers, is exploited to adjust the mechanical attributes of silk- based bioinks. Besides critical rheologi- cal features, swelling ratio, surface ten- sion as well as cell-encapsulating or growth factors doping potency of silk- based bioinks are pertinent from the perspective of cell viability, adhesion, proliferation, differentiation and matura- tion 5 . Choosing appropriate cell-seeding technology as well as optimization of degradation rate of the constructs with respect to the speed of neo-tissue forma- tion are critical. Nevertheless, availabili- ty of considerably large amount of silk sources (silkworm cocoons in particular), intrinsic hydration properties and re- markable strength and toughness of silk, availability of diverse protocols for cross- linking or sol-to-gel induction, tunable biodegradation, ease of modifying the structure and surface chemistry, and