Calcium transforms silk hydrogels into a versatile platform that prints precisely, stores for months, and fuses after sterilization, overcoming barriers to practical biomedical use.
(Nanowerk Spotlight) Hydrogels are soft, water-rich materials that resemble a sponge, and scientists are exploring them for a wide range of medical uses, from growing replacement tissues to soothing damaged skin. Silk has emerged as a promising base for these gels because silk fibroin, the main protein from silkworm cocoons, is tough, compatible with the body, and easy to process in water.
Yet shaping pure silk into precise three dimensional forms has been difficult. Light-driven printing methods promise speed and detail, but silk tends to form stiff protein crystals called beta sheets during curing. That process can warp printed structures and cloud the material. Many silk inks also require cold storage and lose their ability to print over time. Other research has shown that calcium salts can make silk films softer and more adhesive, but those methods often depend on harsh solvents or slow enzymatic steps that limit design flexibility.
These challenges have slowed progress in turning silk hydrogels into practical tools for medicine. The question has been whether a single system could combine elasticity, clarity, long shelf life, and the ability to bond or fuse into complex structures.
A new study in Advanced Functional Materials (“Calcium Imparts Advanced Functionalities to Silk Hydrogels for Biofabrication and Biomedical Innovation”) presents an approach that answers this question. The researchers describe a visible light process that combines calcium ions with a standard photoinitiator pair made of ruthenium and sodium persulfate. Light activates this pair, linking tyrosine amino acids in silk into di tyrosine bonds that hold the material together. Calcium changes how silk chains pack as this reaction occurs, creating transparent, stretchable gels that print cleanly and remain stable at room temperature.
Overview of the fabrication methods for Ca2+-containing silk materials. The method developed in this study is outlined in red. (Image: Reprinted from DOI:10.1002/adfm.202508572, CC BY)
The chemistry offers insight into how this system works. Using photo rheology, which measures gel formation under light, the team found that pure silk gels formed in about 22 seconds. With calcium, gelation slowed slightly to about 34 seconds. The delay comes from temporary ionic bridges between calcium and acidic amino acids that limit chain movement before bonds form. Despite the slower onset, both systems cured fast enough for 3D printing.
Structural analysis revealed why calcium improves the material. Fourier transform infrared spectroscopy showed that calcium based gels contained fewer beta sheets than pure silk. Beta sheets strengthen the network but also make it brittle and cloudy. Calcium reduces this crystallization, likely by binding to charged residues in silk and by reacting with persulfate to form calcium sulfate. This reaction removes free sulfate ions that otherwise encourage the protein to assemble tightly.
These changes were visible in the finished gels. Calcium gels kept their size after curing, while pure silk gels shrank as close packing forced water out. Because calcium strongly attracts water, it countered shrinkage and preserved clarity. Light transmission through the calcium gels remained above 80 percent across most of the visible spectrum and reached about 90 percent around 600 nanometers. Pure silk gels transmitted less than 80 percent, limiting their use where transparency is required.
Mechanical tests reinforced the difference. Calcium gels withstood 76 percent strain and 135 kilopascals in compression before failure, while pure silk endured only 52 percent strain and 35 kilopascals. In stretching tests, calcium gels elongated to about 60 percent beyond their original length and were more than twice as tough as pure silk. They also bent and flexed more easily without cracking. Lower crystallinity and a greater fraction of flexible coil segments allowed the network to absorb and recover from stress.
These properties translated into better printing performance. With digital light processing, the team printed structures with sub millimeter precision. They also used volumetric printing, which projects rotating light fields to form entire objects at once. In four to five minutes they created complex shapes such as a torus, a lattice cube, and a human heart model. The calcium gels produced designs that closely matched the intended geometry, while calcium free gels thickened unpredictably during exposure.
Stability of the liquid ink proved to be another advantage. Calcium supplemented silk remained a clear liquid for months at room temperature. Pure silk gradually gelled during storage. After nine months, calcium inks printed with the same fidelity as fresh batches, and hydrogels cast from eleven month old solutions showed the same compressive strength as new samples. Calcium binding to silk’s side chains appears to prevent premature assembly, removing the need for refrigeration and simplifying storage for research and clinical use.
The gels also acted as effective bonding layers. A fresh calcium silk solution cured between two pre formed pure silk gels created a bond that held during stretching until the bulk material itself failed. Without calcium, the joint separated cleanly. The ions at the interface likely formed salt bridges and exposed unreacted silk groups for new chemical links. This allowed the researchers to combine brittle and ductile silk formats into single constructs without separation.
Humidity further expanded the material’s capabilities. Calcium attracts water, so the gels expanded and contracted reversibly with changes in humidity. In dry conditions they became more stretchable, reaching up to 178 percent strain, and could even be tied into a knot. They stiffened compared to their hydrated state, and two dry gels adhered to each other or to surfaces such as iron, silicone, and plastic. In humid conditions adhesion weakened because a thin layer of water formed at the interface. Electrical conductivity also fell when water evaporated but recovered when humidity rose again.
Permanent fusion was achieved with autoclaving, the standard sterilization process using pressurized steam. Calcium gel pieces pressed together and autoclaved formed continuous, seamless interfaces that held under stress and agitation. Fusion was possible even at lower calcium concentrations, although opacity and shrinkage increased. This process both fused and sterilized constructs in one step, a direct benefit for clinical workflows.
Thermal behavior added another dimension. When cooled with liquid nitrogen, calcium gels resisted freezing throughout their structure and thawed faster than pure silk gels. When heated, they absorbed more energy and cooled more quickly. On the back of a glove in sunlight, the covered area stayed several degrees cooler. Applied to a hot surface mimicking a burn, the gel lowered the temperature to near body levels within minutes.
Electrical properties supported possible biomedical use. Calcium gels showed ionic conductivity of about 7 millisiemens per centimeter at 1 kilohertz, compared to 3 millisiemens for pure silk. The gels could power a small light emitting diode. Their conductivity was similar to many soft tissues, which suggests they could transmit signals without interfering with surrounding biology.
Safety tests in mice confirmed that the gels are biocompatible. After four weeks under the skin, calcium gels produced tissue responses comparable to pure silk. Some calcium gels fragmented, which the team linked to faster degradation due to lower crystallinity. Depending on the application, this property could be tuned to either promote resorption or maintain longer term stability.
Together these findings present a versatile material system. Calcium suppresses unwanted crystallization, preserves clarity, and improves elasticity. It stabilizes liquid inks for months at room temperature, enables precise light based printing, supports humidity controlled adhesion, and allows permanent fusion through autoclaving. The gels also conduct ions, manage heat effectively, and remain compatible with living tissue.
This work shows how a simple addition can shift the balance between order and flexibility in a protein network, leading to materials that align with the practical needs of laboratories and clinics alike.
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Jelena Rnjak-Kovacina (University of New South Wales)
, 0000-0001-6121-4676 corresponding author
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