Off-the-shelf platelet lysate hydrogel shows early promise for cartilage repair

Cartilage has a frustrating habit of refusing to heal. Its dense extracellular matrix is starved of blood vessels, and once the tissue degenerates, current treatments mostly manage pain rather than rebuild structure. A team of researchers in Iceland, Norway, Sweden and elsewhere has now reported a step toward a regenerative alternative: a gelatin-based hydrogel loaded with a standardized, freeze-dried form of human platelet lysate that can be stored at room temperature and mixed on demand. The study, published in Frontiers in Bioengineering and Biotechnology, describes both the material science and the early laboratory tests of this bioactive matrix.

The key ingredient is lyophilized, pathogen-inactivated platelet lysate, abbreviated L-PIPL. Platelet lysate is produced by rupturing platelets to release their cargo of growth factors, which include platelet-derived growth factor, transforming growth factor beta and epidermal growth factor, molecules known to regulate cell proliferation, matrix synthesis and tissue remodeling. Unlike platelet-rich plasma, which contains intact concentrated platelets, lysate is a cell-free extract. The researchers obtained their starting material from expired pathogen-inactivated platelet concentrates at the Icelandic Blood Bank, each prepared from buffy coats of eight ABO-matched donors and suspended in platelet additive solution. The units had been treated with the INTERCEPT pathogen-reduction system and stored under agitation at 22 degrees Celsius for up to seven days before being frozen at minus 80 degrees.

Turning those concentrates into a shelf-stable powder involved a closed-bag workflow designed to preserve sterility. The frozen bags were subjected to three freeze-thaw cycles, each thawing in a 37 degree water bath followed by immediate refreezing. After the third thaw, the lysates were pooled irrespective of ABO group into 600 milliliter bags, which increased the number of donors represented and reduced the influence of any single donor’s biology. The pooled lysate was centrifuged at 4,975 g for ten minutes to strip out platelet debris and aggregates, then filtered, aliquoted and freeze-dried for roughly three days at a primary drying pressure of 0.04 millibar with an ice condenser at minus 88 degrees. The resulting powder was stored protected from light and moisture at room temperature for up to about two months before use.

The carrier matrix was a commercially available cartilage bioink composed of gelatin methacryloyl, sodium alginate, xanthan gum, hyaluronic acid methacrylate and laminin-521, a mixture intended to mimic the cartilage extracellular matrix. The methacrylated components allow photo-crosslinking initiated by a photoinitiator called LAP, while the alginate permits ionic crosslinking with calcium chloride. The team mixed 0.05, 0.10 or 0.15 grams of L-PIPL powder into 0.95, 0.90 or 0.85 milliliters of bioink, producing formulations containing 5, 10 and 15 percent L-PIPL by weight per volume. Constructs were shaped by extrusion through a blunt needle and double-crosslinked with 30 seconds of 405 nanometer blue light followed by 15 minutes in a 50 millimolar calcium chloride solution.

Physicochemical characterization revealed a clear concentration-dependent pattern. Scanning electron microscopy showed that the plain hydrogel had a comparatively smooth matrix with limited pores, while 5 and 10 percent enrichment produced progressively more heterogeneous, porous and fibrillar architectures, with the 10 percent formulation displaying an interconnected porous network the authors considered potentially favorable for nutrient diffusion and cell infiltration. At 15 percent, however, the microstructure turned dense and irregular, with apparent pore collapse or excessive aggregation, possibly from saturation of the matrix with particulate content. Infrared spectroscopy confirmed that L-PIPL increased protein-associated amide signals without introducing new peaks or shifting the characteristic functional groups of the base network, meaning the enrichment preserved the core chemistry while boosting protein content.

Rheology told a similar story of a beneficial window that closes at high loading. In amplitude sweeps on uncrosslinked samples, all formulations behaved like gels at low strain, with the storage modulus exceeding the loss modulus. The 10 and 15 percent formulations showed higher stiffness at small deformations, but increasing L-PIPL content reduced resistance to large deformation, and the 15 percent material showed an earlier transition toward network disruption. In frequency sweeps, the control and 5 percent hydrogels maintained gel-like behavior across most of the tested range, the 10 percent formulation reached the highest storage modulus at higher frequencies, and the 15 percent hydrogel was viscous-dominated at low frequencies. The authors concluded that excessive loading increases strain sensitivity and weakens the uncrosslinked network.

Biological testing used two cell types: C20A4 human chondrocytes for cytocompatibility screening and human adipose-derived mesenchymal stromal cells for early chondrogenic assays. Live-dead staining on days 1 and 5 showed that viability was well maintained up to 10 percent enrichment, with the 5 percent group reaching the highest live-cell coverage by day 5, while the 15 percent group remained significantly lower with higher mortality. Growth factor release was measured by tracking transforming growth factor beta 1 in the culture medium over roughly 94.5 hours. The unenriched control stayed below 200 picograms per milliliter throughout, whereas enriched hydrogels released far more, with the 15 percent group reaching around 20,000 picograms per milliliter by the final time point. Release followed a burst pattern within the first 4.5 hours, most evident at 10 and 15 percent, before gradually plateauing, confirming the hydrogel acts as a tunable reservoir of bioactive factors.

The chondrogenic experiments produced the study’s most nuanced finding. In AD-MSC-laden discs cultured for seven days in serum-free chondrogenic medium, L-PIPL enrichment increased metabolic activity, with glucose consumption and lactate accumulation rising in a concentration-dependent manner and the 10 percent formulation showing the highest metabolic and protein signals. Yet higher metabolism did not translate into stronger cartilage commitment. Immunofluorescence for SOX9, a transcription factor central to chondrogenic signaling, was most prominent in the 5 percent formulation, and COMP, a cartilage-associated matrix protein secreted into the medium, was highest in the 5 percent group on days 4 and 6. The authors caution that the SOX9 signal is a qualitative indicator rather than a standalone quantitative endpoint, but the combination of SOX9 staining and COMP release points to 5 percent as the most favorable condition for early chondrogenic commitment, while 10 percent shifts the response toward general metabolic stimulation.

The authors are explicit about the limits of what these results show. The biological assessment was short, the panel of chondrogenic readouts was narrow, and the culture conditions were static in vitro systems, so the early SOX9 and COMP responses may not translate into sustained cartilage matrix deposition. Ionized calcium measurements were interpreted as reflecting the combined ionic environment of the system, including citrate from the platelet additive solution and the calcium chloride crosslinking step, rather than cellular calcium use alone. Future work, they suggest, should include longer experiments, a broader marker panel, dynamic bioreactor culture, testing under osteoarthritis-like inflammatory conditions, ex vivo adhesion testing and ultimately in vivo studies in clinically relevant cartilage defect models. They also note that higher-loading formulations might still suit acellular applications focused purely on localized factor delivery.

Even with those caveats, the study addresses a genuine translational bottleneck. Platelet-derived products are attractive for cartilage repair, but direct injection suffers from rapid clearance, and many platelet-enriched hydrogels rely on variably processed derivatives with poor batch-to-batch consistency and storage stability. By coupling a standardized Blood Bank workflow with pathogen inactivation and lyophilization, the L-PIPL format offers dry-state stability, room-temperature transport and simple reconstitution, enabling standardized dosing in a defined commercial hydrogel. Within the tested range, moderate enrichment of 5 to 10 percent balanced bioactivity against structural integrity, with 5 percent emerging as the most promising formulation for cell-laden cartilage constructs. The work remains an early in vitro proof of concept, but it maps a reproducible path from blood-bank byproducts to a bioactive, off-the-shelf matrix for cartilage tissue engineering.

Subject of Research: A lyophilized, pathogen-inactivated platelet lysate enriched gelatin hydrogel for cartilage tissue engineering

Article Title: Lyophilized pathogen-inactivated platelet lysate enriched hydrogel for cartilage tissue engineering

Article References: Boretti, G., Tiainen, H., Marczak, A., Hamzehpour, H., Simonsson, S., Gargiulo, P., & Sigurjónsson, Ó. E. (2026). Lyophilized pathogen-inactivated platelet lysate enriched hydrogel for cartilage tissue engineering. Frontiers in Bioengineering and Biotechnology, 14, Article 1986446. https://doi.org/10.3389/fbioe.2026.1986446

Image Credits: AI Generated

DOI: 10.3389/fbioe.2026.1986446

Keywords: cartilage tissue engineering, platelet lysate, hydrogel, lyophilization, pathogen inactivation, growth factor release, chondrogenesis, mesenchymal stromal cells, bioprinting bioink, rheology, regenerative medicine, blood bank