Citric acid

Barium (II) nitrate (99.95%), scandium (III) nitrate hydrate (99.9%), iron (III) nitrate nonahydrate (98+%), citric acid (99+%), tetraethylorthosilicate (99%), sodium hydroxide (98%), 3-aminopropyl)triethoxysilane (APTES) (98%), and sodium alendronate (a source of AL) (97%) were purchased from Alfa Aesar (Lancashire, UK). Nitric acid was purchased from Sigma Aldrich (Burlington, MA, USA).
Precise concentrations of the metals in the salts were determined with inductively coupled plasma (ICP-OES, Agilent 720, Santa Clara, CA, USA). Deionized water was used in all experiments. Barium hexaferrite nanoplatelets partly substituted with Sc³⁺ (BSHF NPLs) were synthesized hydrothermally as previously described [9 (link)], followed by washing with Nitric acid and water. The obtained BSHF NPLs were colloidally stable in water after tuning the pH to pH = 2 with Nitric acid.
The BSHF NPLs showed a typical hexagonal plate-like shape with a magnetoplumbite crystal structure (Figure 1a). A selected-area electron diffraction (SAED) of the sample is shown in Figure S1 in the Supplementary Materials. The detailed structural analysis of BSHF NPLs is given in [10 (link)]. Figure 1 represents magnetic hysteresis (Figure 1a) with saturation magnetization of Ms~35 Am²kg⁻¹ and a significant coercivity of Hc = 97 kAm⁻¹ typical for hard magnetic materials such as barium hexaferrite (Figure 1b).

Before starting each batch of experiments, at least three samples were used to evaluate cell viability and the effective cell density. In the monolayers, cells were detached from the culture surface with trypsin-EDTA (Lonza^®, Basel, Switzerland); spheroids were left in a solution of 55 mM citric acid (Alfa Aesar^®, Haverhill, Massachusetts, USA) and incubated until complete dissolution of the hydrogel matrix. Then, the cells were centrifuged for 5 min at 900 rpm (Centrifuge 5702, Eppendorf^®, Hamburg, Germany) and resuspended in 200 μl of DMEM.
A sample of 30 μl was then mixed with the same volume of trypan blue (Thermo Fisher Scientific^®, Walthan, Massachusetts, USA), and an automated cell counter (Countess II FL, Thermo Fisher Scientific^®) was used to determine cell number and viability. O₂ measurements were only performed on batches in which the percentage of viable cells was > 90%.
The cell surface density was obtained knowing the size of a 96-well bottom. The volumetric cell density was instead determined from images of the spheroids acquired to compute their volume. More specifically, construct equivalent radii were estimated as the mean of those returned by using three different measurement tools in ImageJ.³⁸ (link)

For the synthesis of the γ-Fe₂O₃/SiO₂/RhB NPs, reagent grade chemicals were used as received from the manufacturers. Iron (III) sulfate hydrate, iron (II) sulfate heptahydrate (ACS, 99%), citric acid (CA, 99%), tetraethoxysilane (TEOS, 99.9%) and NH₄OH (28–30%) were supplied by Alfa Aesar (Lancashire, UK). Acetone (AppliChem GmbH) and absolute ethanol (Carlo Erba, reagent - USP) were used as received. (3-Aminopropyl)triethoxysilane (APS; silane-NH₂, 99%), tetraethoxysilane (TEOS; 98%), ethyl acetate (EA), dichloromethane (DCM), dimethylformamide (DMF), Rhodamine B isothiocyanate (RhB), poly(acrylic acid) solution (25 wt. % in water) and polyvinyl pyrrolidone (PVP, 40 kDa) were obtained from Sigma-Aldrich (St. Louis, MO, USA).
Cellulose acetate (CA, Mn = 30000 g/mol) was obtained from Sigma-Aldrich and used without further purification. Acetone (Technical Grade 99.5% - Panaska Trading CO) was the solvent used in the preparation of CA polymer solutions that were further electrospun.

Bovin serum albumin, catalase from bovine liver lyophilised powder (2000–5000 units/mg protein), ethanol, hydrogen peroxide solution (30% w/w in H₂O), methanol, polyvinylpyrrolidone (55 kDa), potassium hydroxide, Purpald^®, sodium chloride, sodium citrate tribasic dihydrate, and sodium phosphate monobasic monohydrate were supplied by Sigma-Aldrich (Steinheim, Germany). Citric acid, L-arginine monohydrochloride, dextran (~70 kDa), and 2,2,2-trifluoroethanol (>99%) were provided by Alfa Aesar (Heysham, UK). Ethylene diaminetetraacetic acid disodium salt dihydrate (EDTA), sodium phosphate dibasic dihydrate, aqueous formaldehyde solution (35 wt%) and hydrochloric acid (36.5–38% in water) were obtained from Honeywell (Seelze, Germany). Potassium periodate was purchased from Scientific Laboratory Suppliers Ltd. (Nottingham, UK). Ficoll PM70 (polysucrose 70) was obtained from Cytiva (Uppsala, Sweden). The Bradford 5× reagent was obtained from Serva (Heidelberg, Germany) and a microBCA protein assay kit was sourced from ThermoFisher (Rockford, IL, USA). Super refined Polysorbate 20^® and super refined Polysorbate 80^® were obtained from Croda (Goole, UK). α,α-Trehalose dihydrate was purchased from Pfanstiehl (Waukegan, IL, USA).