GoKawiilSample preparation of VPS hydrogels
A typical sample preparation process involves two key stages: (1) preparing a stable suspension of aerogel particles and (2) inducing VPS and filling with hydrogels.
Stage 1: to suspend the aerogel particles in water and facilitate VPS in subsequent steps, amphiphilic triblock-copolymer Pluronic F-127 (surfactant, Mw approximately 12 kDa), hydrophilic PAA (non-adsorbing polymer, Mw approximately 4,000 kDa) and hydrophilic PVA (suspension thickener, Mw approximately 146 kDa) were added while mixing superhydrophobic aerogel particles with water. In a typical procedure, 3.2 g of 10 wt% PVA solution, 3.2 g of 1 wt% PAA solution, 48 mg of 20 wt% F-127 solution and either 240 mg (denoted as 2 wt% aerogel content; Supplementary Table 5) or 480 mg (denoted as 4 wt% aerogel content; Supplementary Table 5) of aerogel particles were combined in a Thinky AR-100 mixer and mixed for 2 min. Subsequently, an extra 5.4 g of 1 wt% PAA solution was added, followed by another 2-min mixing. The resulting aerogel particle suspension is a white, viscous emulsion with low fluidity.
Stage 2: to induce VPS for the aerogel network, the aerogel suspension was poured into a customized acrylic mould sandwiched between two rigid acrylic plates with holes and a layer of dialysis tubing (molecular weight cut-off = 12–14 kDa). The VPS was triggered by immersing the samples into a 26 wt% NaCl solution at 60 °C. After varying immersion times, the samples were frozen at −5 °C for 3 h to induce physical crosslinking and then rinsed in deionized water three times (2 h per rinse). This process yielded a soft yet elastic VPS aerogel network. To fill the VPS aerogel network with polymers to yield VPS hydrogels, the dialysis tubing was replaced with new tubing (molecular weight cut-off ≈ 300 kDa), much higher than the molecular weight of the polymer used for filling. The encapsulated samples were then immersed in pre-gel solutions of PVA, gelatin, alginate, agarose or chitosan for 12 h. Finally, gelation was induced as described in Supplementary Information section 3.1, yielding VPS PVA, gelatin, alginate, agarose or chitosan hydrogels. After preparation, all samples were swollen in PBS solution three times before use. The detailed set-ups for VPS and filling have are illustrated in Supplementary Fig. 21. All samples used for the air-permeability measurements (including VPS gelatin, alginate, agarose and chitosan hydrogels in Fig. 3c) were prepared with a 4 wt% aerogel content.
Oxygen permeability tests
The oxygen permeability of hydrogels was tested with the polarographic method following the instructions of ISO 18369-4:2017 with a Rehder single-chamber system. Before the tests, the hydrogels were swollen in PBS buffer for 12 h. During the test, to keep the hydrogels moistened, the tests were carried out at 95% relative humidity throughout. Also, the testing sample was sandwiched by lens cleaning paper, which serves as the ‘aqueous bridge’ between the sample and electrode. The signal measured by the polarographic method is the current generated by an oxygen sensor over time I(t), in which t is time in minutes. The current at a steady state I s is defined as the I(t) at t (min) when \(\frac{I(t)}{I(t+5)} < 0.995\). As such, the preliminary oxygen permeability Dk pre is defined as
$${{Dk}}_{{\rm{pre}}}=\frac{0.278T({I}_{{\rm{s}}}-{I}_{{\rm{d}}})}{A}$$
in which T is the thickness of hydrogel (mm), I d is the dark current generated by the oxygen sensor when the oxygen level is zero (A) and A is the area of the cathode (cm2).
To obtain the corrected oxygen permeability of hydrogel samples, two more effects are required to be considered, including edge effects and boundary effect. The edge effects originate from the geometry of the electrode and hydrogel sample. For flat cathode and hydrogel samples, the oxygen permeability after considering edge effects Dk edge is expressed as:
$${{Dk}}_{{\rm{edge}}}=\frac{{DT}}{D+1.89T}{{Dk}}_{{\rm{pre}}}$$
... continue reading