SENSOR FUNCTIONALISED BIOINK

Abstract

The present invention relates to 3D printable composition comprising a cross-linkable component, a non-cross-linkable polymer, and analyte sensor particles, and to a method of fabricating a scaffold for living cells, a scaffold for a living cell, and a kit of parts comprising components for performing the method to obtain the scaffold. The scaffold is useful for prolonged culture of living cells and allows monitoring a metabolite throughout the culture with high spatial resolution of the metabolite in the scaffold.

Claims

1. A 3D printable composition comprising a cross-linkable component, a non-cross-linkable polymer, and analyte sensor particles.

2. The 3D printable composition according to claim 1, wherein the analyte detectable by the analyte sensor particles is selected from the list consisting of: O.sub.2, CO.sub.2, H.sub.2S, H.sub.2O.sub.2, pH, irradiation and temperature.

3. The 3D printable composition according to claim 1, wherein the cross-linkable component is a carbohydrate polymer with acid groups selected from the list consisting of: alginate, pectin, and carrageenan, or a mixture thereof.

4. The 3D printable composition according to claim 1, wherein the non-cross-linkable polymer is a cellulosic polymer.

5. The 3D printable composition according to claim 1, wherein the ratio of the cross-linkable component to the non-cross-linkable polymer is in the range of 1:1 to 1:5.

6. The 3D printable composition according to claim 1, wherein the analyte sensor particles have a size in the range of 10 nm to 500 nm.

7. The 3D printable composition according to claim 1, wherein the analyte sensor particles employ optical detection principles.

8. The 3D printable composition according to claim 1 further comprising an aqueous medium.

9. The 3D printable composition according to claim 1 further comprising a living cell.

10. A method of fabricating a scaffold for living cells, the method comprising the steps of: providing a cross-linkable component, providing a non-cross-linkable polymer, providing analyte sensor particles, providing living cells, mixing the cross-linkable component, the non-cross-linkable polymer, the analyte sensor particles and the living cells in an aqueous medium to provide a bioink, 3D printing the scaffold from the bioink.

11. The method of preparing a scaffold for living cells according to claim 10, wherein the analyte detectable by the analyte sensor particles is selected from the list consisting of: O.sub.2, CO.sub.2, H.sub.2S, H.sub.2O.sub.2, pH, irradiation and temperature.

12. A kit of parts comprising a cross-linkable component, a non-cross-linkable polymer, analyte sensor particles, and a cross-linking agent.

13. The kit of parts according to claim 12, wherein the cross-linkable component, the non-cross-linkable polymer, and the analyte sensor particles are contained in a 3D printable composition.

14. A scaffold for a living cell, the scaffold comprising a cross-linked hydrogel with a non-cross-linkable polymer, and analyte sensor particles distributed in the hydrogel.

15. The scaffold according to claim 14 further comprising a living cell.

16. The method of preparing a scaffold for living cells according to claim 10, the method further comprising a step of cross-linking the cross-linkable component.

17. The method of preparing a scaffold for living cells according to claim 10, wherein cross-linkable component is a carbohydrate polymer with acid groups selected from the list consisting of: alginate, pectin, and carrageenan, or a mixture thereof.

18. The method of preparing a scaffold for living cells according to claim 10, wherein the non-cross-linkable polymer is a cellulosic polymer.

19. The method of preparing a scaffold for living cells according to claim 10, wherein the ratio of the cross-linkable component to the non-cross-linkable polymer is in the range of 1:1 to 1:5.

20. The method of preparing a scaffold for living cells according to claim 10, wherein the analyte sensor particles have a size in the range of 10 nm to 500 nm.

21. The method of preparing a scaffold for living cells according to claim 10, wherein the analyte sensor particles employ optical detection principles.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] In the following the invention will be explained in greater detail with the aid of an example and with reference to the schematic drawings, in which

[0035] FIG. 1 shows the viscosity of a bioink of the invention with sensor nanoparticles and of a bioink without sensor nanoparticles;

[0036] FIG. 2 shows photographs of 3D printed hydrogel scaffolds of the invention containing optical O.sub.2 sensor nanoparticles at different O.sub.2 levels;

[0037] FIG. 3 shows the calibration of 3D printed hydrogel scaffolds of the invention containing optical O.sub.2 sensor nanoparticles;

[0038] FIG. 4 shows the mapping of O.sub.2 dynamics in a 3D bioprinted scaffold containing mammalian hTERT cells and O.sub.2 sensitive nanoparticles when exposed to a decreasing O.sub.2 level (21-5% O.sub.2) in the incubator atmosphere;

[0039] FIG. 5 shows the viability of the microalga Chlorella sorokiniana and the mammalian cell line hTERT in a scaffold of the invention;

[0040] FIG. 6 shows microscopic images showing the viability of the microalga C. sorokiniana (A) and the mammalian cell line hTERT (B) when immobilised in 3D bioprinted hydrogel scaffolds containing O.sub.2 sensor nanoparticles;

[0041] FIG. 7 shows variable chlorophyll fluorescence imaging of 3D bioprinted hydrogel scaffold containing the green alga C. sorokiniana in all layers;

[0042] FIG. 8 shows the time course of O.sub.2 concentrations in a scaffold of the invention;

[0043] FIG. 9 shows lateral profiles of O.sub.2 concentrations in a scaffold of the invention.

[0044] Reference to the figures serves to explain the invention and should not be construed as limiting the features to the specific embodiments as depicted.

DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention relates to 3D printable composition, a method of fabricating a scaffold for living cells, a scaffold for a living cell, and a kit of parts comprising components for performing the method to obtain the scaffold. Thus, the 3D printable composition comprises a cross-linkable component, a non-cross-linkable polymer, and analyte sensor particles.

[0046] These components are relevant also for the kit of parts, which also comprises a cross-linking agent. Further details of the aspects of the invention are disclosed above with additional details provided below.

[0047] The composition of the invention is a “3D printable” composition. In the context of the invention “3D printing” is understood in its broadest sense. 3D printing may also be referred to as “additive manufacturing” and describes fabrication of three-dimensional (“3D”) objects by additive deposition, additive agglomeration or additive layering, and also stereolithography or selective laser sintering.

[0048] In the context of the present invention a 3D printable composition may also be referred to as a “bioink”, and the two terms may be used interchangeably. The term “bioink” may be used regardless of the presence of living cells or analyte sensor particles during 3D printing and the term generally implies that a scaffold or structure fabricated from the bioink is suited for culturing living cells.

[0049] The scaffold of the invention allows non-invasive monitoring a metabolite relevant to living cells in the scaffold during culture. In the context of the invention the term “culture” is to be understood broadly, and implies that the living cells will be alive in the scaffold. The term culture does not imply that the cells are dividing, so that the living cells may also be considered to be “maintained” and the scaffold is suitable for “maintenance” of living cells.

[0050] The scaffold of the invention is suited for culture or maintenance of a living cells. In general, the term “a living cell” will refer to a plurality of living cells, although it is also contemplated that a single living cell may be included in the bioink or applied to the scaffold after fabrication.

[0051] In the context of the invention the “cross-linkable component” may be a polymer or a monomer capable of being polymerised. For polymerisable monomers any polymerisation chemistry is contemplated.

[0052] The 3D printable composition also comprises a “non-cross-linkable polymer”. Any polymer that cannot be cross-linked under conditions employed in the 3D printing process and during the optional cross-linking of the cross-linkable component is contemplated for the invention.

[0053] In the context of the present invention a structure obtainable in the method of the invention is considered to constitute a “hydrogel”. Preferred hydrogels are obtained when both the cross-linkable component and also the non-cross-linkable polymer are carbohydrate based polymers, and even more preferred hydrogels are obtained when the cross-linkable polymer, e.g. a carbohydrate based polymer with acidic groups, is cross-linked using a cross-linking agent, e.g. divalent metal ions, such as Ca.sup.2+.

[0054] The method may employ a cross-linking agent, and the kit of parts contains a cross-linking agent. In the context of the present invention a “cross-linking agent” is any chemical agent that can induce cross-linking of the cross-linkable component. It is, however, preferred that the cross-linking does not involve covalent reactions between the cross-linking agent and the cross-linkable component, or between two molecules of the cross-linkable component. Thus, a preferred cross-linking agent is a salt with a divalent metal ion, e.g. CaCl.sub.2), CaBr.sub.2, CaI.sub.2, etc. It is however also contemplated that other divalent metals may be employed, e.g. magnesium or strontium, or transition metals. Moreover, trivalent metal ions, e.g. in the form of salts with trivalent aluminium, are also contemplated as cross-linking agents. Divalent metal ions are especially useful for cross-linking carbohydrate polymers with acidic groups.

[0055] The invention employs analyte sensor particles, and in the invention the “analyte sensor particle” uses a “detection principle” for sensing, or detecting, an analyte, in particular a metabolite. The analyte or metabolite is therefore considered to be “detectable” by the analyte sensor particle. Analyte sensor particles may also be referred to as indicator probes. In general, the detection principle is specific for a single analyte, e.g. the analyte sensor particle may be an O.sub.2 sensor particle, although the analyte sensor particle may also be specific for additional analytes, e.g. an analyte sensor particle may also include a pH sensor. The analyte sensor particle will normally have a reference compound that improves the data, e.g. for quantification, obtained from the analyte sensor particle. However, analyte sensor particles without a reference compound are also included in the invention.

[0056] The analyte sensor particles will generally be “small”, i.e. they can be classified as “nanoparticles” or “microparticles”. In the context of the present invention a nanoparticle is a particle with a size in the range of 10 nm to 500 nm, and a microparticle is a particle with a size in the range of 0.5 μm to 10 μm. However, the analyte sensor particles may also be a mixture of nanoparticles and microparticles, e.g. the analyte sensor particles may have a size in the range of 10 nm to 10 μm. It is preferred that the analyte sensor particles have a monodisperse size distribution.

[0057] The invention will now be described in the following non-limiting example.

Example

[0058] Preparation of O.sub.2 Sensitive Nanoparticles

[0059] Oxygen (O.sub.2) was selected as an appropriate metabolite, and O.sub.2 sensitive nanoparticles were prepared. The O.sub.2-sensitive indicator Platinum(II) meso(2,3,4,5,6-pentafluoro)phenyl porphyrin (PtTFPP) was bought from Frontier Scientific (www.frontiersci.com). A reference dye Bu.sub.3Coum was generously provided by Dr. Sergey Borisov (Graz University of Technology). A styrene maleic anhydride copolymer (PSMA with 8% MA, Mw: 250000 g mol.sup.−1) XIRAN® was provided from Polyscope (http://www.polyscope.eu). Tetrahydrofuran (THF) was obtained from Sigma-Aldrich.

[0060] The O.sub.2-sensitive sensor nanoparticles were prepared according to Mistlberger et al. (2010). Briefly, 200 mg of PSMA, 3 mg of Bu.sub.3Coum (reference dye) and 3 mg of PtTFPP (O.sub.2 indicator) were dissolved in 20 g of THF. This mixture was quickly poured into 200 mL of vigorously stirred distilled water. After evaporating the THF under an air stream, the particle suspension was concentrated at elevated temperature (60° C.) until a concentration of 5 mg mL.sup.−1 was reached. The particle concentration was checked by drying and subsequent weighing of 1 mL of the particle suspension. The obtained particles have a size of several hundred nm and exhibit a strongly negative zeta potential of around −30 mV as shown elsewhere (Mistlberger et al., 2010). The particles (both O.sub.2 indicator and reference dye) can be excited by blue light (within a spectral range of 400-475 nm) and emit an O.sub.2-dependent luminescence in the red spectral region (625-720 nm with a distinct peak at 650 nm) along with an O.sub.2-independent fluorescence from the reference dye in the green spectral region (475-550 nm) (Mistlberger et al. 2010). The particle suspension could be stored over several weeks without any signs of sedimentation, aggregation, colour change or change in the calibration characteristics.

[0061] Preparation and Characterisation of a Hydrogel

[0062] An alginate (3%)/methylcellulose (9%) blend has previously shown to have excellent printing fidelity and good biocompatibility for printing hydrogel scaffolds of microalgae and human cell lines (Schütz et al. 2017, Lode et al. 2015) and was used in this Example.

[0063] A bioink was prepared by dissolving 30 mg mL.sup.−1 alginic acid sodium salt (Sigma-Aldrich, Taufkirchen, Germany) either in distilled water in case of microalgae printing or in phosphate buffered saline (PBS) in case of mammalian cell printing followed by addition of 90 mg mL.sup.−1 methylcellulose (Sigma-Aldrich; approximately MW=88 kDa). After thorough mixing, the mixture was incubated at room temperature for 1-2 h enabling swelling of the methylcellulose prior to printing. When preparing printing material containing the O.sub.2 sensitive nanoparticles, equal amounts of a 6% (60 mg mL.sup.−1) alginate solution and a 5 mg mL.sup.−1 stock solution of the nanoparticles were mixed before addition of the methylcellulose, yielding a hydrogel matrix with the same final alginate concentration (3%) as in the hydrogels without nanoparticles.

[0064] The viscosity of individual blends, e.g. 3% alginate/9% methylcellulose (in distilled water or PBS) with and without nanoparticles was measured using a rotary rheometer (Rheotest® RN 4.1, Medingen, Germany) with a 1° cone/plate. A constant shear rate of 10 s.sup.−1 was applied for 100 s with a plate distance of 0.1 mm, and the corresponding viscosity was obtained for the individual blends. Shear thinning behaviour of the blends was also observed using a 1° cone/plate with a 0.1 mm plate distance. Incremental shear rate was applied over the range of 0-100 s.sup.−1 (with an increment of 0.5 s.sup.−1), and corresponding changes in the viscosity were quantified. A plot of shear rate and viscosity provided information about the shear thinning behaviour of the blends. See FIG. 1.

[0065] Fabrication of Hydrogel Scaffolds

[0066] Simple cross-layered, wood-pile structured hydrogel scaffolds (˜15×15×1.08 mm) were printed on a 3D plotting system (BioScaffolder 2.1, GeSiM mbH, Radeberg, Germany) as previously described (Lode et al. 2015). A 610 μm conical needle (Globaco GmbH, Rodermark, Germany) was used to print the scaffolds at a printing speed of 10 mm s.sup.−1, using an extrusion pressure of ˜80 kPa and ˜300 kPa for the printing of mammalian cells and microalgae, respectively. Strand spacing was always maintained at 2 mm for all the scaffolds. An outline, e.g. a continuous strand, covering 4 sides of the scaffold was printed at an offset of 0.3 mm. The printed hydrogel scaffolds were cross-linked in a 100 mM CaCl.sub.2) solution for 10 min before washing in PBS (for human telomerase reverse transcriptase mesenchymal stem cell scaffolds) or TAP medium (for microalgae scaffolds) and were then transferred into DMEM or TAP medium for further cultivation or calibration, respectively. While all printed scaffolds had the same overall dimensions in terms of printing geometry and thickness, scaffolds of varying composition encompassing i) pure hydrogel scaffolds, ii) scaffolds with added O.sub.2 sensors nanoparticles, and iii) scaffolds loaded with microalgae or mammalian cells with/without added metabolite sensor nanoparticles were printed. More complex scaffolds with a pattern of crossed hydrogel layers with/without sensor nanoparticles and/or with/without cells were also printed. This included patterned mammalian/algal co-culture scaffolds (as first shown in Lode et al. 2015) of hydrogel containing O.sub.2 sensor nanoparticles.

[0067] 3D printed hydrogels containing green microalgae (Chlorella sorokiniana, strain 211-8k, obtained from the Culture Collection of Algae, University of Goettingen, Germany) or cells of an immortalised human mesenchymal stem cell line expressing hTERT (human telomerase reverse transcriptase) (Nicker et al. 2008): hTERT-MSC were prepared. Prior to printing, the microalgae were proliferated in liquid TAP (tris-acetate-phosphate) medium (50 mL in a 250 mL shaking flask) at room temperature and under a photon irradiance of 100 μmol photons m.sup.−2 s.sup.−1. The hTERT-MSC were proliferated in cell culture medium (DMEM+10% fetal calf serum+100 U/ml Penicillin and 100 μg/ml Streptomycin) at 37° C. and 5% CO.sub.2. After harvesting, the respective alginate/methylcellulose blend was mixed with cells at a cell density of 5×10.sup.7 microalgal cells g.sup.−1 and 5×10.sup.6 hTERT-MSC cells g.sup.−1 of bioink, which was used for printing the cell-containing scaffolds. Similar bioink blends were used for printing scaffolds containing both cell types. After printing, hTERT-MSC scaffolds were cultivated in DMEM medium, whereas microalgal scaffolds were incubated in TP medium, i.e., TAP medium without acetic acid. Co-cultures of microalgae and hTERT-MSCs were cultivated in an adapted medium consisting of DMEM and TAP media at 37° C., 5% CO.sub.2 and defined O.sub.2 concentrations (1-21%) with and without illumination (white light, 450 μmol photons m.sup.−2s.sup.−1).

[0068] Viable cells (both hTERT-MSCs and microalgae) in printed scaffolds with nanoparticles were quantified by performing respective live-dead staining procedures immediately after printing, 24 and 72 hours after printing. Scaffolds with hTERT-MSCs and nanoparticles were incubated with 2 μM Calcein AM and 4 μM Ethidium homodimer (LIVE/DEAD Viability/Cytotoxicity Kit, for mammalian cells, Invitrogen, Eugene, Oreg., USA) for 30 min to stain live (green) and dead (red) cells. In case of scaffolds consisting of microalgae and nanoparticles, the scaffolds were incubated in TAP medium containing 5 μM SYTOX Green (dead cells stained in green) (Molecular Probes, Eugene, Oreg., USA) for 15 min at room temperature in the dark. Live cells were detected by autofluorescence of chlorophyll. After the staining procedures, z-stack images of the scaffolds were acquired using a Leica TCS SP5 confocal microscope, located at the Core Facility Cellular Imaging (CFCI) of Technische Universitat Dresden. The percentage of live and dead cells was quantified using ImageJ V1.44p (NIH; Schindelin et al 2012). See FIG. 5 and FIG. 6.

[0069] O.sub.2 Imaging

[0070] The photosynthetic activity of the microalgae immobilised in hydrogel scaffold with O.sub.2 sensor particles was monitored via variable chlorophyll fluorescence measurements (Schreiber 2004) using a commercial pulse-amplitude modulated imaging fluorometer (I-PAM/GFP, Walz GmbH, Effeltrich, Germany) as described in detail elsewhere (Ralph et al. 2005; Kuhl and Polerecky 2008). The system employed blue (470 nm) LED light for weak (<1 μmol photons m.sup.−2 s.sup.−1) modulated measuring light pulses, strong (0.8 s at >2500 μmol photons m.sup.−2 s.sup.−1) saturating light pulses, and defined levels of actinic irradiance, as measured with a calibrated irradiance meter at the level of bioprinted scaffolds (ULM, Walz, Effeltrich, Germany). Based on imaging the fluorescence yield under ambient light conditions, F, and under a strong light pulse completely saturating photosynthesis, F′.sub.m, the effective quantum yield of photosystem (PS) II activity under a given photon irradiance (PAR) was determined as:

YII=(F′.sub.m−F)/F′.sub.m

[0071] Based on such YII images, a measure of the relative photosynthetic electron transport associated with PSII was calculated as rETR=PAR×YII. In absence of light, all photosynthetic reaction centres of the microalgae are open causing a minimal fluorescent yield, F.sub.0, in darkness and a maximum fluorescent yield, F.sub.m, during the brief saturation pulse. From these measurements, the maximum PSII quantum yield was calculated as:

φ.sub.max=(F.sub.m−F.sub.0)/F.sub.m

[0072] This parameter is often used as an index for fitness of photosynthetic organisms (Schreiber 2004). Besides images showing the distribution of different variable chlorophyll fluorescence-derived parameters over the 3D bioprinted hydrogel scaffolds, data could also be averaged over particular areas of interest that could be freely defined by help of the imaging systems software (ImagingWIn, Walz, Effeltrich, Germany).

[0073] A ratiometric RGB camera system was used for O.sub.2 imaging (Larsen et al., 2011; Koren et al., 2015). The system consisted of a SLR camera (EOS 1000D, Canon, Japan) combined with a macro objective (Macro 100 f2.8 D, Tokina, Japan) equipped with a 530 nm long pass filter (Uqgoptics.com). Excitation of sensor particles was achieved with a custom-built 445 nm multichip LED equipped with a bandpass filter (NT43-156, Edmundoptics.com). The LED was powered by a USB-controlled LED driver unit for fluorescence imaging applications (http://imaging.fish-n-chips.de). Image acquisition control of the SLR and LED were done with the software look@RGB (http://imaging.fish-n-chips.de). All images of printed hydrogel scaffolds were acquired with the camera system mounted inside an incubator (HeraCell 240i Thermo Scientific, USA) kept at a constant temperature of 37° C. The incubator was capable of controlling the internal gas composition, and different defined O.sub.2 levels could thus be adjusted. White light illumination for the scaffolds containing microalgae was provided by fiber-optic LED lamp (KL2500 LED, Schott GmbH, Germany), which could be interfaced and controlled by a PC via a USB interface, and provided defined levels of incident photon irradiance onto the scaffolds in the incubator, as measured with a calibrated photon irradiance meter (ULM, Walz GmbH, Effeltrich, Germany). All cables were guided to the outside of the incubator, which was kept closed during experiments, and all measurements were PC-controlled from the outside.

[0074] Image analysis and calibration: RGB images acquired upon LED excitation were split into red, green, and blue channels and analysed using the freeware ImageJ (http://rsbweb.nih.gov/ij/) (Koren et al. 2015). In order to obtain O.sub.2 concentration images the following steps were performed: First the red channel (O.sub.2 sensitive emission of the PtTFPP) and green channel (constant emission of the reference dye) images were divided using the ImageJ plugin Ratio Plus (http://rsb.info.nih.gov/ij/plugins/ratio-plus.html) in order to get the ratio image (R), which is a proxy for the O.sub.2 dependent luminescence. In order to get O.sub.2 concentration images from the measured ratios, a calibration curve was measured for a range of different preset O.sub.2 levels, which was then used to convert ratio images into calibrated O.sub.2 concentration images. Calibration was done by printing a scaffold containing only nanosensors in the bioink. The scaffold was placed in a water-filled dish inside the incubator and the O.sub.2 levels in the incubator were changed (for zero O.sub.2, sodium sulphite was added to the sample). A calibrated optical O.sub.2 sensor (OXROB3 probe connected to a Firesting GO2 meter; both from PyroScience GmbH, Aachen, Germany) was also put into the incubator (in a similar volume of water) to ensure that the O.sub.2 levels were correct and enough time was given to achieve full change of O.sub.2. Calibration images were recorded after equilibration at each set O.sub.2 concentration (for >2 hours). Using the curve fitting function of ImageJ all ratio images were then converted to O.sub.2 images.

[0075] Calibration curves of optical O.sub.2 sensors immobilised in a polymer matrix describe the collisional quenching of the indicator luminescence by O.sub.2. The measured luminescence intensity (I) or decay time (τ) shows a non-linear decrease with O.sub.2 concentration, [O.sub.2], that can be described by a modified Stern-Volmer relation (Bacon and Demas 1987, Kühl 2005):

[00001]$\begin{array}{cc}\frac{I}{I_0}=\frac{\tau }{{\tau }_0}=\frac{R}{R_0}=\frac{1-\alpha }{1+K_{S.Math.V}.Math.\left[O_2\right]}+\alpha & \mathrm{Eq}..Math.1\end{array}$

[0076] where, I.sub.0 and τ.sub.0 are the luminescence intensity and decay time, respectively, of the indicator in the absence of O.sub.2, K.sub.sv is a characteristic quenching coefficient of the optical O.sub.2 indicator, and a describes the non-quenchable fraction of the indicator when immobilises in a given polymer matrix. In this imaging approach, the ratio between the red and green channels was used in the acquired images, R, as a proxy for the O.sub.2 concentration dependent luminescence.

[0077] Results

[0078] The O.sub.2 sensitive nanoparticles dispersed well in the alginate/metylcellulose mixture and formed a homogenous orange coloured printing paste that was easy to print using similar 3D printer settings previously used for bioprinting cells in similar hydrogels (Lode et al. 2015, Krujatz et al. 2015). Compared to blends without nanoparticles, addition of nanoparticles resulted in minimal increase and decrease in viscosity of 3% alginate (in distilled water)/9% methylcellulose or 3% alginate (in PBS)/9% methylcellulose, respectively. In addition, shear thinning behaviour of the blends was not affected by addition of the nanoparticles (FIG. 1), indicating that the blends are printable.

[0079] 3D printed hydrogel scaffolds containing O.sub.2 sensor particles at different O.sub.2 levels are depicted in FIG. 2, and a calibration is shown in FIG. 3. FIG. 2 displays O.sub.2 concentrations after a light-dark shift of a two-layered 3D bio-printed hydrogel scaffold where the vertical lanes contain microalgae and O.sub.2 sensor particles, and the horizontal lanes contain only O.sub.2 sensor particles; the left panel shows a photograph taken at natural daylight, and the middle and right panels show oxygen distribution in a light exposed state, and after 60 minutes and in darkness, respectively. A 3D printed hydrogel scaffolds containing only the O.sub.2 sensitive nanoparticles showed a non-linear decrease in the ratio of O.sub.2 dependent luminescence (detected in the red image channel) over the constant reference emission (detected in the green image channel of the camera) with increasing O.sub.2 levels (FIG. 3) ranging from anoxia to full atmospheric saturation (21% O.sub.2). The reaching of a new equilibrium throughout the bioprinted hydrogel scaffold after a change in external O.sub.2 level was slow and required incubation for >4 hours for each calibration point, even with cells in the scaffold (FIG. 4). No leakage of nanoparticles from printed hydrogels scaffold into the surrounding medium was observed over several days of incubation, and we observed no significant photobleaching under the experimental irradiance levels used.

[0080] The calibration curve (FIG. 3) done at 37° C. could be fitted well by Eq. (1) (r.sup.2>0.999), yielding an apparent K.sub.sv luminescence quenching constant of 0.077, and a non-quenchable fraction of the immobilized indicator of σ=0.258.

[0081] FIG. 5 compares the viability of the microalga C. sorokiniana and the mammalian cell line hTERT when immobilised in 3D bioprinted hydrogel scaffolds containing O.sub.2 sensor nanoparticles. The cells were cultured in a scaffold of the invention for a prolonged period of time, and results for the first three days are shown. As judged from the proportion of live versus dead cells in the bioprinted hydrogels (FIG. 5, FIG. 6), the presence of the O.sub.2 sensitive nanoparticles in the bioprinted hydrogel scaffolds did not reduce the viability of the immobilised microalgae and mammalian cells even after several days of incubation. The general viability of the hTERT cells was marginally lower than the microalgal cells but remained more or less constant over 3 days, while the microalgae exhibited a high but slowly decreasing viability that was, however, still >85% after 3 days (FIG. 5).

[0082] Variable chlorophyll fluorescence imaging of 3D bioprinted hydrogel scaffold containing the green alga C. sorokiniana in all layers is shown in FIG. 7. The top panel shows images of the effective quantum yield of photosystem (PS) II activity, YII, measured at increasing incident irradiance levels of blue actinic light (numbers in panels denote μmol photons m.sup.−2 s.sup.−1). Values of YSII are false colour-coded according to the coloured scale bar. In darkness, the measures YII equals the maximum PSII quantum yield, F.sub.v/F.sub.m. The lower panels show a plot of YII versus photon irradiance (PAR in μmol photons m.sup.−2 s.sup.−1) (left) and the derived proxy of relative photosynthetic activity, rETR=YII×PAR, as integrated over the central vertical strand in the scaffold, showing onset of photosynthesis saturation at an irradiance of ˜250 μmol photons m.sup.−2 s.sup.−1. Good viability of microalgae in the hydrogel was confirmed by the variable chlorophyll fluorescence measurements (FIG. 7) showing a uniform high maximum PSII quantum yield of >0.6 all over the bioprinted hydrogel scaffold, and a high photosynthetic PSII activity that exhibited a typical saturation behaviour of rETR with increasing irradiance, approaching saturation above ˜250 mmol photons m.sup.−2 s.sup.−1.

[0083] The chlorophyll in the microalgae can be excited by the same wavelength (445 nm) used for excitation of the O.sub.2 sensitive nanoparticles causing chlorophyll (Chl) fluorescence emission (680-720 nm), which can be partly detected in the red channel of the camera system, where the O.sub.2-dependent sensor luminescence is also recorded (max emission at 650 nm). Comparison of i) Chl fluorescence from a 3D bioprinted scaffold containing only microalgae with ii) O.sub.2 sensor luminescence from a similar 3D printed scaffold with the same microalgal density as well as nanoparticles, showed low interference from Chl fluorescence amounting to an uncertainty in the O.sub.2 concentration determination of a few %.

[0084] Bioprinted hydrogel scaffolds with microalgae and O.sub.2 sensor nanoparticles demonstrated the ability of using sensor-laden bioinks in combination with luminescence imaging to map spatio-temporal chemical heterogeneity in 3D bioprinted hydrogel scaffolds. Thus, FIG. 8 shows O.sub.2 concentrations after a light-dark shift at three different locations in a 3D printed scaffold of the invention with regions containing microalgae and O.sub.2 sensor nanoparticles, regions with only O.sub.2 sensor nanoparticles and regions where the two layers cross. FIG. 8 clearly shows how O.sub.2, as a metabolite, can be monitored in distinct regions in a scaffold of the invention.

[0085] Upon incubation under high photon irradiance saturating photosynthesis (cf. FIG. 7), the overall O.sub.2 level in the bioprinted scaffold showed supersaturating concentrations, which were then depleted during subsequent dark incubation. A steady state O.sub.2 distribution in dark incubated scaffolds was reached about 2 hours after onset of darkness, clearly showing lowest O.sub.2 concentrations in the hydrogel strands containing respiring microalgae. Scaffold strands with microalgae and nanoparticles encapsulated in the hydrogel exhibited the strongest O.sub.2 concentration dynamics during experimental light-dark shifts, while O.sub.2 changes in scaffold strands with only hydrogel and sensor nanoparticles showed less dynamics, which was mainly determined by diffusive exchange with the microalgae-laden parts of the scaffold (FIG. 8).

[0086] Local shading of the scaffold, lead to local supersaturating O.sub.2 levels in light exposed parts of the hydrogel scaffold with microalgae, while the shaded parts exhibited lower O.sub.2 levels, especially in the hydrogel strands with microalgae.

[0087] Incorporation of sensor particles in more complex 3D bioprinted hydrogel scaffolds containing hydrogel strands with microalgae, mammalian cells and pure hydrogel demonstrated the ability to map local differences in O.sub.2 due to different metabolic activity in hydrogels compartments with mammalian and microalgal cells. For example, lateral profiles of O.sub.2 concentration between hydrogel layer with microalgae+nanoparticles and a layer hTERT cells+sensor nanoparticles are shown in FIG. 9 (see arrow on inset) measured after 60 min light exposure and 30 min of darkness, respectively.

[0088] At contact points between sensor-laden hydrogels strands with microalgae and mammalian cells, respectively, it was possible to extract lateral O.sub.2 concentration profiles showing how the activity of different cell types affected the O.sub.2 availability and exchange between different compartments in the hydrogel scaffold (FIG. 9). The microalgae showed a higher respiratory activity than the mammalian cells, leading to a distinct O.sub.2 concentration gradients, where hydrogel compartments with microalgae acted as O.sub.2 sinks with a lower concentration than in neighbouring hydrogel compartments with mammalian cells. The O.sub.2 concentration in hydrogel strands reached anoxia about 1 mm from intersections to hydrogel strands with mammalian cells that only depleted O.sub.2 to about 14%. In light, the photosynthetic O.sub.2 production alleviated the strong O.sub.2 depletion in the hydrogel compartments with microalgae, leading to less pronounced concentration differences between compartments with mammalian cells and microalgae, where hydrogel strands with mammalian cells now became slight supersaturated by the O.sub.2 released form the surrounding hydrogel strands with microalgae.

[0089] The above Example illustrates the utility of integrating a metabolite sensor, e.g. O.sub.2 sensitive nanoparticles, in a bioink for 3D printing a scaffold for living cells. The O.sub.2 sensitive nanoparticles can readily be replaced with, or supplemented with, nanoparticles capable of sensing other metabolites, e.g. CO.sub.2, H.sub.2S, H.sub.2O.sub.2, and pH, etc.

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