Soft Sub-Structured Multi-Material Biosensor Hydrogels with Enzymes Retained by Plant Viral Scaffolds.

An all-soft multi-material combination consisting of a hydrogel based on poly(ethylene glycol) (PEG) coated with spatially defined spots of gelatin methacryloyl (GM) containing selectively addressable viral nanorods is presented, and its basic application as a qualitative biosensor with reporter enzymes displayed on the tobacco mosaic virus (TMV) bioscaffolds within the GM is demonstrated. Biologically inert PEG supports are equipped with GM spots serving as biological matrix for enzymes clustered on TMV particles preventing diffusion out of the gel. For this multi-material combination, i) the PEG-based hydrogel surface is modified to achieve a clear boundary between coated and non-coated regions by introducing either isothiouronium or thiol groups. ii) Cross-linking of the GM spots is studied to achieve anchoring to the hydrogel surface. iii) The enzymes horseradish peroxidase or penicillinase (Pen) are conjugated to TMV and integrated into the GM matrix. In contrast to free enzymes, enzyme-decorated TMVs persist in GM spots and show sustained enzyme activity as evidenced by specific color reaction after 7 days of washing, and for Pen after 22 months after dry storage. Therefore, the integration of enzyme-coupled TMV into hydrogel matrices is a promising and versatile approach to obtaining reusable and analyte-specific sensor components.

On the reproducibility of extrusion-based bioprinting: round robin study on standardization in the field.

The outcome of three-dimensional (3D) bioprinting heavily depends, amongst others, on the interaction between the developed bioink, the printing process, and the printing equipment. However, if this interplay is ensured, bioprinting promises unmatched possibilities in the health care area. To pave the way for comparing newly developed biomaterials, clinical studies, and medical applications (i.e. printed organs, patient-specific tissues), there is a great need for standardization of manufacturing methods in order to enable technology transfers. Despite the importance of such standardization, there is currently a tremendous lack of empirical data that examines the reproducibility and robustness of production in more than one location at a time. In this work, we present data derived from a round robin test for extrusion-based 3D printing performance comprising 12 different academic laboratories throughout Germany and analyze the respective prints using automated image analysis (IA) in three independent academic groups. The fabrication of objects from polymer solutions was standardized as much as currently possible to allow studying the comparability of results from different laboratories. This study has led to the conclusion that current standardization conditions still leave room for the intervention of operators due to missing automation of the equipment. This affects significantly the reproducibility and comparability of bioprinting experiments in multiple laboratories. Nevertheless, automated IA proved to be a suitable methodology for quality assurance as three independently developed workflows achieved similar results. Moreover, the extracted data describing geometric features showed how the function of printers affects the quality of the printed object. A significant step toward standardization of the process was made as an infrastructure for distribution of material and methods, as well as for data transfer and storage was successfully established.

Preparation of multifunctional hydrogels with accessible isothiouronium groups via radical cross-linking copolymerization.

Hydrogels can be equipped with functional groups for specific purposes. Isothiouronium groups can enhance adsorptivity, or allow coupling of other functional groups through mild reactions after transformation to thiol groups. Here we present a method to prepare multifunctional hydrogels by introducing isothiouronium groups into poly(ethylene glycol) diacrylate (PEGDA) hydrogels, and convert them into thiol-functionalized hydrogels by the reduction of the isothiouronium groups. For this purpose, the amphiphilic monomer 2-(11-(acryloyloxy)-undecyl)isothiouronium bromide (AUITB), containing an isothiouronium group, was synthesized and copolymerized with PEGDA. In this convenient way, it was possible to incorporate up to 3 wt% AUITB into the hydrogels without changing their equilibrium swelling degree. The successful functionalization was demonstrated by surface analysis of the hydrogels with water contact angle measurements and increased isoelectric points of the hydrogel surfaces from 4.5 to 9.0 due to the presence of the isothiouronium groups. The hydrogels showed a suitability as an adsorbent, as exemplified by the pronounced adsorption of the anionic drug diclofenac. The potential of the functionalization for (bio)conjugation reactions was demonstrated by the reduction of isothiouronium groups to thiols and subsequent immobilization of the functional enzyme horseradish peroxidase on the hydrogels. The results show that fully accessible isothiouronium groups can be introduced into radically cross-linked hydrogels.

Integrated polarization mode interferometer in 220-nm silicon-on-insulator technology.

A compact integrated and high-efficiency polarization mode interferometer in the 220-nm silicon-on-insulator platform is presented. Due to the operation with two polarization modes in a single waveguide, low propagation losses and high sensitivities combined with a small footprint are achieved. The designed and fabricated system with a 5-mm-long sensing region shows a measured excess loss of only 1.5 dB with an extinction ratio up to 30 dB, while its simulated homogeneous bulk sensitivity can exceed 8000 rad/RIU. The combination with a 90° hybrid readout system offers single wavelength operation with unambiguousness for phase shifts up to 2π and constant sensitivity.

New Gelatin-Based Hydrogel Foams for Improved Substrate Conversion of Immobilized Horseradish Peroxidase.

Hydrogel foams provide an aqueous environment that is very attractive for the immobilization of enzymes. To this end, functional polymers are needed that can be converted into hydrogel foams and that enable bioconjugation while maintaining high enzyme activity. The present study demonstrates the potential of biotinylated gelatin methacryloyl (GM10EB) for this purpose. GM10EB is synthesized in a two-step procedure with gelatin methacryloyl (GM10) being the starting point. Successful biotinylation is confirmed by 2,4,6-trinitrobenzene sulfonic acid and 4'-hydroxyazobenzene-2-carboxylic acid/avidin assays. Most importantly, a high methacryloyl group content (DM) is maintained in GM10EB, so that solutions of GM10EB show both a sufficiently low viscosity for microfluidic foaming and a pronounced curing behavior. Thus, foamed and nonfoamed GM10EB hydrogels can be prepared via radical crosslinking of the polymer chains. Within both sample types, biotin groups are available for bioconjugation, as is demonstrated by coupling streptavidin-conjugated horseradish peroxidase to the hydrogels. When assessing the substrate conversion rate rA in foamed hydrogels by enzymatic conversion of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), a value for rA 12 times higher than the value for nonfoamed hydrogels of the same mass is observed. In other words, rA can be successfully tailored by the hydrogel morphology.

Evaluation of novel biomaterials for cartilage regeneration based on gelatin methacryloyl interpenetrated with extractive chondroitin sulfate or unsulfated biotechnological chondroitin.

Gelatin is widely proposed as scaffold for cartilage tissue regeneration due to its high similarities to the extracellular matrix. However, poor mechanical properties and high sensitivity to enzymatic degradation encouraged the scientific community to develop strategies to obtain better performing hydrogels. Gelatin networks, specifically gelatin-methacryloyl (GM), have been coupled to hyaluronan or chondroitin sulfate (CS). In this study, we evaluated the biophysical properties of an innovative photocross-linked hydrogel based on GM with the addition of CS or a new unsulfated biotechnological chondroitin (BC). Biophysical, mechanical, and biochemical characterization have been assessed to compare GM hydrogels to the chondroitin containing networks. Moreover, mesenchymal stem cells (MSCs) were seeded on these biomaterials in order to evaluate the differentiation toward the chondrocyte phenotype in 21 days. Rheological characterization showed that both CS and BC increased the stiffness (G' was about 2-fold), providing a stronger rigid matrix, with respect to GM alone. The biological tests confirmed the onset of MSCs differentiation process starting from 14 days of in vitro culture. In particular, the combination GM + BC resulted to be more effective than GM + CS in the up-regulation of key genes such as collagen type 2A1 (COLII), SOX-9, and aggrecan). In addition, the scanning microscope analyses revealed the cellular adhesion on materials and production of extracellular vesicles. Immunofluorescence staining confirmed an increase of COLII in presence of both chondroitins. Finally, the outcomes suggest that BC entangled within cross-linked GM matrix may represent a promising new biomaterial with potential applications in cartilage regeneration.

Cell-derived and enzyme-based decellularized extracellular matrix exhibit compositional and structural differences that are relevant for its use as a biomaterial.

Due to its availability and minimal invasive harvesting human adipose tissue-derived extracellular matrix (dECM) is often used as a biomaterial in various tissue engineering and healthcare applications. Next to dECM, cell-derived ECM (cdECM) can be generated by and isolated from in vitro cultured cells. So far both types of ECM were investigated extensively toward their application as (bio)material in tissue engineering and healthcare. However, a systematic characterization and comparison of soft tissue dECM and cdECM is still missing. In this study, we characterized dECM from human adipose tissue, as well as cdECM from human adipose-derived stem cells, toward their molecular composition, structural characteristics, and biological purity. The dECM was found to exhibit higher levels of collagens and lower levels of sulfated glycosaminoglycans compared with cdECMs. Structural characteristics revealed an immature state of the fibrous part of cdECM samples. By the identified differences, we aim to support researchers in the selection of a suitable ECM-based biomaterial for their specific application and the interpretation of obtained results.

An Advanced 'clickECM' That Can be Modified by the Inverse-Electron-Demand Diels-Alder Reaction.

The extracellular matrix (ECM) represents the natural environment of cells in tissue and therefore is a promising biomaterial in a variety of applications. Depending on the purpose, it is necessary to equip the ECM with specific addressable functional groups for further modification with bioactive molecules, for controllable cross-linking and/or covalent binding to surfaces. Metabolic glycoengineering (MGE) enables the specific modification of the ECM with such functional groups without affecting the native structure of the ECM. In a previous approach (S. M. Ruff, S. Keller, D. E. Wieland, V. Wittmann, G. E. M. Tovar, M. Bach, P. J. Kluger, Acta Biomater. 2017, 52, 159-170), we demonstrated the modification of an ECM with azido groups, which can be addressed by bioorthogonal copper-catalyzed azide-alkyne cycloaddition (CuAAC). Here, we demonstrate the modification of an ECM with dienophiles (terminal alkenes, cyclopropene), which can be addressed by an inverse-electron-demand Diels-Alder (IEDDA) reaction. This reaction is cell friendly as there are no cytotoxic catalysts needed. We show the equipment of the ECM with a bioactive molecule (enzyme) and prove that the functional groups do not influence cellular behavior. Thus, this new material has great potential for use as a biomaterial, which can be individually modified in a wide range of applications.

Differentiation of physical and chemical cross-linking in gelatin methacryloyl hydrogels.

Gelatin methacryloyl (GM) hydrogels have been investigated for almost 20 years, especially for biomedical applications. Recently, strengthening effects of a sequential cross-linking procedure, whereby GM hydrogel precursor solutions are cooled before chemical cross-linking, were reported. It was hypothesized that physical and enhanced chemical cross-linking of the GM hydrogels contribute to the observed strengthening effects. However, a detailed investigation is missing so far. In this contribution, we aimed to reveal the impact of physical and chemical cross-linking on strengthening of sequentially cross-linked GM and gelatin methacryloyl acetyl (GMA) hydrogels. We investigated physical and chemical cross-linking of three different GM(A) derivatives (GM10, GM2A8 and GM2), which provided systematically varied ratios of side-group modifications. GM10 contained the highest methacryloylation degree (DM), reducing its ability to cross-link physically. GM2 had the lowest DM and showed physical cross-linking. The total modification degree, determining the physical cross-linking ability, of GM2A8 was comparable to that of GM10, but the chemical cross-linking ability was comparable to GM2. At first, we measured the double bond conversion (DBC) kinetics during chemical GM(A) cross-linking quantitatively in real-time via near infrared spectroscopy-photorheology and showed that the DBC decreased due to sequential cross-linking. Furthermore, results of circular dichroism spectroscopy and differential scanning calorimetry indicated gelation and conformation changes, which increased storage moduli of all GM(A) hydrogels due to sequential cross-linking. The data suggested that the total cross-link density determines hydrogel stiffness, regardless of the physical or chemical nature of the cross-links.

Tailoring and visualising pore openings in gelatin-based hydrogel foams.

HYPOTHESIS: While tailoring the pore diameters in hydrogel foams has been demonstrated in numerous studies, fine control over the diameters of the pore openings is still a challenge. We hypothesise that this can be achieved by controlling the size of the thin films which separate the bubbles in the liquid foam template. If this is the case, systematic changes of the template's gas fraction ϕ (the higher ϕ, the larger are the thin films) will lead to corresponding changes of the pore opening diameter. EXPERIMENTS: Since the size of the thin films depends on both bubble size 〈Db〉 and gas fraction ϕ, we need to decouple both parameters to control the film size. Thus, we generated foams with constant bubble sizes via microfluidics and adjusted the gas fractions via two different techniques. The foams were solidified using UV light. Subsequently, they were analysed with confocal fluorescence microscopy. FINDINGS: We were able to change the pore opening diameter 〈dp〉 at a constant pore diameter 〈Dp〉 by adjusting the gas fraction of the foam template. The obtained 〈dp〉/〈Dp〉 ratios are between those obtained theoretically for disordered foams and FCC ordered foams, respectively.

Azido-functionalized gelatin via direct conversion of lysine amino groups by diazo transfer as a building block for biofunctional hydrogels.

Gelatin is one of the most prominent biopolymers in biomedical material research and development. It is frequently used in hybrid hydrogels, which combine the advantageous properties of bio-based and synthetic polymers. To prevent the biological component from leaching out of the hydrogel, the biomolecules can be equipped with azides. Those groups can be used to immobilize gelatin covalently in hydrogels by the highly selective and specific azide-alkyne cycloaddition. In this contribution, we functionalized gelatin with azides at its lysine residues by diazo transfer, which offers the great advantage of only minimal side-chain extension. Approximately 84-90% of the amino groups are modified as shown by 1 H-NMR spectroscopy, 2,4,6-trinitrobenzenesulfonic acid assay as well as Fourier-transform infrared spectroscopy, rheology, and the determination of the isoelectric point. Furthermore, the azido-functional gelatin is incorporated into hydrogels based on poly(ethylene glycol) diacrylate (PEG-DA) at different concentrations (0.6, 3.0, and 5.5%). All hydrogels were classified as noncyctotoxic with significantly enhanced cell adhesion of human fibroblasts on their surfaces compared to pure PEG-DA hydrogels. Thus, the new gelatin derivative is found to be a very promising building block for tailoring the bioactivity of materials.

Precision 3D-Printed Cell Scaffolds Mimicking Native Tissue Composition and Mechanics.

Cellular dynamics are modeled by the 3D architecture and mechanics of the extracellular matrix (ECM) and vice versa. These bidirectional cell-ECM interactions are the basis for all vital tissues, many of which have been investigated in 2D environments over the last decades. Experimental approaches to mimic in vivo cell niches in 3D with the highest biological conformity and resolution can enable new insights into these cell-ECM interactions including proliferation, differentiation, migration, and invasion assays. Here, two-photon stereolithography is adopted to print up to mm-sized high-precision 3D cell scaffolds at micrometer resolution with defined mechanical properties from protein-based resins, such as bovine serum albumin or gelatin methacryloyl. By modifying the manufacturing process including two-pass printing or post-print crosslinking, high precision scaffolds with varying Young's moduli ranging from 7-300 kPa are printed and quantified through atomic force microscopy. The impact of varying scaffold topographies on the dynamics of colonizing cells is observed using mouse myoblast cells and a 3D-lung microtissue replica colonized with primary human lung fibroblast. This approach will allow for a systematic investigation of single-cell and tissue dynamics in response to defined mechanical and bio-molecular cues and is ultimately scalable to full organs.

Azide-Functional Extracellular Matrix Coatings as a Bioactive Platform for Bioconjugation.

In recent years, the development and application of decellularized extracellular matrices (ECMs) for use as biomaterials have grown rapidly. These cell-derived matrices (CDMs) represent highly bioactive and biocompatible materials consisting of a complex assembly of biomolecules. Even though CDMs mimic the natural microenvironment of cells in vivo very closely, they still lack specifically addressable functional groups, which are often required to tailor a biomaterial functionality by bioconjugation. To overcome this limitation, metabolic glycoengineering has emerged as a powerful tool to equip CDMs with chemical groups such as azides. These small chemical handles are known for their ability to undergo bioorthogonal click reactions, which represent a desirable reaction type for bioconjugation. However, ECM insolubility makes its processing very challenging. In this contribution, we isolated both the unmodified ECM and azide-modified clickECM by osmotic lysis. In a first step, these matrices were concentrated to remove excessive water from the decellularization step. Next, the hydrogel-like ECM and clickECM films were mechanically fragmentized, resulting in easy to pipette suspensions with fragment sizes ranging from 7.62 to 31.29 µm (as indicated by the mean d90 and d10 values). The biomolecular composition was not impaired as proven by immunohistochemistry. The suspensions were used for the reproducible generation of surface coatings, which proved to be homogeneous in terms of ECM fragment sizes and coating thicknesses (the mean coating thickness was found to be 33.2 ± 7.3 µm). Furthermore, they were stable against fluid-mechanical abrasion in a laminar flow cell. When primary human fibroblasts were cultured on the coated substrates, an increased bioactivity was observed. By conjugating the azides within the clickECM coatings with alkyne-coupled biotin molecules, a bioconjugation platform was obtained, where the biotin-streptavidin interaction could be used. Its applicability was demonstrated by equipping the bioactive clickECM coatings with horseradish peroxidase as a model enzyme.

Eclectic characterisation of chemically modified cell-derived matrices obtained by metabolic glycoengineering and re-assessment of commonly used methods.

Azide-bearing cell-derived extracellular matrices ("clickECMs") have emerged as a highly exciting new class of biomaterials. They conserve substantial characteristics of the natural extracellular matrix (ECM) and offer simultaneously small abiotic functional groups that enable bioorthogonal bioconjugation reactions. Despite their attractiveness, investigation of their biomolecular composition is very challenging due to the insoluble and highly complex nature of cell-derived matrices (CDMs). Yet, thorough qualitative and quantitative analysis of the overall material composition, organisation, localisation, and distribution of typical ECM-specific biomolecules is essential for consistent advancement of CDMs and the understanding of the prospective functions of the developed biomaterial. In this study, we evaluated frequently used methods for the analysis of complex CDMs. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and (immune)histochemical staining methods in combination with several microscopic techniques were found to be highly eligible. Commercially available colorimetric protein assays turned out to deliver inaccurate information on CDMs. In contrast, we determined the nitrogen content of CDMs by elementary analysis and converted it into total protein content using conversion factors which were calculated from matching amino acid compositions. The amount of insoluble collagens was assessed based on the hydroxyproline content. The Sircol™ assay was identified as a suitable method to quantify soluble collagens while the Blyscan™ assay was found to be well-suited for the quantification of sulphated glycosaminoglycans (sGAGs). Eventually, we propose a series of suitable methods to reliably characterise the biomolecular composition of fibroblast-derived clickECM.

Highly Ordered Gelatin Methacryloyl Hydrogel Foams with Tunable Pore Size.

In this study, we present a fast and convenient liquid foam templating route to generate gelatin methacryloyl (GM) foams. Microfluidic bubbling was used to generate monodisperse liquid foams with bubble sizes ranging from 220 to 390 µm. The continuous phase contained 20 wt % GM and 0.7 wt % lithium phenyl-2,4,6-trimethylbenzoylphosphinate as photoinitiator. Gelation was achieved via UV-initiated radical cross-linking of GM. After cross-linking, the hydrogel foams were either swollen in water or freeze-dried. The pore sizes of the dry foams were 15-20% smaller than the bubble sizes of the liquid templates, whereas the pore sizes of the swollen porous hydrogels were in the range of the bubble sizes of the liquid templates. Compared to commonly used methods for the fabrication of biopolymer scaffolds, our route neither involves cryogenic treatment nor toxic chemicals or organic solvents and potentially allows for the photoencapsulation of cells.

Hydrophobization of Tobacco Mosaic Virus to Control the Mineralization of Organic Templates.

The robust, anisotropic tobacco mosaic virus (TMV) provides a monodisperse particle size and defined surface chemistry. Owing to these properties, it became an excellent bio-template for the synthesis of diverse nanostructured organic/inorganic functional materials. For selective mineralization of the bio-template, specific functional groups were introduced by means of different genetically encoded amino acids or peptide sequences into the polar virus surface. An alternative approach for TMV surface functionalization is chemical coupling of organic molecules. To achieve mineralization control in this work, we developed a synthetic strategy to manipulate the surface hydrophilicity of the virus through covalent coupling of polymer molecules. Three different types of polymers, namely the perfluorinated (poly(pentafluorostyrene) (PFS)), the thermo-responsive poly(propylene glycol) acrylate (PPGA), and the block-copolymer polyethylene-block-poly(ethylene glycol) were examined. We have demonstrated that covalent attachment of hydrophobic polymer molecules with proper features retains the integrity of the virus structure. In addition, it was found that the degree of the virus hydrophobicity, examined via a ZnS mineralization test, could be tuned by the polymer properties.

Physical Interactions Strengthen Chemical Gelatin Methacryloyl Gels.

Chemically cross-linkable gelatin methacryloyl (GM) derivatives are getting increasing attention regarding biomedical applications. Thus, thorough investigations are needed to achieve full understanding and control of the physico-chemical behavior of these promising biomaterials. We previously introduced gelatin methacryloyl acetyl (GMA) derivatives, which can be used to control physical network formation (solution viscosity, sol-gel transition) independently from chemical cross-linking by variation of the methacryloyl-to-acetyl ratio. It is known that temperature dependent physical network formation significantly influences the mechanical properties of chemically cross-linked GM hydrogels. We investigated the temperature sensitivity of GM derivatives with different degrees of modification (GM2, GM10), or similar degrees of modification but different methacryloyl contents (GM10, GM2A8). Rheological analysis showed that the low modified GM2 forms strong physical gels upon cooling while GM10 and GM2A8 form soft or no gels. Yet, compression testing revealed that all photo cross-linked GM(A) hydrogels were stronger if cooling was applied during hydrogel preparation. We suggest that the hydrophobic methacryloyl and acetyl residues disturb triple helix formation with increasing degree of modification, but additionally form hydrophobic structures, which facilitate chemical cross-linking.

Plant virus-based materials for biomedical applications: Trends and prospects.

Nanomaterials composed of plant viral components are finding their way into medical technology and health care, as they offer singular properties. Precisely shaped, tailored virus nanoparticles (VNPs) with multivalent protein surfaces are efficiently loaded with functional compounds such as contrast agents and drugs, and serve as carrier templates and targeting vehicles displaying e.g. peptides and synthetic molecules. Multiple modifications enable uses including vaccination, biosensing, tissue engineering, intravital delivery and theranostics. Novel concepts exploit self-organization capacities of viral building blocks into hierarchical 2D and 3D structures, and their conversion into biocompatible, biodegradable units. High yields of VNPs and proteins can be harvested from plants after a few days so that various products have reached or are close to commercialization. The article delineates potentials and limitations of biomedical plant VNP uses, integrating perspectives of chemistry, biomaterials sciences, molecular plant virology and process engineering.

Beyond the Modification Degree: Impact of Raw Material on Physicochemical Properties of Gelatin Type A and Type B Methacryloyls.

Gelatin methacryloyl (acetyl) (GM(A)) is increasingly investigated for various applications in life sciences and medicine, for example, drug release or tissue engineering. Gelatin type A and type B are utilized for GA M(A) and GB M(A) preparation, but the impact of gelatin raw material on modification reaction and resulting polymer properties is rather unknown so far. Therefore, the degrees of modification (DMA) and physicochemical properties of five GA M(A) and GB M(A) derivatives are compared: The degrees of methacryloylation (0.32-0.98 mmol g-1 ) are indistinguishable for GA M(A) and GB M(A) as are the sol-gel temperatures. Isoelectric points, solution viscosities, and hydrodynamic radii which are distinct for GA and GB, converge with increasing DMA. Interestingly, differences are measured for the storage moduli and equilibrium degrees of swelling of respective GA and GB derivative-based hydrogels, in spite of their comparable DMA. This underlines the importance of GM(A) characterization beyond the modification degree.

Photoinduced Cleavage and Hydrolysis of o-Nitrobenzyl Linker and Covalent Linker Immobilization in Gelatin Methacryloyl Hydrogels.

Light-induced release systems can be triggered remotely and are of interest for many controlled release applications due to the possibility for spatio-temporal release control. In this study a biotin-functionalized photocleavable macromer is incorporated with an o-nitrobenzyl moiety into gelatin methacryloyl(-acetyl) hydrogels via radical cross-linking. Stronger immobilization of streptavidin-coupled horseradish peroxidase occurs in linker-functionalized hydrogels compared to pure gelatin methacryloyl(-acetyl) hydrogels, and a controlled release of the streptavidin conjugate upon UV-irradiation is possible. Liquid chromatography coupled to mass spectrometry (LC-MS) analysis of aqueous linker solutions allows the identification of the main cleavage products and the cleavage kinetics. Thus, it is shown that a significant hydrolysis of the linker occurs at 37 °C. Nevertheless the system reported here is a promising controlled release scaffold for proteins and application in tissue engineering, if background releases of the immobilized drug are tolerable.