Optical glucose sensor for microfluidic cell culture systems.

Glucose is the primary energy source of human cells. Therefore, monitoring glucose inside microphysiological systems (MPS) provides valuable information on the viability and metabolic state of the cultured cells. However, continuous glucose monitoring inside MPS is challenging due to a lack of suitable miniaturized sensors. Here we present an enzymatic, optical glucose sensor element for measurement inside microfluidic systems. The miniaturized glucose sensor (Ø 1 mm) is fabricated together with a reference oxygen sensor onto biocompatible, pressure-sensitive adhesive tape for easy integration inside microfluidic systems. Furthermore, the proposed microfluidic system can be used as plug and play sensor system with existing MPS. It was characterized under cell culture conditions (37 °C and pH 7.4) for five days, exhibiting minor drift (3% day-1). The influence of further cell culture parameters like oxygen concentration, pH, flow rate, and sterilization methods was investigated. The plug-and-play system was used for at-line measurements of glucose levels in (static) cell culture and achieved good agreement with a commercially available glucose sensor. In conclusion, we developed an optical glucose sensor element that can be easily integrated in microfluidic systems and is able to perform stable glucose measurements under cell culture conditions.

Advanced Online Monitoring of In Vitro Human 3D Full-Thickness Skin Equivalents.

Skin equivalents and skin explants are widely used for dermal penetration studies in the pharmacological development of drugs. Environmental parameters, such as the incubation and culture conditions affect cellular responses and thus the relevance of the experimental outcome. However, available systems such as the Franz diffusion chamber, only measure in the receiving culture medium, rather than assessing the actual conditions for cells in the tissue. We developed a sampling design that combines open flow microperfusion (OFM) sampling technology for continuous concentration measurements directly in the tissue with microfluidic biosensors for online monitoring of culture parameters. We tested our design with real-time measurements of oxygen, glucose, lactate, and pH in full-thickness skin equivalent and skin explants. Furthermore, we compared dermal penetration for acyclovir, lidocaine, and diclofenac in skin equivalents and skin explants. We observed differences in oxygen, glucose, and drug concentrations in skin equivalents compared to the respective culture medium and to skin explants.

Innovative Platform for the Advanced Online Monitoring of Three-Dimensional Cells and Tissue Cultures.

The use of 3D cell cultures has gained increasing importance in medical and pharmaceutical research. However, the analysis of the culture medium is hardly representative for the culture conditions within a 3D model which hinders the standardization of 3D cultures and translation of results. Therefore, we developed a modular monitoring platform combining a perfusion bioreactor with an integrated minimally invasive sampling system and implemented sensors that enables the online monitoring of culture parameters and medium compounds within 3D cultures. As a proof-of-concept, primary cells as well as cell lines were cultured on a collagen or gelatin methacryloyl (GelMA) hydrogel matrix, while monitoring relevant culture parameters and analytes. Comparing the interstitial fluid of the 3D models versus the corresponding culture medium, we found considerable differences in the concentrations of several analytes. These results clearly demonstrate that analyses of the culture medium only are not relevant for the development of standardized 3D culture processes. The presented bioreactor with an integrated sampling and sensor platform opens new horizons for the development, optimization, and standardization of 3D cultures. Furthermore, this technology holds the potential to reduce animal studies and improve the transferability of pharmaceutical in vitro studies by gaining more relevant results, bridging the gap towards clinical translation.

Insulin induces a progressive increase in the resistance of subcutaneous tissue to fluid flow: Implications for insulin pump therapy.

AIM: To determine the effect of insulin on the resistance of subcutaneous tissue to the flow of infusion fluids. MATERIALS AND METHODS: Thirty subjects with type 1 diabetes wore two Accu-Chek Spirit Combo insulin pumps with Accu-Chek FlexLink infusion sets (Roche Diabetes Care, Mannheim, Germany) for 7 days. One pump was filled with insulin aspart (Novo Nordisk, Bagsvaerd, Denmark) and used for continuous subcutaneous insulin infusion (CSII). The other pump was filled with insulin diluting medium (IDM; Novo Nordisk) and used to deliver IDM subcutaneously at rates identical to those employed for CSII. Both infusion sites were assessed daily by measuring the pressure required to infuse various bolus amounts of IDM. RESULTS: On day 1, maximum pressure (Pmax ) and tissue flow resistance (TFR; calculated from measured pressure profiles) were similar for both infusion sites (P > 0.20). During the subsequent study days, the Pmax and TFR values observed at the IDM infusion site remained at levels comparable to those seen on day 1 (P > 0.13). However, at the site of CSII, Pmax and TFR progressively increased with CSII duration. By the end of day 7, Pmax and TFR reached 25.8 */2.11 kPa (geometric mean */geometric standard deviation) and 8.64 */3.48 kPa*s/µL, respectively, representing a remarkable 3.5- and 20.6-fold increase relative to the respective Pmax and TFR values observed on day 1 (P < 0.001). CONCLUSION: Our results suggest that insulin induces a progressive increase in the resistance of subcutaneous tissue to the introduction of fluid; this has important implications for the future design of insulin pumps and infusion sets.

3D Printing of Cell Culture Devices: Assessment and Prevention of the Cytotoxicity of Photopolymers for Stereolithography.

3D printing is increasingly important for the rapid prototyping of advanced and tailor-made cell culture devices. In this context, stereolithography represents a method for the rapid generation of prototypes from photocurable polymers. However, the biocompatibility of commercially available photopolymers is largely unknown. Therefore, we evaluated the cytotoxicity of six polymers, two of them certified as biocompatible according to ISO 10993-5:2009, and we evaluated, if coating with Parylene, an inert polymer widely used in medical applications, might shield cells from the cytotoxic effects of a toxic polymer. In addition, we evaluated the processability, reliability, and consistency of the details printed. Human mesenchymal stem cells (MSCs) were used for cytotoxicity testing as they are widely used and promising for numerous applications in regenerative medicine. MSCs were incubated together with printed photopolymers, and the cytotoxicity was assessed. All photopolymers significantly reduced the viability of MSCs while the officially biocompatible resins displayed minor toxic effects. Further, coating with Parylene completely protected MSCs from toxic effects. In conclusion, none of the tested polymers can be fully recommended for rapid prototyping of cell culture devices. However, coating with Parylene can shield cells from toxic effects and thus might represent a viable option until more compatible materials are available.

Recirculation--a novel approach to quantify interstitial analytes in living tissue by combining a sensor with open-flow microperfusion.

We report a novel approach to quantify interstitial analytes in living tissue by combining open-flow microperfusion (OFM) with a sensor and the re-circulation method. OFM is based on the unrestricted exchange of molecules between the interstitial fluid (ISF) and a perfusion medium through macroscopic perforations that enables direct access to the ISF. By re-circulating the perfusate and monitoring the changes of the analytes' concentration with a sensor, the absolute analyte concentration in the ISF can be calculated. In order to validate the new concept, the absolute electrical conductivity of the ISF was identified in six subjects to be 1.33 ± 0.08 S/m (coefficient of variation CV = 6 %), showing the robustness of this approach. The most striking feature of this procedure is the possibility to monitor several compounds simultaneously by applying different sensors which will allow not only the determination of the concentration of a single substance in vivo but also the simultaneous dynamics of different analytes. This will open new fields in analytical chemistry, pharmacology, as well as clinical experimental research.

Dermal PK/PD of a lipophilic topical drug in psoriatic patients by continuous intradermal membrane-free sampling.

BACKGROUND: Methodologies for continuous sampling of lipophilic drugs and high-molecular solutes in the dermis are currently lacking. We investigated the feasibility of sampling a lipophilic topical drug and the locally released biomarker in the dermis of non-lesional and lesional skin of psoriatic patients over 25h by means of membrane-free dermal open-flow microperfusion probes (dOFM) and novel wearable multi-channel pumps. METHODS: Nine psoriatic patients received a topical p-38 inhibitor (BCT194, 0.5% cream) on a lesional and a non-lesional application site once daily for 8 days. Multiple dOFM sampling was performed for 25 h from each site on day 1 and day 8. Patients were mobile as dOFM probes were operated by a novel light-weight push-pull pump. Ultrasound was used to verify intradermal probe placement, cap-LC-MS/MS for BCT194 and ELISA for TNFα analysis. RESULTS: dOFM was well tolerated and demonstrated significant drug concentrations in lesional as well as non-lesional skin after 8 days, but did not show significant differences between tissues. On day 8, TNFα release following probe insertion was significantly reduced compared to day 1. CONCLUSIONS: Novel membrane-free probes and wearable multi-channel pumps allowed prolonged intradermal PK/PD profiling of a lipophilic topical drug in psoriatic patients. This initial study shows that dOFM overcomes limitations of microdialysis sampling methodology, and it demonstrates the potential for PK/PD studies of topical products and formulations in a clinical setting.