Passive Diffusion of Transdermal Glucose: Noninvasive Glucose Sensing Using a Fluorescent Glucose Binding Protein

Development of skin sebum medium and inhibition of lipase activity in Cutibacterium acnes by oleic acid

Cutibacterium acnes is associated with the exacerbated inflammation of acne vulgaris, which occurs through the immune induction and pathogenicity factor production. Sebum, which is not present in the growth medium currently used to study acne, is present in acne pustules in differing concentrations among the pathological stages, such as the initial formation and inflammatory phase. To evaluate the effect of C. acnes on inflammation exacerbation in acne pustules in vitro, we developed an skin sebum medium containing artificial sebum and studied the growth and pathogenicity factor production of C. acnes in the skin sebum medium.

The growth and lipase activity of C. acnes ATCC11828 were tested using skin sebum medium containing different sebum concentrations. Only lipase activity decreased in the skin sebum medium culture containing 0.5 % sebum when compared with that without sebum, while both growth and lipase activity decreased in cultures with 1.0 % sebum. Therefore, the growth and lipase activity of C. acnes changed in the presence of sebum. Furthermore, when the growth and lipase activity of C. acnes were tested in skin sebum medium containing sebum components, unsaturated fatty acids, such as oleic acid and triolein, led to a decrease in lipase activity without inducing a change in growth. In the presence of oleic acid, C. acnes lipase activity decreased noncompetitively in a concentration-dependent manner.

Our data showed that C. acnes growth and lipase activity changed upon sebum addition to the skin sebum medium, and acne inflammation caused by C. acnes needs to be studied under conditions similar to those in acne pustules.

Transdermal sensing: in-situ non-invasive techniques for monitoring of human biochemical status

Improving life expectancy necessitates prevention and early diagnosis of any disease state based on active self-monitoring of symptoms and longitudinal biochemical profiling. Non-invasive and continuous measurement of molecular biomarkers that reflect metabolism and health must however be established to realize this plan. Human samples non-invasively obtained via the skin are suitable in this context for in-situ biochemical monitoring. We present a brief classification of transdermal sampling in aqueous and gaseous phases and then introduce a new generation of transdermal monitoring devices for rapid and accurate assessment of important parameters. Finally, we have summarized the diversity of body-wide skin characteristics that have possible effects for transdermal sampling. Because of its passive nature, in-situ biochemical monitoring via transdermal sampling will potentially lead to a greater understanding of important biochemical markers and their temporal variation.

Noninvasive Epidermal Metabolite Profiling

A buffer placed in brief contact in the skin was assayed by ¹H NMR spectroscopy. We found that this passive extraction of the skin surface yields abundant metabolites. Metabolites of the skin surface originate from a variety of sources, including the sweat gland, which produces lactate from the glucose received from its capillary bed. Little is known about how metabolites resident on and within the skin surface respond to a metabolic or hemodynamic perturbation. As a possible application of epidermal metabolite profiling, we asked whether metabolites extracted from the skin surface are indicative of heart failure. The levels of lactate and other molecules were significantly lower in patients in heart failure than in individuals who reported healthy heart function, possibly due to reduced blood flow to the sweat gland resulting in a lack of tissue perfusion. Most amino acids were unchanged in levels, except for glycine and serine that increased as a percentage of all amino acids. These results have the potential in the long term to help decide the extent to which a patient has heart failure for which objective measures are lacking. Moreover, the results suggest that epidermal metabolite profiling may be useful for other assessments of human health.

Minimally invasive technique for measuring transdermal glucose with a fluorescent biosensor

There is a need for blood glucose monitoring techniques that eliminate the painful and invasive nature of current methods, while maintaining the reliability and accuracy of established medical technology. This research aims to ultimately address these shortcomings in critically ill pediatric patients. Presented in this work is an alternative, minimally invasive technique that uses microneedles (MN) for the collection of transdermal glucose (TG). Due to their comparable skin properties, diffusion studies were performed on full thickness Yucatan miniature pig skin mounted to an in-line diffusion flow cell and on different skin sites of human subjects. Collected TG samples were measured with a L255C mutant of the E. coli glucose-binding protein (GBP) with an attached fluorescent probe. The binding constant (K_d = 0.67 μM) revealed the micromolar sensitivity and high selectivity of the his-tagged GBP biosensor for glucose, making it suitable for TG measurements. In both the animal and human models, skin permeability and TG diffusion across the skin increased with MN application. For intact and MN-treated human skin, a significant positive linear correlation (r > 0.95, p < 0.01) existed between TG and BG. The micromolar sensitivity of GBP minimized the volume required for interstitial fluid glucose analysis allowing MN application time (30 s) to be shortened compared to other studies. This time reduction can help in eliminating skin irritation issues and improving practical use of the technique by caregivers in the hospital. In addition, the his-tagged optical biosensor used in this work can be immobilized and used with a portable sensing fluorometer device at the point of care (POC) making this minimally invasive technology more ideal for use in the pediatric intensive care unit. Graphical abstract ᅟ.

Epidermal Microfluidic Electrochemical Detection System: Enhanced Sweat Sampling and Metabolite Detection

Despite tremendous recent efforts, noninvasive sweat monitoring is still far from delivering its early analytical promise. Here, we describe a flexible epidermal microfluidic detection platform fabricated through hybridization of lithographic and screen-printed technologies, for efficient and fast sweat sampling and continuous, real-time electrochemical monitoring of glucose and lactate levels. This soft, skin-mounted device judiciously merges lab-on-a-chip and electrochemical detection technologies, integrated with a miniaturized flexible electronic board for real-time wireless data transmission to a mobile device. Modeling of the device design and sweat flow conditions allowed optimization of the sampling process and the microchannel layout for achieving attractive fluid dynamics and rapid filling of the detection reservoir (within 8 min from starting exercise). The wearable microdevice thus enabled efficient natural sweat pumping to the electrochemical detection chamber containing the enzyme-modified electrode transducers. The fabricated device can be easily mounted on the epidermis without hindrance to the wearer and displays resiliency against continuous mechanical deformation expected from such epidermal wear. Amperometric biosensing of lactate and glucose from the rapidly generated sweat, using the corresponding immobilized oxidase enzymes, was wirelessly monitored during cycling activity of different healthy subjects. This ability to monitor sweat glucose levels introduces new possibilities for effective diabetes management, while similar lactate monitoring paves the way for new wearable fitness applications. The new epidermal microfluidic electrochemical detection strategy represents an attractive alternative to recently reported colorimetric sweat-monitoring methods, and hence holds considerable promise for practical fitness or health monitoring applications.

Measuring transdermal glucose levels in neonates by passive diffusion: an in vitro porcine skin model

Current glucose monitoring techniques for neonates rely heavily on blood glucose monitors which require intermittent blood collection through skin-penetrating pricks on the heel or fingers. This procedure is painful and often not clinically conducive, which presents a need for a noninvasive method for monitoring glucose in neonates. Our motivation for this study was to develop an in vitro method for measuring passive diffusion of glucose in premature neonatal skin using a porcine skin model. Such a model will allow us to initially test new devices for noninvasive glucose monitoring without having to do in vivo testing of newborns. The in vitro model is demonstrated by comparing uncompromised and tape-stripped skin in an in-line flow-through diffusion apparatus with glucose concentrations that mimic the hypo-, normo-, and hyper-glycemic conditions in the neonate (2.0, 5.0, and 20 mM, respectively). Transepidermal water loss (TEWL) of the tape-stripped skin was approximately 20 g m^-2 h^-1, which closely mimics TEWL for neonatal skin at about 190 days post-conceptional age. The tape-stripped skin showed a >15-fold increase in glucose diffusion compared to the uncompromised skin. The very small concentrations of collected glucose were measured with a highly selective and highly sensitive fluorescent glucose biosensor based on the glucose binding protein (GBP). The demonstrated method of glucose determination is noninvasive and painless, which makes it especially desirable for glucose testing in neonates and children. This study is an important step towards an in vitro model for noninvasive real-time glucose monitoring that may be easily transferred to the clinic for glucose monitoring in neonates. Graphical Abstract Glucose diffusion through model skin was measured using an in-line flow-through diffusion apparatus with glucose solutions mimicking hypo-, normo- and hyperglycemia in the neonate. Phosphate buffered saline was added to the top chamber and the glucose that diffused through the model skin into the buffer was measured using a fluorescent glucose binding protein biosensor.

The effect of pH on the glucose response of the glucose-galactose binding protein L255C labeled with Acrylodan

The glucose-galactose binding protein (GGBP) is used as an optical biosensor in medical and bioprocess applications. This paper investigates the effect of pH on the behavior of GGBP-L255C labeled with Acrylodan for the purpose of finding the optimum conditions for sensing purposes as well as for protein preparation, purification and storage. The Acrylodan-GGBP fluorescence response in absence and presence of glucose was measured under varying buffer and pH conditions. Dissociation constants (Kd) and Gibbs free energies (ΔG) for the protein-glucose binding were calculated. Binding was found to be energetically favored at slightly acidic to neutral conditions, specifically close to the pI of GBP (∼ 5.0). Minimal fluorescence response to glucose was exhibited at pH 3.0 accompanied by a blue shift in the steady state fluorescence spectrum. In contrast, an almost 45% response to glucose was shown at pH 4.5-9.0 with a 13-nm red shift. Frequency domain lifetime measurements and quenching with KI suggest that at highly acidic conditions both the glucose-free and the glucose-bound protein are in a conformation distinct from those observed at higher pH values.