Extended Data Fig. 2: Evaluation of MNP application with different applicators and MNP architectures.

(a) MNPs were applied to ex vivo pig skin by hand (left), commercial (Micropoint, Shenzhen, China) (middle) and custom spring-loaded applicators (right). (b) NIR images of MNPs with two different needle tip angles show that bits are better transferred when applied with an applicator than by hand. (c) Histological imaging of pig skin where MNPs did not penetrate more than 250 μm in depth, with hand application, leaving most of the dyes deposited near the epidermis layer, Imaging was performed >30 times. (d) In vivo NIR images of shallowly applied MNPs on day 0 and day 36 post-application in pigs demonstrated a dramatic decrease after 1 month, leading to the assumption that they are shed off with the top layer of the epidermis and that a deeper deposition of the dye can lead to a longer NIR signal durability. (e) The Micropoint and five custom-designed spring-loaded applicators with tunable impact velocities and holding pressures were tested for MNPs with two different tip angles. (f) Applicators were tested on pig skin (left). After the application, tissue was fixed in formalin and embedded in paraffin for cross-sectional evaluation of the maximum needle penetration and dye deposition depths (middle). Furthermore, parts of the skin tissue were frozen and fixed in Optimal Cutting Temperature compound for cross-sectional imaging to detect the presence of the NIR bits in the dermis, showing penetration of a 10-needle array (right). (g) NIR bit transfer and needle dissolution results for 2 different needle tip angles and for different application parameters (needles have 1.5 mm height, 0.4 mm base and 1 mm pitch), n = >4, biological, S.D. (h) Four different microneedle tip angles were tested for MNP architecture optimization, n = >5, biological. (i) Four different pitches, 0.5 mm, 1 mm, 1.5 mm, and 3 mm, were tested for MNP architecture optimization. (j) For needles with 8° tip angle, the dye does not reach the very ends of the needle tips (pointed out with yellow arrows), and the needles are more prone to breakage upon removal from the PDMS negative mold because of their thin structures at the tips (pointed out with blue arrows). (k) To assess the mechanical robustness of the MNP upon skin penetration, we performed mechanical compression tests on 10×10 patterned MNPs (n = 5) using Instron 5943 (Norwood, MA). Microneedles must pierce the stratum corneum without rupturing or bending for proper skin penetration (https://link.springer.com/article/10.1007/s40820-021-00611-9). The pressure required to puncture human skin is known to be roughly 100 psi, which is equivalent to 0.689 MPa (https://pubmed.ncbi.nlm.nih.gov/1757138/). Therefore, the minimum force required to puncture human skin with our patterned MNP (roughly 50 microneedles with the needle base dimension of 400 µm x 400 µm) is 5.512 N (Eq. 1), which means one microneedle patch needs to endure minimum of 5.512 N of compression force to pierce human skin. F_min = P x A_max = (6.89 × 10⁵ N/m²) * (8 *10⁻⁶ m²) = 5.512 N (Eq. 1). With Instron 5943, the microneedle patches were compressed at a rate of 5 mm/min, and the maximum load, load at break, and Young’s modulus were measured with Instron static load cell (±500 N) and Instron Bluehill 3 software. For all patches, the compression force measurements reached the upper limit of the load cell (500 N) before the platens reached the base of the needles, indicating that our microneedle patch can endure more than 500 N, easily exceeding the minimum force to endure for human skin penetration.