## 1 Introduction  
Transformation in the case of bacterial cells and--even more importantly--transfection of higher cells including human cell lines in medical research are both key technologies in molecular biology and genetic engineering [1, 2]. Transfection is a procedure to deliver genetic material (either recombinant DNA constructs or RNA molecules) to living cells in a functional form, i.e., it allows for replication, expression of target/marker genes, and access to regulatory factors in the cell [3, 4, 5]. An impressive range of different techniques has been invented in the last 50 years (nicely reviewed in [6, 7]), which can be summarized as belonging to (i) biological methods including virus-mediated gene transfer [8, 9, 10], (ii) physical methods (e.g., direct injection, electroporation, or biolistic particle delivery [11, 12, 13]), and (iii) chemical methods, which have developed into the best established standard techniques [14, 15, 16]. Although nucleic acid-based constructs appear to be promising structures for the manipulation and optimization of cellular functions (as single molecules or higher assembled complexes), naked nucleic acids have an intrinsic limitation in that they are prone to rapid degradation in vivo and their negative charges inhibit efficient binding to cell surfaces and their required subsequent passage through the cell membrane [17]. Hence, protective and efficient facilitators of nucleic acid uptake are generally needed [18; 19; 20; 21]. Such carriers based on liposome-forming cationic lipids have been developed as generally used assemblies of complex nucleic acids prior to their transfective uptake into living cells [22; 23]. Among them, defined mixtures of the lipids DOSPA (2,3-dioleoyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propanaminium trifluoroacetate) and DOPE (1,2-di-(9Z-octadecenoyl)-sn-glycero-3-phosphoethanolamine) commercialized under the name Lipofectamine [24] can be regarded as a \"gold standard\" with mentions in currently over 3580 scientific publications listed in the NCBI PubMed literature database (accessed 16 May 2024). In the background of the complex physiology of mammalian and human cells with different susceptibilities for uptake of foreign nucleic acids (i.e., mediated uptake and intracellular processing of the foreign DNA), efficiencies of such manipulations can be expected to be very different. Depending on the cell line used or the growth conditions applied in the experiment prior to and after transfection, Lipofectamine has received recognition for its broad spectrum of cell lines accessible by this agent [25]. Nevertheless, its cytotoxicity appeared to be considerable [26]. Protein hydrogels based on crosslinked serum albumin (human or bovine; \"HSA\" or \"BSA\") have been developed for applications as different as matrices for 3D cell culture, as functional layers in novel composite wound dressings and as novel passaging tools in cell culture. Moreover, they have proven their low cytotoxicity in these applications [27; 28; 29; 30; 31; 32; 33]. From these materials, nanometer-sized spheres (\"nanogels\" (Ng)) can be fabricated in an inverse-nanoemulsion synthesis technique that we have recently introduced [34]. The nanogels were shown in initial experiments to be suitable for delivering fluorescent dyes, model drug molecules, aptamers, and plasmid DNA to selected human cell lines [34]. Delivery of the GFP-encoding plasmid pEGFP-c3 was demonstrated only with a single breast cancer cell line (MDA-MB-231 (ATCC CRM-HTB-26) [35]), suggesting a principal capability of the nanogels to serve as a transfection agent [34], and we tentatively suggest the nomination \"NanoTrans-gels\" for these functional nanostructures. Here, we present additional evidence for the functionality of nanogel-mediated transfection with the breast cancer cell line MCF7 (ATCC HTB-22) [36; 37] and the lung adenocarcinoma cell line A549 (ATCC CRM-CCL-185) [38; 39] as probably two of the best described and established model human cell lines [40; 41].