This paper focuses on the mechanical properties of polydimethylsiloxane (PDMS) relevant for microelectromechanical system (MEMS) applications. In view of the limited amount of published data, we analyzed the two products most commonly used in MEMS, namely RTV 615 from Bayer Silicones and Sylgard 184 from Dow Corning. With regard to mechanical properties, we focused on the dependence of the elastic modulus on the thinner concentration, temperature and strain rate. In addition, creep and thermal aging were analyzed. We conclude that the isotropic and constant elastic modulus has strong dependence on the hardening conditions. At high hardening temperatures and long hardening time, RTV 615 displays an elastic modulus of 1.91 MPa and Sylgard 184 of 2.60 MPa in a range up to 40% strain.

SummaryThe discovery of induced pluripotent stem cells (iPSCs) and the concurrent development of protocols for their cell-type-specific differentiation have revolutionized our approach to cell therapy. It has now become critical to address the challenges related to the generation of iPSCs under current good manufacturing practice (cGMP) compliant conditions, including tissue sourcing, manufacturing, testing, and storage. Furthermore, regarding the technical challenges, it is very important to keep the costs of manufacturing and testing reasonable and solve logistic hurdles that permit the global distribution of these products. Here we describe our efforts to develop a process for the manufacturing of iPSC master cell banks (MCBs) under cGMPs and announce the availability of such banks.

Protein phosphatase 2A (PP2A) is an essential intracellular serine/threonine phosphatase containing a catalytic subunit that possesses the potential to dephosphorylate promiscuously tyrosine-phosphorylated substrates in vitro. How PP2A acquires its intracellular specificity and activity for serine/threonine-phosphorylated substrates is unknown. Here we report a novel and phylogenetically conserved mechanism to generate active phospho-serine/threonine-specific PP2A in vivo. Phosphotyrosyl phosphatase activator (PTPA), a protein of so far unknown intracellular function, is required for the biogenesis of active and specific PP2A. Deletion of the yeast PTPA homologs generated a PP2A catalytic subunit with a conformation different from the wild-type enzyme, as indicated by its altered substrate specificity, reduced protein stability, and metal dependence. Complementation and RNA-interference experiments showed that PTPA fulfills an essential function conserved from yeast to man. Protein phosphorylation is a posttranslational modification, mostly reversible, that is used in cells for the regulation of multiple processes. Analyses of eukaryotic genomes reveal that the genes coding for protein kinases, the enzymes catalyzing the phosphorylation reaction, outnumber by two-to threefold genes for protein phosphatases, the enzymes catalyzing dephosphorylation (Zolnierowicz 2000). Protein phosphatases counterbalance the activity of the large number of substrate-specific kinases by the combinatorial assembly of holoenzymes with different substrate specificity. Holoenzymes of a certain protein phosphatase family consist of a common catalytic subunit associated with different regulatory subunits that determine substrate targeting and modulate catalytic activity. Hence, the catalytic subunits of the major protein phosphatase families are produced in abundance. For instance, the catalytic subunit (C subunit) of protein phosphatase 2A (PP2A), comprises, dependent on the cell type, 0.3%-1% of total cellular protein (Virshup 2000).Based on its specificity for phosphorylated serine/ threonine residues, the PP2A C subunit belongs to the family of eukaryotic protein-serine/threonine phosphatases (PSTPs). PSTPs possess a catalytic core structure that is distinct from the core of protein tyrosine phosphatases (PTPs) and dual specificity phosphatases (DSPs). In consequence of the structural differences, the different protein phosphatase families use distinct catalytic mechanisms for the hydrolysis of the phosphoester bond. In contrast to PTPs (and the DSP subfamily) PSTPs are metallo-phosphoesterases that require metals in the active site for catalysis and for their structural integrity. When isolated from eukaryotic sources, the PSTP family members protein phosphatase 1 (PP1) and PP2A ("native" PP1 or PP2A) do not require the addition of metal ions for their activity. However, PP1 and PP2A convert into metal-dependent enzymes during long-term storage or on treatment with the phosphatase inhibitors ATP, pyrophosphate (PPi), or NaF (Burche...

Protein phosphatase 2A (PP2A) is a prime example of the multisubunit architecture of protein serine/threonine phosphatases. Until substrate-specific PP2A holoenzymes assemble, a constitutively active, but nonspecific, catalytic C subunit would constitute a risk to the cell. While it has been assumed that the severe proliferation impairment of yeast lacking the structural PP2A subunit, TPD3, is due to the unrestricted activity of the C subunit, we recently obtained evidence for the existence of the C subunit in a low-activity conformation that requires the RRD/PTPA proteins for the switch into the active conformation. To study whether and how maturation of the C subunit is coupled with holoenzyme assembly, we analyzed PP2A biogenesis in yeast. Here we show that the generation of the catalytically active C subunit depends on the physical and functional interaction between RRD2 and the structural subunit, TPD3. The phenotype of the tpd3Δ strain is therefore caused by impaired, rather than increased, PP2A activity. TPD3/RRD2-dependent C subunit maturation is under the surveillance of the PP2A methylesterase, PPE1, which upon malfunction of PP2A biogenesis, prevents premature generation of the active C subunit and holoenzyme assembly by counteracting the untimely methylation of the C subunit. We propose a novel model of PP2A biogenesis in which a tightly controlled activation cascade protects cells from untargeted activity of the free catalytic PP2A subunit.