RESOURCES

Abstract
Bacteria can be programmed to deliver natural materials with defined biological and mechanical properties for controlling cell growth and differentiation. Here, we present an elastic, resilient and bioactive polysaccharide derived from the extracellular matrix of Pantoea sp. BCCS 001. Specifically, it was methacrylated to generate a new photo crosslinkable hydrogel that we coined Pantoan Methacrylate or put simply PAMA. We have used it for the first time as a tissue engineering hydrogel to treat VML injuries in rats. The crosslinked PAMA hydrogel was super elastic with a recovery nearing 100 %, while mimicking the mechanical stiffness of native muscle. After inclusion of thiolated gelatin via a Michaelis reaction with acrylate groups on PAMA we could also guide muscle progenitor cells into fused and aligned tubes – something reminiscent of mature muscle cells. These results were complemented by sarcomeric alpha-actinin immunostaining studies. Importantly, the implanted hydrogels exhibited almost 2-fold more muscle formation and 50 % less fibrous tissue formation compared to untreated rat groups. In vivo inflammation and toxicity assays likewise gave rise to positive results confirming the biocompatibility of this new biomaterial system. Overall, our results demonstrate that programmable polysaccharides derived from bacteria can be used to further advance the field of tissue engineering. In greater detail, they could in the foreseeable future be used in practical therapies against VML.

Abstract
Abstract 3D-printed artificial skeletal muscle, which mimics the structural and functional characteristics of native skeletal muscle, is a promising treatment method for muscle reconstruction. Although various fabrication techniques for skeletal muscle using 3D bio-printers are studied, it is still challenging to build a functional muscle structure. A strategy using microvalve-assisted coaxial 3D bioprinting in consideration of functional skeletal muscle fabrication is reported. The unit (artificial muscle fascicle: AMF) of muscle mimetic tissue is composed of a core filled with medium-based C2C12 myoblast aggregates as a role of muscle fibers and a photo cross-linkable hydrogel-based shell as a role of connective tissue in muscles that enhances printability and cell adhesion and proliferation. Especially, a microvalve system is applied for the core part with even cell distribution and strong cell–cell interaction. This system enhances myotube formation and consequently shows spontaneous contraction. A multi-printed AMF (artificial muscle tissue: AMT) as a piece of muscle is implanted into the anterior tibia (TA) muscle defect site of immunocompromised rats. As a result, the TA-implanted AMT responds to electrical stimulation and represents histologically regenerated muscle tissue. This microvalve-assisted coaxial 3D bioprinting shows a significant step forward to mimicking native skeletal muscle tissue.

Abstract
Conducting polymeric materials have been used to modulate response of cells seeded on their surfaces. However, there is still major improvement to be made related to their biocompatibility, conductivity, stability in biological milieu, and processability toward truly tissue engineered functional device. In this work, conductive polymer, poly(3,4-ethylene-dioxythiophene):polystyrene-sulfonate (PEDOT:PSS), and its possible applications in tissue engineering were explored. In particular PEDOT:PSS solution was inkjet printed onto a gelatin substrate for obtaining a conductive structure. Mechanical and electrical characterizations, structural stability by swelling and degradation tests were carried out on different PEDOT-based samples obtained by varying the number of printed PEDOT layers from 5 to 50 on gelatin substrate. Biocompatibility of substrates was investigated on C2C12 myoblasts, through metabolic activity assay and imaging analysis during a 7-days culture period, to assess cell morphology, differentiation and alignment. The results of this first part allowed to proceed with the second part of the study in which these substrates were used for the design of an electrical stimulation device, with the aim of providing the external stimulus (3 V amplitude square wave at 1 and 2 Hz frequency) to guide myotubes alignment and enhance differentiation, having in this way promising applications in the field of muscle tissue engineering.