The hierarchical architecture of skeletal muscle spans from microscale sarcomeres to macroscale myofibers and is integral to its contractile functionality. Pathologies such as volumetric muscle loss (VML) compromise this structure and destroy the native extracellular matrix (ECM), exceeding the regenerative capacity of endogenous repair mechanisms. Here, we present a novel method for tissue engineering biomimetic three-dimensional (3D) skeletal muscle with complex architectures by leveraging freeform reversible embedding of suspended hydrogels (FRESH) 3D bioprinting of the ECM. Collagen type I scaffolds mimicking diverse muscle architectures (including parallel, unipennate, bipennate, multipennate, and convergent) were designed, FRESH printed, and seeded with C2C12 myoblasts to guide myogenesis. Engineered muscle tissues demonstrated scaffold-mediated alignment and fusion into functional myotubes, exhibiting contractile responses to electrical stimulation with architecture-dependent specific force of ~1 kN/m2 and a positive force-frequency relationship. In vivo implantation further revealed scaffold-directed cellular and vascular organization, underscoring the translational potential of this approach. In summary, this study demonstrates the capability to use FRESH 3D bioprinting to engineer physiologically relevant muscle architectures, significantly advancing the design of functional muscle tissues for regenerative medicine and in vitro modeling applications.  ### Competing Interest Statement AWF is employed by and has an equity stake in FluidFormBio, Inc, which is a startup company commercializing FRESH 3D printing. FRESH 3D printing is the subject of patent protection including US Patents 10,150,258, 11,672,887 and others.

The ultimate goal of cardiac tissue engineering is to generate new muscle to repair or replace the damaged heart. This requires advances in stem cell technologies to differentiate billions of cardiomyocytes, together with advanced biofabrication approaches such as 3D bioprinting to achieve the requisite structure and contractile function. In this concise review, we cover recent progress in 3D bioprinting of cardiac tissue using pluripotent stem cell-derived cardiomyocytes, key design criteria for engineering aligned cardiac tissues, and ongoing challenges in the field that must be addressed to realize this goal.

Cardiac Purkinje fibers form the most distal part of the ventricular conduction system. They coordinate contraction and play a key role in ventricular arrhythmias. While many cardiac cell types can be generated from human pluripotent stem cells, methods to generate Purkinje fiber cells remain limited, hampering our understanding of Purkinje fiber biology and conduction system defects. To identify signaling pathways involved in Purkinje fiber formation, we analyzed single cell data from murine embryonic hearts and compared Purkinje fiber cells to trabecular cardiomyocytes. This identified several genes, processes, and signaling pathways putatively involved in cardiac conduction, including Notch signaling. We next tested whether Notch activation could convert human pluripotent stem cell-derived cardiomyocytes to Purkinje fiber cells. Following Notch activation, cardiomyocytes adopted an elongated morphology and displayed altered electrophysiological properties including increases in conduction velocity, spike slope, and action potential duration, all characteristic features of Purkinje fiber cells. RNA-sequencing demonstrated that Notch-activated cardiomyocytes undergo a sequential transcriptome shift, which included upregulation of key Purkinje fiber marker genes involved in fast conduction such as SCN5A, HCN4 and ID2, and downregulation of genes involved in contractile maturation. Correspondingly, we demonstrate that Notch-induced cardiomyocytes have decreased contractile force in bioengineered tissues compared to control cardiomyocytes. We next modified existing in silico models of human pluripotent stem cell-derived cardiomyocytes using our transcriptomic data and modeled the effect of several anti-arrhythmogenic drugs on action potential and calcium transient waveforms. Our models predicted that Purkinje fiber cells respond more strongly to dofetilide and amiodarone, while cardiomyocytes are more sensitive to treatment with nifedipine. We validated these findings in vitro, demonstrating that our new cell-specific in vitro model can be utilized to better understand human Purkinje fiber physiology and its relevance to disease.

Background:Doxorubicin is an essential cancer treatment, but its usefulness is hampered by the occurrence of cardiotoxicity. Nevertheless, the pathophysiology underlying doxorubicin-induced cardiotoxicity and the respective molecular mechanisms are poorly understood. Recent studies have suggested involvement of cellular senescence. Objectives:The aims of this study were to establish whether senescence is present in patients with doxorubicin-induced cardiotoxicity and to investigate if this could be used as a potential treatment target. Methods:Biopsies from the left ventricles of patients with severe doxorubicin-induced cardiotoxicity were compared with control samples. Additionally, senescence-associated mechanisms were characterized in 3-dimensional dynamic engineered heart tissues (dyn-EHTs) and human pluripotent stem cell-derived cardiomyocytes. These were exposed to multiple, clinically relevant doses of doxorubicin to recapitulate patient treatment regimens. To prevent senescence, dyn-EHTs were cotreated with the senomorphic drugs 5-aminoimidazole-4-carboxamide ribonucleotide and resveratrol. Results:Senescence-related markers were significantly up-regulated in the left ventricles of patients with doxorubicin-induced cardiotoxicity. Treatment of dyn-EHTs resulted in up-regulation of similar senescence markers as seen in the patients, accompanied by tissue dilatation, decreased force generation, and increased troponin release. Treatment with senomorphic drugs led to decreased expression of senescence-associated markers, but this was not accompanied by improved function. Conclusions:Senescence was observed in the hearts of patients with severe doxorubicin-induced cardiotoxicity, and this phenotype can be modeled in vitro by exposing dyn-EHTs to repeated clinically relevant doses of doxorubicin. The senomorphic drugs 5-aminoimidazole-4-carboxamide ribonucleotide and resveratrol prevent senescence but do not result in functional improvements. These findings suggest that preventing senescence by using a senomorphic during doxorubicin administration might not prevent cardiotoxicity.

Background: Fat grafting is an effective treatment for craniofacial deformities. Stromal vascular fraction (SVF) is a concentrated form of adipose derived stem cells that can be isolated from fat. The aim of this clinical trial was to assess the impact of SVF enrichment on craniofacial fat grafting. Methods: Twelve subjects with at least two regions of craniofacial volume deficit were enrolled, and they underwent fat grafting with SVF-enriched or standard fat grafting to each area. All patients had bilateral malar regions injected with SVF-enriched graft on one side and control standard fat grafting to the contralateral side. Outcome assessments included demographic information, volume retention determined by CT scans, SVF cell populations assessed by flow cytometry, SVF cell viability, complications, and appearance ratings. Follow-up was 9 months. Results: All patients had improvement in appearance. There were no serious adverse events. There was no significant difference in volume retention between the SVF-enriched and control regions overall (50.3% versus 57.3%, P = 0.269) or comparing malar regions (51.4% versus 56.7%, P = 0.494). Patient age, smoking status, obesity, and diagnosis of diabetes did not impact volume retention. Cell viability was 77.4% ± 7.3%. Cellular subpopulations were 60.1% ± 11.2% adipose derived stem cells, 12.2 ± 7.0% endothelial cells, and 9.2% ± 4.4% pericytes. A strong positive correlation was found between CD146+ CD31-pericytes and volume retention (R = 0.863, P = 0.027). Conclusions: Autologous fat transfer for reconstruction of craniofacial defects is effective and safe, leading to reliable volume retention. However, SVF enrichment does not significantly impact volume retention.

Three-dimensional (3D) bioprinting is a transformative technology for engineering tissues for disease modeling and drug screening and building tissues and organs for repair, regeneration, and replacement. In this Viewpoint, we discuss technological advances in 3D bioprinting, key remaining challenges, and essential milestones toward clinical translation.

Here we report the 3D bioprinting of a simplified model of the heart, similar to that observed in embryonic development, where the heart is a linear tube that pumps blood and nutrients to the growing embryo. To this end, we engineered a bioinspired model of the human heart tube using freeform reversible of embedding of suspended hydrogels 3D bioprinting. The 3D bioprinted heart tubes were cellularized using human stem cell-derived cardiomyocytes and cardiac fibroblasts and formed patent, perfusable constructs. Synchronous contractions were achieved ∼3-4 days after fabrication and were maintained for up to a month. Immunofluorescent staining confirmed large, interconnected networks of sarcomeric alpha actinin-positive cardiomyocytes. Electrophysiology was assessed using calcium imaging and demonstrated anisotropic calcium wave propagation along the heart tube with a conduction velocity of ∼5 cm s-1. Contractility and function was demonstrated by tracking the movement of fluorescent beads within the lumen to estimate fluid displacement and bead velocity. These results establish the feasibility of creating a 3D bioprinted human heart tube and serve as an initial step towards engineering more complex heart muscle structures.

The extensive crosstalk between the developing heart and lung is critical to their proper morphogenesis and maturation. However, there remains a lack of models that investigate the critical cardio-pulmonary mutual interaction during human embryogenesis. Here, we reported a novel stepwise strategy for directing the simultaneous induction of both mesoderm-derived cardiac and endoderm-derived lung epithelial lineages within a single differentiation of human-induced pluripotent stem cells (hiPSCs) via temporal specific tuning of WNT and nodal signaling in the absence of exogenous growth factors. Using 3D suspension culture, we established concentric cardio-pulmonary micro-Tissues (μTs), and expedited alveolar maturation in the presence of cardiac accompaniment. Upon withdrawal of WNT agonist, the cardiac and pulmonary components within each dual-lineage μT effectively segregated from each other with concurrent initiation of cardiac contraction. We expect that our multilineage differentiation model will offer an experimentally tractable system for investigating human cardio-pulmonary interaction and tissue boundary formation during embryogenesis.

Burn injuries are among the most devastating wounds and represent the fourth most common type of trauma worldwide.1 Burns that occur across joints often result in contractures resulting in severe aesthetic and functional impairment due to scar contracture. The current standard of care, scar excision followed by skin grafts or flaps, create donor site morbidity and result in a high rate of recurrence. For these reasons, there has been great interest in treatments that do not involve surgery and the risks associated with grafting autologous donor tissue.

The start codon c.1A>G mutation in KLHL24, encoding ubiquitin-ligase KLHL24, results in the loss of 28 N-terminal amino acids (KLHL24-ΔN28) by skipping the initial start codon. In skin, KLHL24-ΔN28 leads to gain of function, excessively targeting intermediate filament keratin-14 for proteasomal degradation, ultimately causing epidermolysis bullosa simplex (EBS). The majority of these EBS-patients are also diagnosed with dilated cardiomyopathy (DCM), but the pathological mechanism in the heart is unknown. As desmin is the cardiac homologue of keratin-14, we hypothesized that KLHL24-ΔN28 leads to excessive degradation of desmin, resulting in DCM. Dynamically loaded engineered heart tissues (dyn-EHTs) were generated from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes from two patients and three (non)familial controls. Ten-fold lower desmin protein levels were observed in patient-derived dyn-EHTs, in line with diminished desmin levels detected in patients' explanted heart. This was accompanied by tissue dilatation, impaired mitochondrial function, decreased force values and increased cardiomyocyte stress. HEK293 transfection studies confirmed KLHL24-mediated desmin degradation. KLHL24 RNA interference or direct desmin overexpression recovered desmin protein levels, restoring morphology and function in patient-derived dyn-EHTs. To conclude, presence of KLHL24-ΔN28 in cardiomyocytes leads to excessive degradation of desmin, affecting tissue morphology and function, that can be prevented by restoring desmin protein levels.

Graphene with its unique electrical properties is a promising candidate for carbon-based biosensors such as microelectrodes and field effect transistors. Recently, graphene biosensors were successfully used for extracellular recording of action potentials in electrogenic cells; however, intracellular recordings remain beyond their current capabilities because of the lack of an efficient cell poration method. Here, we present a microelectrode platform consisting of out-of-plane grown three-dimensional fuzzy graphene (3DFG) that enables recording of intracellular cardiac action potentials with high signal-to-noise ratio. We exploit the generation of hot carriers by ultrafast pulsed laser for porating the cell membrane and creating an intimate contact between the 3DFG electrodes and the intracellular domain. This approach enables us to detect the effects of drugs on the action potential shape of human-derived cardiomyocytes. The 3DFG electrodes combined with laser poration may be used for all-carbon intracellular microelectrode arrays to allow monitoring of the cellular electrophysiological state.

Objective: This study aimed to prospectively assess outcomes for surgical autologous fat transfer (AFT) applied for traumatic and postsurgical craniofacial deformities. The minimally invasive nature of AFT has potential for reduced risk and superior outcomes compared with current reconstructive options. Background: Craniofacial deformities have functional and psychosocial sequelae and can profoundly affect quality of life. Traditional reconstructive options are invasive, invasive, complex, and often lack precision in outcomes. Although AFT is safe, effective, and minimally invasive, only anecdotal evidence exists for reconstruction of craniofacial deformities. Methods: In this Institutional Review Board-approved prospective cohort study, 20 subjects underwent AFT (average volume: 23.9 ± 13.2 mL). Volume retention over time was determined using high-resolution computed tomography. Flow cytometry was used to assess cellular subpopulations and viability in the stromal vascular fraction. Quality of life assessments were performed. After the completion of 9-month follow-up, 5 subjects were enrolled for a second treatment. Results: No serious adverse events occurred. Volume retention averaged 63 ± 17% at 9 months. Three-month retention strongly predicted 9-month retention (r=0.996, P < 0.0001). There was no correlation between the total volume injected and retention. Patients undergoing a second procedure had similar volume retention as the first ( P = 0.05). Age, sex, body mass index, and stromal vascular fraction cellular composition did not impact retention. Surprisingly, former smokers had greater volume retention at 9 months compared with nonsmokers (74.4% vs 56.2%, P = 0.009). Satisfaction with physical appearance ( P = 0.002), social relationships ( P = 0.02), and social functioning quality of life ( P = 0.05) improved from baseline to 9 months. Conclusions: For craniofacial defects, AFT is less invasive and safer than traditional reconstructive options. It is effective, predictable, and reaches volume stability at 3 months. Patient-reported outcomes demonstrate a positive life-changing impact.

The role that mechanical forces play in shaping the structure and function of the heart is critical to understanding heart formation and the etiology of disease but is challenging to study in patients. Engineered heart tissues (EHTs) incorporating human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have the potential to provide insight into these adaptive and maladaptive changes. However, most EHT systems cannot model both preload (stretch during chamber filling) and afterload (pressure the heart must work against to eject blood). Here, we have developed a new dynamic EHT (dyn-EHT) model that enables us to tune preload and have unconstrained contractile shortening of >10%. To do this, three-dimensional (3D) EHTs were integrated with an elastic polydimethylsiloxane strip providing mechanical preload and afterload in addition to enabling contractile force measurements based on strip bending. Our results demonstrated that dynamic loading improves the function of wild-type EHTs on the basis of the magnitude of the applied force, leading to improved alignment, conduction velocity, and contractility. For disease modeling, we used hiPSC-derived cardiomyocytes from a patient with arrhythmogenic cardiomyopathy due to mutations in the desmoplakin gene. We demonstrated that manifestation of this desmosome-linked disease state required dyn-EHT conditioning and that it could not be induced using 2D or standard 3D EHT approaches. Thus, a dynamic loading strategy is necessary to provoke the disease phenotype of diastolic lengthening, reduction of desmosome counts, and reduced contractility, which are related to primary end points of clinical disease, such as chamber thinning and reduced cardiac output.

Cardiac two-dimensional tissues were engineered using biomimetic micropatterns based on the fibronectin-rich extracellular matrix (ECM) of the embryonic heart. The goal of this developmentally-inspired, in vitro approach was to identify cell–cell and cell-ECM interactions in the microenvironment of the early 4-chambered vertebrate heart that drive cardiomyocyte organization and alignment. To test this, biomimetic micropatterns based on confocal imaging of fibronectin in embryonic chick myocardium were created and compared to control micropatterns designed with 2 or 20 µm wide fibronectin lines. Results show that embryonic chick cardiomyocytes have a unique density-dependent alignment on the biomimetic micropattern that is mediated in part by N-cadherin, suggesting that both cell–cell and cell-ECM interactions play an important role in the formation of aligned myocardium. Human induced pluripotent stem cell-derived cardiomyocytes also showed density-dependent alignment on the biomimetic micropattern but were overall less well organized. Interestingly, the addition of human adult cardiac fibroblasts and conditioning with T3 hormone were both shown to increase human cardiomyocyte alignment. In total, these results show that cardiomyocyte maturation state, cardiomyocyte-cardiomyocyte and cardiomyocyte-fibroblast interactions, and cardiomyocyte-ECM interactions can all play a role when engineering anisotropic cardiac tissues in vitro and provides insight as to how these factors may influence cardiogenesis in vivo.

This protocol details the step-by-step fabrication process to create an extracellular matrix (ECM) protein-based nanomechanical biosensor (NMBS) via an adaptation of the surface-initiated assembly (SIA) procedure 1 . The NMBS is a fluorescently labeled, ultra-thin fibronectin lattice-mesh with spatial resolution tailored by adjusting the width and spacing of the lattice from 2-100 µm. By applying the NMBS to the surface of cells and tissues one can directly measure deformation from subcellular to tissue length-scales. The procedure outlined here covers all aspects of NMBS construction from pattern design, photolithography for mold creation, casting of polydimethylsiloxane (PDMS) silicone stamps, conjugation of fluorescent dye to fibronectin, microcontact printing of fibronectin, creation of gelatin carriers 2 , transfer of the NMBS to gelatin carriers, and the use of gelatin carriers for application of the NMBS onto cells and tissue. The protocol can be broken down into three phases: material preparation (2 days), NMBS patterning (4 hours), and transfer to cell or tissue (1 hour). Material preparation and NMBS patterning can be performed ahead of time and the patterned NMBS can be stored for up to 1 year prior to use.

Microelectrode arrays (MEAs) have enabled investigation of cellular networks at sub-millisecond temporal resolution. However, current MEAs are limited by the large electrode footprint since reducing the electrode's geometric area to sub-cellular dimensions leads to a significant increase in impedance thus affecting its recording capabilities. We report a breakthrough ultra-microelectrodes platform by leveraging the outstanding surface-to-volume ratio of nanowire-templated out-of-plane synthesized three-dimensional fuzzy graphene (NT-3DFG). The enormous surface area of NT-3DFG leads to 140-fold reduction in electrode impedance compared to bare Au microelectrodes, thus enabling scaling down the geometric size by 625-fold to ca. 2 μm × 2 μm. The out-of-plane morphology of NT-3DFG leads to a tight seal with the cell membrane thus enabling recording of electrical signals with high signal-to-noise ratio (SNR) of > 6. This work highlights the possibility to push the limits of the conventional MEA technology to enable electrophysiological investigation at sub-cellular level without the need of any surface coatings. This presented approach would greatly impact our basic understanding of signal transduction within a single cell as well as complex cellular assemblies.